My Bibliography for Research on Autonomous Driving

Motivation

In this document, I would like to share some personal notes about the latest exciting trends in research about decision making for autonomous driving. I keep on updating it 👷 🚧 😃

Template:

"title" [ Year ] [📝 (paper)] [ (code)] [🎞️ (video)] [ 🎓 University X ] [ 🚗 company Y ] [ related, concepts ]

Categories:

Architecture and Map
Behavioural Cloning, End-To-End and Imitation Learning
Inverse Reinforcement Learning, Inverse Optimal Control and Game Theory
Prediction and Manoeuvre Recognition
Rule-based Decision Making
Model-Free Reinforcement Learning
Model-Based Reinforcement Learning
Planning and Monte Carlo Tree Search

Besides, I reference additional publications in some parallel works:

Looking forward your reading suggestions!

`Architecture` and `Map`

"A Review of Motion Planning for Highway Autonomous Driving"

[ 2019 ] [📝] [ 🎓 French Institute of Science and Technology for Transport ] [ 🚗 VEDECOM Institute ]

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The review divides motion-planning into five parts. The `decision-making` part contains `risk evaluation`, `criteria minimization`, and `constraint submission`. In the last part, a low-level reactive planner deforms the generated motion from the high-level planner. Source.


The review offers two detailed tools for comparing methods for motion planning for highway scenarios. Criteria for the generated motion include: `feasible`, `safe`, `optimal`, `usable`, `adaptive`, `efficient`, `progressive` and `interactive`. The authors stressed the importance of spatiotemporal consideration and emphasize that highway motion-planning is highly structured. Source.

Contrary to solve-algorithms methods, set-algorithm methods require a complementary algorithm should be added to find the feasible motion. Depending on the importance of the generation (iv) and deformation (v) part, approaches are more or less reactive or predictive. Finally, based on their work on AI-based algorithms, the authors define four subfamilies to compare to human: logic, heuristic, approximate reasoning, and human-like. Source.

The review also offers overviews for possible space configurations, i.e. the choices for decomposition of the evolution space (sampling-based decomposition, connected cells decomposition and lattice representation) as well as path-finding algorithms (e.g. Dijkstra, A*, and RRT). attractive and repulsive forces, parametric and semi-parametric curves, numerical optimization and artificial intelligence are also developed. Source.

Authors: Claussmann, L., Revilloud, M., Gruyer, D., & Glaser, S.

"A Survey of Deep Learning Applications to Autonomous Vehicle Control"

[ 2019 ] [📝] [ 🎓 University of Surrey ] [ 🚗 Jaguar Land Rover ]

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Challenges for learning-based control methods. Source.

Authors: Kuutti, S., Bowden, R., Jin, Y., Barber, P., & Fallah, S.

Three categories are examined:
- lateral control alone.
- longitudinal control alone.
- longitudinal and lateral control combined.
Two quotes:
- "While lateral control is typically achieved through vision, the longitudinal control relies on measurements of relative velocity and distance to the preceding/following vehicles. This means that ranging sensors such as RADAR or LIDAR are more commonly used in longitudinal control systems.".
- "While lateral control techniques favour supervised learning techniques trained on labelled datasets, longitudinal control techniques favour reinforcement learning methods which learn through interaction with the environment."

"Longitudinal Motion Planning for Autonomous Vehicles and Its Impact on Congestion: A Survey"

[ 2019 ] [📝] [ 🎓 Georgia Institute of Technology ]

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`mMP` refers to machine learning methods for longitudinal motion planning. Source.

Authors: Zhou, H., & Laval, J.

This review has been completed at a school of "civil and environmental engineering".
- It does not have any scientific contribution, but offers a quick overview about some current trends in decision-making.
- The authors try to look at industrial applications (e.g. Waymo, Uber, Tesla), i.e. not just focussing on theoretical research. Since companies do no communicate explicitly about their approaches, most of their publications should be considered as research side-projects, rather than "actual state" of the industry.
One focus of the review: the machine learning approaches for decision-making for longitudinal motion.
- About the architecture and representation models. They mention the works of DeepDriving and (H. Xu, Gao, Yu, & Darrell, 2016).
  - Mediated perception approaches parse an entire scene to make a driving decision.
  - Direct perception approaches first extract affordance indicators (i.e. only the information that are important for driving in a particular situation) and then map them to actions.
    - "Only a small portion of detected objects are indeed related to the real driving reactions so that it would be meaningful to reduce the number of key perception indicators known as learning affordances".
  - Behavioural reflex approaches directly map an input image to a driving action by a regressor.
    - This end-to-end paradigm can be extended with auxiliary tasks such as learning semantic segmentation (this "side task" should further improves the model), leading to Privileged training.
- About the learning methods:
  - BC, RL, IRL and GAIL are considered.
  - The authors argue that their memory and prediction abilities should make them stand out from the rule-based approaches.
  - "Both BC and IRL algorithms implicitly assume that the demonstrations are complete, meaning that the action for each demonstrated state is fully observable and available."
  - "We argue that adopting RL transforms the problem of learnt longitudinal motion planning from imitating human demonstrations to searching for a policy complying a hand-crafted reward rule [...] No studies have shown that a genuine reward function for human driving really exists."
About congestion:
- "The AV industry has been mostly focusing on the long tail problem caused by corner errors related to safety, while the impact of AVs on traffic efficiency is almost ignored."
- It reminds me the finding of (Kellett, J., Barreto, R., Van Den Hengel, A. & Vogiatzis. N., 2019) in "How Might Autonomous Vehicles Impact the City? The Case of Commuting to Central Adelaide": driverless cars could lead to more traffic congestion.

"Design Space of Behaviour Planning for Autonomous Driving"

[ 2019 ] [📝] [ 🎓 University of Waterloo ]

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Some figures:


The focus is on the `BP` module, together with its predecessor (`environment`) and its successor (`LP`) in a modular architecture. Source.


Classification for Question `1` - environment representation. A combination is possible. In black, my notes giving examples. Source.


Classification for Question `2` - on the architecture. Source.


Classification for Question `3` - on the decision logic representation. Source.

Authors: Ilievski, M., Sedwards, S., Gaurav, A., Balakrishnan, A., Sarkar, A., Lee, J., Bouchard, F., De Iaco, R., & Czarnecki K.

The authors divide their review into three sections:

Question 1: How to represent the environment? (relation with predecessor of BP)
- Four representations are compared: raw data, feature-based, grid-based and latent representation.
Question 2: How to communicate with other modules, especially the local planner (LP)? (relation with successor (LP) of BP)
- A first sub-question is the relevance of separation BP / LP.
  - A complete separation (top-down) can lead to computational redundancy (both have a collision checker).
  - One idea, close to sampling techniques, could be to invert the traditional architecture for planning, i.e. generate multiple possible local paths (~LP) then selects the best manoeuvre according to a given cost function (~BP). But this exasperates the previous point.
- A second sub-question concerns prediction: Should the BP module have its own dedicated prediction module?
  - First, three kind of prediction are listed, depending on what should be predicted (marked with ->):
    - Physics-based (-> trajectory).
    - Manoeuvre-based (-> low-level motion primitives).
    - Interaction-aware (-> intent).
  - Then, the authors distinguish between explicitly-defined and implicitly-defined prediction models:
    - Explicitly-defined can be:
      - Integrated with the motion planning process (called Internal prediction models) such as belief-based decision making (e.g. POMDP). Ideal for planning under uncertainty.
      - Decoupled from the planning process (called External prediction models). There is a clear interface between prediction and planning, which aids modularity.
    - Implicitly-defined, such as RL techniques.
Question 3: How to make BP decisions? (BP itself)
- A first distinction in representation of decision logic is made depending based on non-learnt / learnt:
  - Using a set of explicitly programmed production rules can be divided into:
    - Imperative approaches, e.g. state machines.
    - Declarative approaches often based on some probabilistic system.
      - The decision-tree structure and the (PO)MDP formulation makes it more robust to uncertainty.
      - Examples include MCTS and online POMDP solvers.
  - Logic representation can also rely on mathematical models with parameters learned a priori.
    - A distinction is made depending on "where does the training data come from and when is it created?".
    - In other words, one could think of supervised learning (learning from example) versus reinforcement learning (learning from interaction).
    - The combination of both seems beneficial:
      - An initial behaviour is obtained through imitation learning (learning from example). Also possible with IRL.
      - But improvements are made through interaction with a simulated environment (learning from interaction).
        
        By the way, the learning from interaction techniques raise the question of the origin of the experience (e.g. realism of the simulator) and its sampling efficiency.
    - Another promising direction is hierarchical RL where the MDP is divided into sub-problems (the lower for LP and the higher for BP)
      - The lowest level implementation of the hierarchy approximates a solution to the control and LP problem ...
      - ... while the higher level selects a manoeuvre to be executed by the lower level implementations.
- As mentioned in my the section on Scenarios and Datasets, the authors mention the lack of benchmark to compare and evaluate the performance of BP technologies.

One quote about the representation of decision logic:

As identified in my notes about IV19, the combination of learnt and non-learnt approaches looks the most promising.
"Without learning, traditional robotic solutions cannot adequately handle complex, dynamic human environments, but ensuring the safety of learned systems remains a significant challenge."
"Hence, we speculate that future high performance and safe behaviour planning solutions will be hybrid and heterogeneous, incorporating modules consisting of learned systems supervised by programmed logic."

"A Behavioral Planning Framework for Autonomous Driving"

[ 2014 ] [📝] [ 🎓 Carnegie Mellon University ] [ 🚗 General Motor ]
[ behavioural planning, sampling-based planner, decision under uncertainty, TORCS ]

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Some figures:


Comparison and fusion of the hierarchical* and parallel architectures. Source.*


The `PCB` algorithm implemented in the `BP` module. Source.


Related work by (Xu, Pan, Wei, & Dolan, 2014) - Grey ellipses indicate the magnitude of the uncertainty of state. Source.

Authors: Wei, J., Snider, J. M., & Dolan, J. M.

Note: I find very valuable to get insights from the CMU (Carnegie Mellon University) Team, based on their experience of the DARPA Urban Challenges.

Related works:
- A prediction- and cost function-based algorithm for robust autonomous freeway driving. 2010 by (Wei, Dolan, & Litkouhi, 2010).
  - They introduced the "Prediction- and Cost-function Based (PCB) algorithm" used.
  - The idea is to generate-forward_simulate-evaluate a set of manoeuvres.
  - The planner can therefore take surrounding vehicles’ reactions into account in the cost function when it searches for the best strategy.
  - At the time, the authors rejected the option of a POMDP formulation (computing the control policy over the space of the belief state, which is a probability distribution over all the possible states) deemed as computationally expensive. Improvements in hardware and algorithmic have been made since 2014.
- Motion planning under uncertainty for on-road autonomous driving. 2014 by (Xu, Pan, Wei, & Dolan, 2014).
  - An extension of the framework to consider uncertainty (both for environment and the others participants) in the decision-making.
  - The prediction module is using a Kalman Filter (assuming constant velocity).
  - For each candidate trajectory, the uncertainty can be estimated using a Linear-Quadratic Gaussian (LQG) framework (based on the noise characteristics of the localization and control).
  - Their Gaussian-based method gives some probabilistic safety guaranty (e.g. likelihood 2% of collision to occur).
Proposed architecture for decision-making:
- First ingredient: Hierarchical architecture.
  - The hierarchy mission -> manoeuvre -> motion 3M concept makes it very modular but can raise limitations:
  - "the higher-level decision making module usually does not have enough detailed information, and the lower-level layer does not have authority to re-evaluate the decision."
- Second ingredient: Parallel architecture.
  - This is inspired from ADAS engineering.
  - The control modules (ACC, Merge Assist, Lane Centreing) are relatively independent and work in parallel.
  - In some complicated cases needing cooperation, this framework may not perform well.
    - This probably shows that just extending the common ADAS architectures cannot be enough to reach the level-5 of autonomy.
- Idea of the proposed framework: combine the strengths of the hierarchical and parallel architectures.
  - This relieves the path planner and the control module (the search space is reduced).
  - Hence the computational cost shrinks (by over 90% compared to a sample-based planner in the spatio-temporal space).
One module worth mentioning: Traffic-free Reference Planner.
- Its input: lane-level sub-missions from the Mission Planning.
- Its output: kinematically and dynamically feasible paths and a speed profile for the Behavioural Planner (BP).
  - It assumes there is no traffic on the road, i.e. ignores dynamic obstacles.
  - It also applies traffic rules such as speed limits.
- This guides the BP layer which considers both static and dynamic obstacles to generate so-called "controller directives" such as:
  - The lateral driving bias.
  - The desired leading vehicle to follow.
  - The aggressiveness of distance keeping.
  - The maximum speed.

`Behavioural Cloning` `End-To-End` and `Imitation Learning`

"Robust Imitative Planning : Planning from Demonstrations Under Uncertainty"

[ 2019 ] [📝] [ 🎓 University of Oxford, UC Berkeley, Carnegie Mellon University ]
[ epistemic uncertainty, risk-aware decision-making, CARLA ]

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Illustration of the `state distribution shift` in behavioural cloning (`BC`) approaches. The models (e.g. neural networks) usually fail to generalize and instead extrapolate confidently yet incorrectly, resulting in arbitrary outputs and dangerous outcomes. Not to mention the compounding (or cascading) errors, inherent to the sequential decision making. Source.


Testing behaviours on scenarios such as roundabouts* that are not present in the training set. Source.*

Above - in their previous work, the authors introduced Deep imitative models (IM). The imitative planning objective is the log posterior probability of a state trajectory, conditioned on satisfying some goal G. The state trajectory that has the highest likelihood w.r.t. the expert model q(S given φ; θ) is selected, i.e. maximum a posteriori probability (MAP) estimate of how an expert would drive to the goal. This captures any inherent aleatoric stochasticity of the human behaviour (e.g., multi-modalities), but only uses a point-estimate of θ, thus q(s given φ;θ) does not quantify model (i.e. epistemic) uncertainty. φ denotes the contextual information (3 previous states and current LIDAR observation) and s denotes the agent’s future states (i.e. the trajectory). Bottom - in this works, an ensemble of models is used: q(s given φ; θk) where θk denotes the parameters of the k-th model (neural network). The Aggregation Operator operator is applied on the posterior p(θ given D). The previous work is one example of that, where a single θi is selected. Source.

To save computation and improve runtime to real-time, the authors use a trajectory library: they perform K-means clustering of the expert plan’s from the training distribution and keep 128 of the centroids, allegedly reducing the planning time by a factor of 400. During optimization, the trajectory space is limited to only that trajectory library. It makes me think of templates sometimes used for path-planning. I also see that as a way restrict the search in the trajectory space, similar to injecting expert knowledge about the feasibility of cars trajectories. Source.


Estimating the uncertainty is not enough. One should then forward that estimate to the `planning` module. This reminds me an idea of (McAllister et al., 2017) about the key benefit of propagating uncertainty throughout the AV framework. Source.

Authors: Tigas, P., Filos, A., Mcallister, R., Rhinehart, N., Levine, S., & Gal, Y.

Previous work: "Deep Imitative Models for Flexible Inference, Planning, and Control" (see below).
- The idea was to combine the benefits of imitation learning (IL) and goal-directed planning such as model-based RL (MBRL).
  - In other words, to complete planning based on some imitation prior, by combining generative modelling from demonstration data with planning.
  - One key idea of this generative model of expert behaviour: perform context-conditioned density estimation of the distribution over future expert trajectories, i.e. score the "expertness" of any plan of future positions.
- Limitations:
  - It only uses a point-estimate of θ. Hence it fails to capture epistemic uncertainty in the model’s density estimate.
  - "Plans can be risky in scenes that are out-of-training-distribution since it confidently extrapolates in novel situations and lead to catastrophes".
Motivations here:
- 1- Develop a model that captures epistemic uncertainty.
- 2- Estimating uncertainty is not a goal at itself: one also need to provide a mechanism for taking low-risk actions that are likely to recover in uncertain situations.
  - I.e. both aleatoric and epistemic uncertainty should be taken into account in the planning objective.
  - This reminds me the figure of (McAllister et al., 2017) about the key benefit of propagating uncertainty throughout the AV framework.
One quote about behavioural cloning (BC) that suffers from state distribution shift (co-variate shift):
- "Where high capacity parametric models (e.g. neural networks) usually fail to generalize, and instead extrapolate confidently yet incorrectly, resulting in arbitrary outputs and dangerous outcomes".
One quote about model-free RL:
- "The specification of a reward function is as hard as solving the original control problem in the first place."
About epistemic and aleatoric uncertainties:
- "Generative models can provide a measure of their uncertainty in different situations, but robustness in novel environments requires estimating epistemic uncertainty (e.g., have I been in this state before?), where conventional density estimation models only capture aleatoric uncertainty (e.g., what’s the frequency of times I ended up in this state?)."
How to capture uncertainty about previously unseen scenarios?
- Using an ensemble of density estimators and aggregate operators over the models’ outputs.
  - "By using demonstration data to learn density models over human-like driving, and then estimating its uncertainty about these densities using an ensemble of imitative models".
- The idea it to take the disagreement between the models into consideration and inform planning.
  - "When a trajectory that was never seen before is selected, the model’s high epistemic uncertainty pushes us away from it. During planning, the disagreement between the most probable trajectories under the ensemble of imitative models is used to inform planning."

"End-to-end Interpretable Neural Motion Planner"

[ 2019 ] [📝] [ 🎓 University of Toronto ] [ 🚗 Uber ]
[ interpretability, trajectory sampling ]

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The visualization of `3D` detection, motion forecasting as well as learned cost-map volume offers interpretability. A set of candidate trajectories is sampled, first considering the geometrical path and then then speed profile. The trajectory with the minimum learned cost is selected. Source.


Source.

Authors: Zeng W., Luo W., Suo S., Sadat A., Yang B., Casas S. & Urtasun R.

Motivation is to bridge the gap between the traditional engineering stack and the end-to-end driving frameworks.
- 1- Develop a learnable motion planner, avoiding the costly parameter tuning.
- 2- Ensure interpretability in the motion decision. This is done by offering an intermediate representation.
- 3- Handle uncertainty. This is allegedly achieved by using a learnt, non-parametric cost function.
- 4- Handle multi-modality in possible trajectories (e.g changing lane vs keeping lane).
One quote about RL and IRL:
- "It is unclear if RL and IRL can scale to more realistic settings. Furthermore, these methods do not produce interpretable representations, which are desirable in safety critical applications".
Architecture:
- Input: raw LIDAR data and a HD map.
- 1st intermediate result: An "interpretable" bird’s eye view representation that includes:
  - 3D detections.
  - Predictions of future trajectories (planning horizon of 3 seconds).
  - Some spatio-temporal cost volume defining the goodness of each position that the self-driving car can take within the planning horizon.
- 2nd intermediate result: A set of diverse physically possible trajectories (candidates).
  - They are Clothoid curves being sampled. First building the geometrical path. Then the speed profile on it.
  - "Note that Clothoid curves can not handle circle and straight line trajectories well, thus we sample them separately."
- Final output: The trajectory with the minimum learned cost.
Multi-objective:
- 1- Perception Loss - to predict the position of vehicles at every time frame.
  - Classification: Distinguish a vehicle from the background.
  - Regression: Generate precise object bounding boxes.
- 2- Planning Loss.
  - "Learning a reasonable cost volume is challenging as we do not have ground-truth. To overcome this difficulty, we minimize the max-margin loss where we use the ground-truth trajectory as a positive example, and randomly sampled trajectories as negative examples."
  - As stated, the intuition behind is to encourage the demonstrated trajectory to have the minimal cost, and others to have higher costs.
  - The model hence learns a cost volume that discriminates good trajectories from bad ones.

"Learning from Interventions using Hierarchical Policies for Safe Learning"

[ 2019 ] [📝] [ 🎓 University of Rochester, University of California San Diego ]
[ hierarchical, sampling efficiency, safe imitation learning ]

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The main idea is to use Learning from Interventions (`LfI`) in order to ensure safety and improve data efficiency, by intervening on sub-goals rather than trajectories. Both `top-level` policy (that generates sub-goals) and `bottom-level` policy are jointly learnt. Source.

Authors: Bi, J., Dhiman, V., Xiao, T., & Xu, C.

Motivations:
- 1- Improve data-efficiency.
- 2- Ensure safety.
One term: "Learning from Interventions" (LfI).
- One way to classify the "learning from expert" techniques is to use the frequency of expert’s engagement.
  - High frequency -> Learning from Demonstrations.
  - Medium frequency -> learning from Interventions.
  - Low frequency -> Learning from Evaluations.
- Ideas of LfI:
  - "When an undesired state is detected, another policy is activated to take over actions from the agent when necessary."
  - Hence the expert overseer only intervenes when it suspects that an unsafe action is about to be taken.
- Two issues:
  - 1- LfI (as for LfD) learn reactive behaviours.
    - "Learning a supervised policy is known to have 'myopic' strategies, since it ignores the temporal dependence between consecutive states".
    - Maybe one option could be to stack frames or to include the current speed in the state. But that makes the state space larger.
  - 2- The expert only signals after a non-negligible amount of delay.
One idea to solve both issues: Hierarchy.
- The idea is to split the policy into two hierarchical levels, one that generates sub-goals for the future and another that generates actions to reach those desired sub-goals.
- The motivation is to intervene on sub-goals rather than trajectories.
- One important parameter: k
  - The top-level policy predicts a sub-goal to be achieved k steps ahead in the future.
  - It represents a trade-off between:
    - The ability for the top-level policy to predict sub-goals far into the future.
    - The ability for the bottom-level policy to follow it correctly.
- One question: How to deal with the absence of ground- truth sub-goals ?
  - One solution is "Hindsight Experience Replay", i.e. consider an achieved goal as a desired goal for past observations.
  - The authors present additional interpolation techniques.
  - They also present a Triplet Network to train goal-embeddings (I did not understand everything).

"Urban Driving with Conditional Imitation Learning"

[ 2019 ] [📝] [🎞️] [ 🚗 Wayve ]
[ end-to-end, conditional IL, robust BC ]

Click to expand

The encoder is trained to reconstruct RGB, depth and segmentation, i.e. to learn scene understanding. It is augmented with optical flow for temporal information. As noted, such representations could be learned simultaneously with the driving policy, for example, through distillation. But for efficiency, this was pre-trained (Humans typically also have ~30 hours of driver training before taking the driving exam. But they start with huge prior knowledge). Interesting idea: the navigation command is injected as multiple locations of the control part. Source.


Driving data is inherently heavily imbalanced, where most of the captured data will be driving near-straight in the middle of a lane. Any naive training will collapse to the dominant mode present in the data. No data augmentation is performed. Instead, during training, the authors sample data uniformly across `lateral` and `longitudinal` control dimensions. Source.

Authors: Hawke, J., Shen, R., Gurau, C., Sharma, S., Reda, D., Nikolov, N., Mazur, P., Micklethwaite, S., Griffiths, N., Shah, A. & Kendall, A.

Motivations:
- 1- Learn both steering and speed via Behavioural Cloning.
- 2- Use raw sensor (camera) inputs, rather than intermediate representations.
- 3- Train and test on dense urban environments.
Why "conditional"?
- A route command (e.g. turn left, go straight) resolves the ambiguity of multi-modal behaviours (e.g. when coming at an intersection).
- "We found that inputting the command multiple times at different stages of the network improves robustness of the model".
Some ideas:
- Provide wider state observability through multiple camera views (single camera disobeys navigation interventions).
- Add temporal information via optical flow.
  - Another option would be to stack frames. But it did not work well.
- Train the primary shared encoders and auxiliary independent decoders for a number of computer vision tasks.
  - "In robotics, the test data is the real-world, not a static dataset as is typical in most ML problems. Every time our cars go out, the world is new and unique."
One concept: "Causal confusion".
- "Spurious correlations cannot be distinguished from true causes in the demonstrations. [...] For example, inputting the current speed to the policy causes it to learn a trivial identity mapping, making the car unable to start from a static position."
- Two ideas during training:
  - Using flow features to make the model use explicit motion information without learning the trivial solution of an identity mapping for speed and steering.
  - Add random noise and use dropout on it.
- One alternative is to explicitly maintain a causal model.
- Another alternative is to learn to predict the speed, as detailed in "Exploring the Limitations of Behavior Cloning for Autonomous Driving".
Output:
- The model decides of a "motion plan", i.e. not directly the low-level control?
- Concretely, the network gives one prediction and one slope, for both speed and steering, leading to two parameterised lines.
Two types of tests:
- 1- Closed-loop (i.e. go outside and drive).
  - The number and type of safety-driver interventions.
- 2- Open-loop (i.e., evaluating on an offline dataset).
  - The weighted mean absolute error for speed and steering.
    - As noted, this can serve as a proxy for real world performance.
- "As discussed by [34] and [35], the correlation between offline open-loop metrics and online closed-loop performance is weak."
About the training data:
- As stated, they are two levers to increase the performance:
  - 1- Algorithmic innovation.
  - 2- Data.
- For this IL approach, 30 hours of demonstrations.
- "Re-moving a quarter of the data notably degrades performance, and models trained with less data are almost undriveable."
Next steps:
- I find the results already impressive. But as noted:
  - "The learned driving policies presented here need significant further work to be comparable to human driving".
- Ideas for improvements include:
  - Add some predictive long-term planning model. At the moment, it does not have access to long-term dependencies and cannot reason about the road scene.
  - Learn not only from demonstration, but also from mistakes.
    - This reminds me the concept of ChauffeurNet about "simulate the bad rather than just imitate the good".
  - Continuous learning: Learning from corrective interventions would also be beneficial.
- The last point goes in the direction of adding learning signals, which was already done here.
  - Imitation of human expert drivers (supervised learning).
  - Safety driver intervention data (negative reinforcement learning) and corrective action (supervised learning).
  - Geometry, dynamics, motion and future prediction (self-supervised learning).
  - Labelled semantic computer vision data (supervised learning).
  - Simulation (supervised learning).

"Application of Imitation Learning to Modeling Driver Behavior in Generalized Environments"

[ 2019 ] [📝] [ 🎓 Stanford ]
[ GAIL, RAIL, domain adaption, NGSIM ]

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The `IL` models were trained on a straight road and tested on roads with high curvature. `PS-GAIL` is effective only while surrounded by other vehicles, while the `RAIL` policy remained stably within the bounds of the road thanks to the additional rewards terms included into the learning process.. Source.

Authors: Lange, B. A., & Brannon, W. D.

One motivation: Compare the robustness (domain adaptation) of three IL techniques:
- 1- Generative Adversarial Imitation Learning (GAIL).
- 2- Parameter Sharing GAIL (PS-GAIL).
- 3- Reward Augmented Imitation Learning (RAIL).
One take-away: This student project builds a good overview of the different IL algorithms and why these algorithms came out.
- Imitation Learning (IL) aims at building an (efficient) policy using some expert demonstrations.
- Behavioural Cloning (BC) is a sub-class of IL. It treats IL as a supervised learning problem: a regression model is fit to the state/action space given by the expert.
  - Issue of distribution shift: "Because data is not infinite nor likely to contain information about all possible state/action pairs in a continuous state/action space, BC can display undesirable effects when placed in these unknown or not well-known states."
  - "A cascading effect is observed as the time horizon grows and errors expand upon each other."
- Several solutions (not exhaustive):
  - 1- DAgger: Ask the expert to say what should be done in some encountered situations. Thus iteratively enriching the demonstration dataset.
  - 2- IRL: Human driving behaviour is not modelled inside a policy, but rather capture into a reward/cost function.
    - Based on this reward function, an (optimal) policy can be derived with classic RL techniques.
    - One issue: It can be computationally expensive.
  - 3- GAIL (I still need to read more about it):
    - "It fits distributions of states and actions given by an expert dataset, and a cost function is learned via Maximum Causal Entropy IRL."
    - "When the GAIL-policy driven vehicle was placed in a multi-agent setting, in which multiple agents take over the learned policy, this algorithm produced undesirable results among the agents."
- PS-GAIL is therefore introduced for multi-agent driving models (agents share a single policy learnt with PS-TRPO).
  - "Though PS-GAIL yielded better results in multi-agent simulations than GAIL, its results still led to undesirable driving characteristics, including unwanted trajectory deviation and off-road duration."
- RAIL offers a fix for that: the policy-learning process is augmented with two types of reward terms:
  - Binary penalties: e.g. collision and hard braking.
  - Smoothed penalties: "applied in advance of undesirable actions with the theory that this would prevent these actions from occurring".
  - I see that technique as a way to incorporate knowledge.
About the experiment:
- The three policies were originally trained on the straight roadway: cars only consider the lateral distance to the edge.
- In the "new" environment, a road curvature is introduced.
- Findings:
  - "None of them were able to fully accommodate the turn in the road."
  - PS-GAIL is effective only while surrounded by other vehicles.
  - The smoothed reward augmentation helped RAIL, but it was too late to avoid off-road (the car is already driving too fast and does not dare a hard brake which is strongly penalized).
  - The reward function should therefore be updated (back to reward engineering 😅), for instance adding a harder reward term to prevent the car from leaving the road.

"Learning by Cheating"

[ 2019 ] [📝] [] [ 🎓 UT Austin ] [ 🚗 Intel Labs ]
[ on-policy supervision, DAgger, conditional IL, mid-to-mid, CARLA ]

Click to expand


The main idea is to decompose* the imitation learning (`IL`) process into two stages: `1-` Learn to act. `2-` Learn to see. Source.*

mid-to-mid learning: Based on a processed bird’s-eye view map, the privileged agent predicts a sequence of waypoints to follow. This desired trajectory is eventually converted into low-level commands by two PID controllers. It is also worth noting how this privileged agent serves as an oracle that provides adaptive on-demand supervision to train the sensorimotor agent across all possible commands. Source.


Example of privileged map* supplied to the first agent. And details about the lateral `PID` controller that produces `steering` commands based on a list of target waypoints. Source.*

Authors: Chen, D., Zhou, B., Koltun, V. & Krähenbühl, P

One motivation: decomposing the imitation learning (IL) process into two stages:
- Direct IL (from expert trajectories to vision-based driving) conflates two difficult tasks:
  - 1- Learning to see.
  - 2- Learning to act.
One term: "Cheating".
- 1- First, train an agent that has access to privileged information:
  - "This privileged agent cheats by observing the ground-truth layout of the environment and the positions of all traffic participants."
  - Goal: The agent can focus on learning to act (it does not need to learn to see because it gets direct access to the environment’s state).
- 2- Then, this privileged agent serves as a teacher to train a purely vision-based system (abundant supervision).
  - Goal: Learning to see.
1- First agent (privileged agent):
- Input: A processed bird’s-eye view map (with ground-truth information about lanes, traffic lights, vehicles and pedestrians) together with high-level navigation command and current speed.
- Output: A list of waypoints the vehicle should travel to.
- Hence mid-to-mid learning approach.
- Goal: imitate the expert trajectories.
- Training: Behaviour cloning (BC) from a set of recorded expert driving trajectories.
  - Augmentation can be done offline, to facilitate generalization.
  - The agent is thus placed in a variety of perturbed configurations to learn how to recover
  - E.g. facing the sidewalk or placed on the opposite lane, it should find its way back onto the road.
2- Second agent (sensorimotor agent):
- Input: Monocular RGB image, current speed, and a high-level navigation command.
- Output: A list of waypoints.
- Goal: Imitate the privileged agent.
One idea: "White-box" agent:
- The internal state of the privileged agent can be examined at will.
  - Based on that, one could test different high-level commands: "What would you do now if the command was [follow-lane] [go left] [go right] [go straight]".
- This relates to conditional IL: all conditional branches are supervised during training.
Another idea: "online learning" and "on-policy supervision":
- "“On-policy” refers to the sensorimotor agent rolling out its own policy during training."
  - Here, the decision of the second agents are directly implemented (close-loop).
  - And an oracle is still available for the newly encountered situation (hence on-policy), which also accelerates the training.
  - This is an advantage of using a simulator: it would be difficult/impossible in the physical world.
- Here, the second agent is first trained off-policy (on expert demonstration) to speed up the learning (offline BC), and only then go on-policy:
  - "Finally, we train the sensorimotor agent on-policy, using the privileged agent as an oracle that provides adaptive on-demand supervision in any state reached by the sensorimotor student."
  - The sensorimotor agent can thus be supervised on all its waypoints and across all commands at once.
- It resembles the Dataset aggregation technique of DAgger:
  - "This enables automatic DAgger-like training in which supervision from the privileged agent is gathered adaptively via online rollouts of the sensorimotor agent."
About the two benchmarks:
- 1- Original CARLA benchmark (2017).
- 2- NoCrash benchmark (2019).
- Interesting idea for timeout:
  - "The time limit corresponds to the amount of time needed to drive the route at a cruising speed of 10 km/h".
Another idea: Do not directly output low-level commands.
- Instead, predict waypoints and speed targets.
- And rely on two PID controllers to implement them.
  - 1- "We fit a parametrized circular arc to all waypoints using least-squares fitting and then steer towards a point on the arc."
  - 2- "A longitudinal PID controller tries to match a target velocity as closely as possible [...] We ignore negative throttle commands, and only brake if the predicted velocity is below some threshold (2 km/h)."

"Deep Imitative Models for Flexible Inference, Planning, and Control"

[ 2019 ] [📝] [🎞️] [ 🎓 Carnegie Mellon University, UC Berkeley ]
[ conditional IL, model-based RL, CARLA ]

Click to expand


The main motivation is to combine the benefits* of `IL` (to imitate some expert demonstrations) and `goal-directed planning` (e.g. `model-based RL`). Source.*


`φ` represents the scene. It consists of the current `lidar scan`, `previous states` in the trajectory as well as the current `traffic light state`. Source.


From left to right: `Point`, `Line-Segment` and `Region` (small and wide) Final State Indicators* used for planning. Source.*


Comparison of features and implementations. Source.

Authors: Rhinehart, N., McAllister, R., & Levine, S.

Main motivation: combine the benefits of imitation learning (IL) and goal-directed planning such as model-based RL (MBRL).
- Especially to generate interpretable, expert-like plans with offline learning and no reward engineering.
- Neither IL nor MBRL can do so.
- In other words, it completes planning based on some imitation prior.
One concept: "Imitative Models"
- They are _"probabilistic predictive models able to plan interpretable expert-like trajectories to achieve new goals".
- As for IL -> use expert demonstration:
  - It generates expert-like behaviors without reward function crafting.
  - The model is learnt "offline" also means it avoids costly online data collection (contrary to MBRL).
  - It learns dynamics desirable behaviour models (as opposed to learning the dynamics of possible behaviour done by MBRL).
- As for MBRL -> use planning:
  - It achieves new goals (goals that were not seen during training). Therefore, it avoids the theoretical drift shortcomings (distribution shift) of vanilla behavioural cloning (BC).
  - It outputs (interpretable) plan to them at test-time, which IL cannot.
  - It does not need goal labels for training.
- Binding IL and planning:
  - The learnt imitative model q(S|φ) can generate trajectories that resemble those that the expert might generate.
    - These manoeuvres do not have a specific goal. How to direct our agent to goals?
  - General tasks are defined by a set of goal variables G.
    - At test time, a route planner provides waypoints to the imitative planner, which computes expert-like paths for each candidate waypoint.
  - The best plan is chosen according to the planning objective (e.g. prefer routes avoiding potholes) and provided to a low-level PID-controller in order to produce steering and throttle actions.
  - In other words, the derived plan (list of set-points) should be:
    - Maximizing the similarity to the expert demonstrations (term with q)
    - Maximizing the probability of reaching some general goals (term with P(G)).
  - How to represent goals?
    - dim=0 - with points: Final-State Indicator.
    - dim=1 - with lines: Line-Segment Final-State Indicator.
    - dim=2 - with areas (regions): Final-State Region Indicator.
How to deal with traffic lights?
- The concept of smart waypointer is introduced.
- "It removes far waypoints beyond 5 meters from the vehicle when a red light is observed in the measurements provided by CARLA".
- "The planner prefers closer goals when obstructed, when the vehicle was already stopped, and when a red light was detected [...] The planner prefers farther goals when unobstructed and when green lights or no lights were observed."
About interpretability and safety:
- "In contrast to black-box one-step IL that predicts controls, our method produces interpretable multi-step plans accompanied by two scores. One estimates the plan’s expertness, the second estimates its probability to achieve the goal."
  - The imitative model can produce some expert probability distribution function (PDF), hence offering superior interpretability to one-step IL models.
  - It is able to score how likely a trajectory is to come from the expert.
  - The probability to achieve a goal is based on some "Goal Indicator methods" (using "Goal Likelihoods"). I must say I did not fully understand that part
- The safety aspect relies on the fact that experts were driving safely and is formalized as a "plan reliability estimation":
  - "Besides using our model to make a best-effort attempt to reach a user-specified goal, the fact that our model produces explicit likelihoods can also be leveraged to test the reliability of a plan by evaluating whether reaching particular waypoints will result in human-like behavior or not."
  - Based on this idea, a classification is performed to recognize safe and unsafe plans, based on the planning criterion.
About the baselines:
- Obviously, the proposed approach is compared to the two methods it aims at combining.
- About MBRL:
  - 1- First, a forward dynamics model is learnt using given observed expert data.
    - It does not imitate the expert preferred actions, but only models what is physically possible.
  - 2- The model then is used to plan a reachability tree through the free-space up to the waypoint while avoiding obstacles:
    - Playing with the throttle action, the search expands each state node and retains the 50 closest nodes to the target waypoint.
    - The planner finally opts for the lowest-cost path that ends near the goal.
  - "The task of evoking expert-like behavior is offloaded to the reward function, which can be difficult and time-consuming to craft properly."
- About IL: It used Conditional terms on States, leading to CILS.
  - S for state: Instead of emitting low-level control commands (throttle, steering), it outputs set-points for some PID-controller.
  - C for conditional: To navigate at intersections, waypoints are classified into one of several directives: {Turn left, Turn right, Follow Lane, Go Straight}.
    - This is inspired by "End-to-end driving via conditional imitation learning" - (Codevilla et al. 2018) - detailed below.

"Conditional Vehicle Trajectories Prediction in CARLA Urban Environment"

[ 2019 ] [📝] [🎞️] [ 🚗 Valeo ]
[ conditional IL, CARLA, distributional shift problem ]

Click to expand

Some figures:


`End-to-``Mid` approach: `3` inputs with different levels of abstraction are used to predict the future positions on a fixed `2s`-horizon of the ego vehicle and the neighbours. The ego trajectory is be implemented by an external `PID` controller - Therefore, not `end-to-``end`. Source.

The past 3D-bounding boxes of the road users in the current reference are projected back in the current camera space. The past positions of ego and other vehicles are projected into some grid-map called proximity map. The image and the proximity map are concatenated to form context feature vector C. This context encoding is concatenated with the ego encoding, then fed into branches corresponding to the different high-level goals - conditional navigation goal. Source.


Illustration of the distribution shift* in imitation learning. Source.*


`VisualBackProp` highlights the image pixels which contributed the most to the final results - Traffic lights and their colours are important, together with highlights lane markings and curbs when there is a significant lateral deviation. Source.

Authors: Buhet, T., Wirbel, E., & Perrotton, X.

Previous works:
- "Imitation Learning for End to End Vehicle Longitudinal Control with Forward Camera" - (George, Buhet, Wirbel, Le-Gall, & Perrotton, 2018).
- "End to End Vehicle Lateral Control Using a Single Fisheye Camera" (Toromanoff, M., Wirbel, E., Wilhelm, F., Vejarano, C., Perrotton, X., & Moutarde, F. 2018).
One term: "End-To-Middle".
- It is opposed to "End-To-End", i.e. it does not output "end" control signals such as throttle or steering but rather some desired trajectory, i.e. a mid-level representation.
  - Each trajectory is described by two polynomial functions (one for x, the other for y), therefore the network has to predict a vector (x0, ..., x4, y0, ..., y4) for each vehicle.
  - The desired ego-trajectory is then implemented by an external controller (PID). Therefore, not end-to-end.
- Advantages of end-to-mid: interpretability for the control part + less to be learnt by the net.
- This approach is also an instance of "Direct perception":
  - "Instead of commands, the network predicts hand-picked parameters relevant to the driving (distance to the lines, to other vehicles), which are then fed to an independent controller".
- Small digression: if the raw perception measurements were first processed to form a mid-level input representation, the approach would be said mid-to-mid. An example is ChauffeurNet, detailed on this page as well.
About Ground truth:
- The expert demonstrations do not come from human recordings but rather from CARLA autopilot.
- 15 hours of driving in Town01 were collected.
- As for human demonstrations, no annotation is needed.
One term: "Conditional navigation goal".
- Together with the RGB images and the past positions, the network takes as input a navigation command to describe the desired behaviour of the ego vehicle at intersections.
- Hence, the future trajectory of the ego vehicle is conditioned by a navigation command.
  - If the ego-car is approaching an intersection, the goal can be left, right or cross, else the goal is to keep lane.
  - That means lane-change is not an option.
- "The last layers of the network are split into branches which are masked with the current navigation command, thus allowing the network to learn specific behaviours for each goal".
Three ingredients to improve vanilla end-to-end imitation learning (IL):
- 1- Mix of high and low-level input (i.e. hybrid input):
  - Both raw signal (images) and partial environment abstraction (navigation commands) are used.
- 2- Auxiliary tasks:
  - One head of the network predicts the future trajectories of the surrounding vehicles.
    - It differs from the primary task which should decide the 2s-ahead trajectory for the ego car.
    - Nevertheless, this secondary task helps: "Adding the neighbours prediction makes the ego prediction more compliant to traffic rules."
  - This refers to the concept of "Privileged learning":
    - "The network is partly trained with an auxiliary task on a ground truth which is useful to driving, and on the rest is only trained for IL".
- 3- Label augmentation:
  - The main challenge of IL is the difference between train and online test distributions. This is due to the difference between
    - Open-loop control: decisions are not implemented.
    - Close-loop control: decisions are implemented, and the vehicle can end in a state absent from the train distribution, potentially causing "error accumulation".
  - Data augmentation is used to reduce the gap between train and test distributions.
    - Classical randomization is combined with label augmentation: data similar to failure cases is generated a posteriori.
  - Three findings:
  - "There is a significant gap in performance when introducing the augmentation."
  - "The effect is much more noticeable on complex navigation tasks." (Errors accumulate quicker).
  - "Online test is the real significant indicator for IL when it is used for active control." (The common offline evaluation metrics may not be correlated to the online performance).
Baselines:
- Conditional Imitation learning (CIL): "End-to-end driving via conditional imitation learning" video - (Codevilla, F., Müller, M., López, A., Koltun, V., & Dosovitskiy, A. 2017).
  - CIL produces instantaneous commands.
- Conditional Affordance Learning (CAL): "Conditional affordance learning for driving in urban environments" video - (Sauer, A., Savinov, N. & Geiger, A. 2018).
  - CAL produces "affordances" which are then given to a controller.
One word about the choice of the simulator.
- A possible alternative to CARLA could be DeepDrive or the LGSVL simulator developed by the Advanced Platform Lab at the LG Electronics America R&D Centre. This looks promising.

"Uncertainty Quantification with Statistical Guarantees in End-to-End Autonomous Driving Control"

[ 2019 ] [📝] [ 🎓 Oxford University ]
[ uncertainty-aware decision, Bayesian inference, CARLA ]

Click to expand

One figure:


The trust* or uncertainty in one decision can be measured based on the `probability mass function` around its mode. Source.*


The measures of uncertainty based on mutual information* can be used to issue warnings to the driver and perform safety / emergency manoeuvres. Source.*


As noted by the authors: while the `variance` can be useful in collision avoidance, the wide variance of `HMC` causes a larger proportion of trajectories to fall outside of the safety boundary when a new weather is applied. Source.

Authors: Michelmore, R., Wicker, M., Laurenti, L., Cardelli, L., Gal, Y., & Kwiatkowska, M

One related work:
- NVIDIA’s PilotNet [DAVE-2] where expert demonstrations are used together with supervised learning to map from images (front camera) to steering command.
- Here, human demonstrations are collected in the CARLA simulator.
One idea: use distribution in weights.
- The difference with PilotNet is that the neural network applies the "Bayesian" paradigm, i.e. each weight is described by a distribution (not just a single value).
- The authors illustrate the benefits of that paradigm, imagining an obstacle in the middle of the road.
  - The Bayesian controller may be uncertain on the steering angle to apply (e.g. a 2-tail or M-shape distribution).
  - A first option is to sample angles, which turns the car either right or left, with equal probability.
  - Another option would be to simply select the mean value of the distribution, which aims straight at the obstacle.
  - The motivation of this work is based on that example: "derive some precise quantitative measures of the BNN uncertainty to facilitate the detection of such ambiguous situation".
One definition: "real-time decision confidence".
- This is the probability that the BNN controller is certain of its decision at the current time.
- The notion of trust can therefore be introduced: the idea it to compute the probability mass in a ε−ball around the decision π(observation) and classify it as certain if the resulting probability is greater than a threshold.
  - It reminds me the concept of trust-region optimisation in RL.
  - In extreme cases, all actions are equally distributed, π(observation) has a very high variance, the agent does not know what to do (no trust) and will randomly sample an action.
How to get these estimates? Three Bayesian inference methods are compared:
- Monte Carlo dropout (MCD).
- Mean-field variational inference (VI).
- Hamiltonian Monte Carlo (HMC).
What to do with this information?
- "This measure of uncertainty can be employed together with commonly employed measures of uncertainty, such as mutual information, to quantify in real time the degree that the model is confident in its predictions and can offer a notion of trust in its predictions."
  - I did not know about "mutual information" and liked the explanation of Wikipedia about the link of MI to entropy and KL-div.
    - I am a little bit confused: in what I read, the MI is function of two random variables. What are they here? The authors rather speak about the predictive distribution exhibited by the predictive distribution.
- Depending on the uncertainty level, several actions are taken:
  - mutual information warnings slow down the vehicle.
  - standard warnings slow down the vehicle and alert the operator of potential hazard.
  - severe warnings cause the car to safely brake and ask the operator to take control back.
Another definition: "probabilistic safety", i.e. the probability that a BNN controller will keep the car "safe".
- Nice, but what is "safe"?
- It all relies on the assumption that expert demonstrations were all "safe", and measures the how much of the trajectory belongs to this "safe set".
- I must admit I did not fully understand the measure on "safety" for some continuous trajectory and discrete demonstration set:
  - A car can drive with a large lateral offset from the demonstration on a wide road while being "safe", while a thin lateral shift in a narrow street can lead to an "unsafe" situation.
  - Not to mention that the scenario (e.g. configuration of obstacles) has probably changed in-between.
  - This leads to the following point with an interesting application for scenario coverage.
One idea: apply changes in scenery and weather conditions to evaluate model robustness.
- To check the generalization ability of a model, the safety analysis is re-run (offline) with other weather conditions.
- As noted in conclusion, this offline safety probability can be used as a guide for active learning in order to increase data coverage and scenario representation in training data.

"Exploring the Limitations of Behavior Cloning for Autonomous Driving"

[ 2019 ] [📝] [🎞️] [] [ 🎓 CVC, UAB, Barcelona ] [ 🚗 Toyota ]
[ distributional shift problem, off-policy data collection, CARLA, conditional imitation learning, residual architecture, reproducibility issue, variance caused by initialization and sampling ]

Click to expand

One figure:


Conditional Imitation Learning is extended with a ResNet architecture and Speed prediction (`CILRS`). Source.

Authors: Codevilla, F., Santana, E., Antonio, M. L., & Gaidon, A.

One term: “CILRS” = Conditional Imitation Learning extended with a ResNet architecture and Speed prediction.
One Q&A: How to include in E2E learning information about the destination, i.e. to disambiguate imitation around multiple types of intersections?
- Add a high-level navigational command (e.g. take the next right, left, or stay in lane) to the tuple <observation, expert action> when building the dataset.
One idea: learn to predict the ego speed (mediated perception) to address the inertia problem stemming from causal confusion (biased correlation between low speed and no acceleration - when the ego vehicle is stopped, e.g. at a red traffic light, the probability it stays static is indeed overwhelming in the training data).
Another idea: The off-policy (expert) driving demonstration is not produced by a human, but rather generated from an omniscient "AI" agent.
One quote:

"The more common the vehicle model and color, the better the trained agent reacts to it. This raises ethical challenges in automated driving".

"Conditional Affordance Learning for Driving in Urban Environments"

[ 2018 ] [📝] [🎞️] [🎞️] [] [ 🎓 CVC, UAB, Barcelona ] [ 🚗 Toyota ]
[ CARLA, end-to-mid, direct perception ]

Click to expand

Some figures:


Examples of affordances, i.e. attributes of the environment* which limit the space of allowed actions. `A1`, `A2` and `A3` are predefined observation areas. Source.*


The presented `direct perception` method predicts a low-dimensional intermediate representation of the environment - affordance - which is then used in a conventional control algorithm. The affordance is conditioned for goal-directed navigation, i.e. before each intersection, it receives an instruction such as `go straight`, `turn left` or `turn right`. Source.


The feature maps produced by a `CNN` feature extractor are stored in a memory and consumed by task-specific layers (one affordance has one task block). Every task block has its own specific temporal receptive field - it decides how much of the memory it needs. This figure also illustrates how the navigation command is used as switch between trained submodules. Source.

Authors: Sauer, A., Savinov, N., & Geiger, A.

One term: "Direct perception" (DP):
- The goal of DP methods is to predict a low-dimensional intermediate representation of the environment which is then used in a conventional control algorithm to manoeuvre the vehicle.
- With this regard, DP could also be said end-to-mid. The mapping to learn is less complex than end-to-end (from raw input to controls).
- DP is meant to combine the advantages of two other commonly-used approaches: modular pipelines MP and end-to-end methods such as imitation learning IL or model-free RL.
- Ground truth affordances are collected using CARLA. Several augmentations are performed.
Related work on affordance learning and direct perception.
- Deepdriving: Learning affordance for direct perception in autonomous driving by (Chen, Seff, Kornhauser, & Xiao, 2015).
- Deepdriving works on highway.
- Here, the idea is extended to urban scenarios (with traffic signs, traffic lights, junctions) considering a sequence of images (not just one camera frame) for temporal information.
One term: "Conditional Affordance Learning" (CAL):
- "Conditional": The actions of the agent are conditioned on a high-level command given by the navigation system (the planner) prior to intersections. It describes the manoeuvre to be performed, e.g., go straight, turn left, turn right.
- "Affordance": Affordances are one example of DP representation. They are attributes of the environment which limit the space of allowed actions. Only 6 affordances are used for CARLA urban driving:
  - Distance to vehicle (continuous).
  - Relative angle (continuous and conditional).
  - Distance to centre-line (continuous and conditional).
  - Speed Sign (discrete).
  - Red Traffic Light (discrete - binary).
  - Hazard (discrete - binary).
    - The Class Weighted Cross Entropy is the loss used for discrete affordances to put more weights on rare but important occurrences (hazard occurs rarely compared to traffic light red).
- "Learning": A single neural network trained with multi-task learning (MTL) predicts all affordances in a single forward pass (~50ms). It only takes a single front-facing camera view as input.
About the controllers: The path-velocity decomposition is applied. Hence two controllers are used in parallel:
- 1- throttle and brake
  - Based on the predicted affordances, a state is "rule-based" assigned among: cruising, following, over limit, red light, and hazard stop (all are mutually exclusive).
  - Based on this state, the longitudinal control signals are derived, using PID or threshold-predefined values.
  - It can handle traffic lights, speed signs and smooth car-following.
  - Note: The Supplementary Material provides details insights on controller tuning (especially PID) for CARLA.
- 2- steering is controlled by a Stanley Controller, based on two conditional affordances: distance to centreline and relative angle.
One idea: I am often wondering what timeout I should set when testing a scenario with CARLA. The author computes this time based on the length of the pre-defined path (which is actually easily accessible):
- "The time limit equals the time needed to reach the goal when driving along the optimal path at 10 km/h"
Another idea: Attention Analysis.
- For better understanding on how affordances are constructed, the attention of the CNN using gradient-weighted class activation maps (Grad-CAMs).
- This "visual explanation" reminds me another technique used in end-to-end approaches, VisualBackProp, that highlights the image pixels which contributed the most to the final results.
Baselines and results:
- Compared to CARLA-based Modular Pipeline (MP), Conditional Imitation Learning (CIL) and Reinforcement Learning (RL), CAL particularly excels in generalizing to the new town.
Where to provide the high-level navigation conditions?
- The authors find that "conditioning in the network has several advantages over conditioning in the controller".
- In addition, in the net, it is preferable to use the navigation command as switch between submodules rather than an input:
  - "We observed that training specialized submodules for each directional command leads to better performance compared to using the directional command as an additional input to the task networks".

"Variational Autoencoder for End-to-End Control of Autonomous Driving with Novelty Detection and Training De-biasing"

[ 2018 ] [📝] [🎞️] [🎞️] [ 🎓 MIT ] [ 🚗 Toyota ]
[ VAE, uncertainty estimation, sampling efficiency, augmentation ]

Click to expand

Some figures:


One particular latent variable `^y` is explicitly supervised to predict steering control. Another interesting idea: augmentation is based on domain knowledge - if a method used to the middle-view is given some left-view image, it should predict some correction to the right. Source.


For each new image, empirical uncertainty estimates are computed by sampling from the variables of the latent space. These estimates lead to the `D` statistic that indicates whether an observed image is well captured by our trained model, i.e. `novelty detection`. Source.


In a subsequent work, the `VAE` is conditioned onto the road topology. It serves multiple purposes such as localization and `end-to-end` navigation. The routed or unrouted map given as additional input goes toward the `mid-to-end` approach where processing is performed and/or external knowledge is embedded. Source. See this video temporal for evolution of the predictions.

Authors: Amini, A., Schwarting, W., Rosman, G., Araki, B., Karaman, S., & Rus, D.

One issue raised about vanilla E2E:
- The lack a measure of associated confidence in the prediction.
- The lack of interpretation of the learned features.
- Having said that, the authors present an approach to both understand and estimate the confidence of the output.
- The idea is to use a Variational Autoencoder (VAE), taking benefit of its intermediate latent representation which is learnt in an unsupervised way and provides uncertainty estimates for every variable in the latent space via their parameters.
One idea for the VAE: one particular latent variable is explicitly supervised to predict steering control.
- The loss function of the VAE has therefore 3 parts:
  - A reconstruction loss: L1-norm between the input image and the output image.
  - A latent loss: KL-divergence between the latent variables and a unit Gaussian, providing regularization for the latent space.
  - A supervised latent loss: MSE between the predicted and actual curvature of the vehicle’s path.
One contribution: "Detection of novel events" (which have not been sufficiently trained for).
- To check if an observed image is well captured by the trained model, the idea is to propagate the VAE’s latent uncertainty through the decoder and compare the result with the original input. This is done by sampling (empirical uncertainty estimates).
- The resulting pixel-wise expectation and variance are used to compute a sort of loss metric D(x, ˆx) whose distribution for the training-set is known (approximated with a histogram).
- The image x is classified as novel if this statistic is outside of the 95th percentile of the training distribution and the prediction can finally be "untrusted to produce reliable outputs".
- "Our work presents an indicator to detect novel images that were not contained in the training distribution by weighting the reconstructed image by the latent uncertainty propagated through the network. High loss indicates that the model has not been trained on that type of image and thus reflects lower confidence in the network’s ability to generalize to that scenario."
A second contribution: "Automated debiasing against learned biases".
- As for the novelty detection, it takes advantage of the latent space distribution and the possibility of sampling from the most representative regions of this space.
- Briefly said, the idea it to increase the proportion of rarer datapoints by dropping over-represented regions of the latent space to accelerate the training (sampling efficiency).
- This debiasing is not manually specified beforehand but based on learned latent variables.
One reason to use single frame prediction (as opposed to RNN):
- ""Note that only a single image is used as input at every time instant. This follows from original observations where models that were trained end-to-end with a temporal information (CNN+LSTM) are unable to decouple the underlying spatial information from the temporal control aspect. While these models perform well on test datasets, they face control feedback issues when placed on a physical vehicle and consistently drift off the road.""
One idea about augmentation (also met in the Behavioral Cloning Project of the Udacity Self-Driving Car Engineer Nanodegree):
- "To inject domain knowledge into our network we augmented the dataset with images collected from cameras placed approximately 2 feet to the left and right of the main centre camera. We correspondingly changed the supervised control value to teach the model how to recover from off-centre positions."
One note about the output:
- "We refer to steering command interchangeably as the road curvature: the actual steering angle requires reasoning about road slip and control plant parameters that change between vehicles."
Previous and further works:
- "Spatial Uncertainty Sampling for End-to-End control" - (Amini, Soleimany, Karaman, & Rus, 2018)
- "Variational End-to-End Navigation and Localization" - (Amini, Rosman, Karaman, & Rus, 2019)
  - One idea: incorporate some coarse-grained roadmaps with raw perceptual data.
    - Either unrouted (just containing the drivable roads). Output = continuous probability distribution over steering control.
    - Or routed (target road highlighted). Output = deterministic steering control to navigate.
  - How to evaluate the continuous probability distribution over steering control given the human "scalar" demonstration?
    - "For a range of z-scores over the steering control distribution we compute the number of samples within the test set where the true (human) control output was within the predicted range."
  - About the training dataset: 25 km of urban driving data.

"ChauffeurNet: Learning to Drive by Imitating the Best and Synthesizing the Worst"

[ 2018 ] [📝] [🎞️] [🎞️] [ 🚗 Waymo ]
[ imitation learning, distributional shift problem ]

Click to expand

Two figures:


Different layers composing the `mid-level representation`. Source.


Training architecture around `ChauffeurNet` with the different loss terms, that can be grouped into `environment` and `imitation` losses. Source.

Authors: Bansal, M., Krizhevsky, A., & Ogale, A.

One term: "mid-level representation"
- The decision-making task (between perception and control) is packed into one single "learnable" module.
  - Input: the representation divided into several image-like layers:
    - Map features such as lanes, stop signs, cross-walks...; Traffic lights; Speed Limit; Intended route; Current agent box; Dynamic objects; Past agent poses.
    - Such a representation is generic, i.e. independent of the number of dynamic objects and independent of the road geometry/topology.
    - I discuss some equivalent representations seen at IV19.
  - Output: intended route, i.e. the future poses recurrently predicted by the introduced ChauffeurNet model.
- This architecture lays between E2E (from pixels directly to control) and fully decomposed modular pipelines (decomposing planning in multiple modules).
- Two notable advantages over E2E:
  - It alleviates the burdens of learning perception and control:
    - The desired trajectory is passed to a controls optimizer that takes care of creating the low-level control signals.
    - Not to mention that different types of vehicles may possibly utilize different control outputs to achieve the same driving trajectory.
  - Perturbations and input data from simulation are easier to generate.
One key finding: "pure imitation learning is not sufficient", despite the 60 days of continual driving (30 million examples).
- One quote about the "famous" distribution shift (deviation from the training distribution) in imitation learning:
"The key challenge is that we need to run the system closed-loop, where errors accumulate and induce a shift from the training distribution."
- The training data does not have any real collisions. How can the agent efficiently learn to avoid them if it has never been exposed during training?
- One solution consists in exposing the model to non-expert behaviours, such as collisions and off-road driving, and in adding extra loss functions.
  - Going beyond vanilla cloning.
    - Trajectory perturbation: Expose the learner to synthesized data in the form of perturbations to the expert’s driving (e.g. jitter the midpoint pose and heading)
      - One idea for future works is to use more complex augmentations, e.g. with RL, especially for highly interactive scenarios.
    - Past dropout: to prevent using the history to cheat by just extrapolating from the past rather than finding the underlying causes of the behaviour.
    - Hence the concept of tweaking the training data in order to “simulate the bad rather than just imitate the good”.
  - Going beyond the vanilla imitation loss.
    - Extend imitation losses.
    - Add environment losses to discourage undesirable behaviour, e.g. measuring the overlap of predicted agent positions with the non-road regions.
    - Use imitation dropout, i.e. sometimes favour the environment loss over the imitation loss.

"Imitating Driver Behavior with Generative Adversarial Networks"

[ 2017 ] [📝] [] [ 🎓 Stanford ]
[ adversarial learning, distributional shift problem, cascading errors, IDM, NGSIM, rllab ]

Click to expand

Some figures:

The state consists in 51 features divided into 3 groups: The core features include hand-picked features such as Speed, Curvature and Lane Offset. The LIDAR-like beams capture the surrounding objects in a fixed-size representation independent of the number of vehicles. Finally, 3 binary indicator features identify when the ego vehicle encounters undesirable states - collision, drives off road, and travels in reverse. Source.

As for common adversarial approaches, the objective function in GAIL includes some sigmoid cross entropy terms. The objective is to fit ψ for the discriminator. But this objective function is non-differentiable with respect to θ. One solution is to optimize πθ separately using RL. But what for reward function? In order to drive πθ into regions of the state-action space similar to those explored by the expert πE, a surrogate reward ˜r is generated from D_ψ based on samples and TRPO is used to perform a policy update of πθ. Source.

Authors: Kuefler, A., Morton, J., Wheeler, T., & Kochenderfer, M.

One term: the problem of "cascading errors" in behavioural cloning (BC).
- BC, which treats IL as a supervised learning problem, tries to fit a model to a fixed dataset of expert state-action pairs. In other words, BC solves a regression problem in which the policy parameterization is obtained by maximizing the likelihood of the actions taken in the training data.
- But inaccuracies can lead the stochastic policy to states that are underrepresented in the training data (e.g., an ego-vehicle edging towards the side of the road). And datasets rarely contain information about how human drivers behave in such situations.
- The policy network is then forced to generalize, and this can lead to yet poorer predictions, and ultimately to invalid or unseen situations (e.g., off-road driving).
- "Cascading Errors" refers to this problem where small inaccuracies compound during simulation and the agent cannot recover from them.
  - This issue is inherent to sequential decision making.
- As found by the results:
  - "The root-weighted square error results show that the feedforward BC model has the best short-horizon performance, but then begins to accumulate error for longer time horizons."
  - "Only GAIL (and of course IDM+MOBIL) are able to stay on the road for extended stretches."
One idea: RL provides robustness against "cascading errors".
- RL maximizes the global, expected return on a trajectory, rather than local instructions for each observation. Hence more appropriate for sequential decision making.
- Also, the reward function r(s_t, a_t) is defined for all state-action pairs, allowing an agent to receive a learning signal even from unusual states. And these signals can establish preferences between mildly undesirable behaviour (e.g., hard braking) and extremely undesirable behaviour (e.g., collisions).
  - In contrast, BC only receives a learning signal for those states represented in a labelled, finite dataset.
  - Because handcrafting an accurate RL reward function is often difficult, IRL seems promising. In addition, the imitation (via the recovered reward function) extends to unseen states: e.g. a vehicle that is perturbed toward the lane boundaries should know to return toward the lane centre.
Another idea: use GAIL instead of IRL:
- "IRL approaches are typically computationally expensive in their recovery of an expert cost function. Instead, recent work has attempted to imitate expert behaviour through direct policy optimization, without first learning a cost function."
- "Generative Adversarial Imitation Learning" (GAIL) implements this idea:
  - "Expert behaviour can be imitated by training a policy to produce actions that a binary classifier mistakes for those of an expert."
  - "GAIL trains a policy to perform expert-like behaviour by rewarding it for “deceiving” a classifier trained to discriminate between policy and expert state-action pairs."
- One contribution is to extend GAIL to the optimization of recurrent neural networks (GRU in this case).
One concept: "Trust Region Policy Optimization".
- Policy-gradient RL optimization with "Trust Region" is used to optimize the agent's policy πθ, addressing the issue of training instability of vanilla policy-gradient methods.
  - "TRPO updates policy parameters through a constrained optimization procedure that enforces that a policy cannot change too much in a single update, and hence limits the damage that can be caused by noisy gradient estimates."
- But what reward function to apply? Again, we do not want to do IRL.
- Some "surrogate" reward function is empirically derived from the discriminator. Although it may be quite different from the true reward function optimized by expert, it can be used to drive πθ into regions of the state-action space similar to those explored by πE.
One finding: Should previous actions be included in the state s?
- "The previous action taken by the ego vehicle is not included in the set of features provided to the policies. We found that policies can develop an over-reliance on previous actions at the expense of relying on the other features contained in their input."
- But on the other hand, the authors find:
- "The GAIL GRU policy takes similar actions to humans, but oscillates between actions more than humans. For instance, rather than outputting a turn-rate of zero on straight road stretches, it will alternate between outputting small positive and negative turn-rates".
- "An engineered reward function could also be used to penalize the oscillations in acceleration and turn-rate produced by the GAIL GRU".
Some interesting interpretations about the IDM and MOBIL driver models (resp. longitudinal and lateral control).
- These commonly-used rule-based parametric models serve here as baselines:
- "The Intelligent Driver Model (IDM) extended this work by capturing asymmetries between acceleration and deceleration, preferred free road and bumper-to-bumper headways, and realistic braking behaviour."
- "MOBIL maintains a utility function and 'politeness parameter' to capture intelligent driver behaviour in both acceleration and turning."

`Inverse Reinforcement Learning` `Inverse Optimal Control` and `Game Theory`

"Accelerated Inverse Reinforcement Learning with Randomly Pre-sampled Policies for Autonomous Driving Reward Design"

[ 2019 ] [📝] [ 🎓 UC Berkeley, Tsinghua University, Beijin ]
[ max-entropy ]

Click to expand


Instead of the costly `RL` optimisation step at each iteration of the vanilla `IRL`, the idea is to randomly sample a massive of policies in advance and then to pick one of them as the optimal policy. In case the sampled policy set does not contain the optimal policy, exploration of policy is introduced as well for supplement. Source.


The approximation* used in Kuderer et al. (2015) is applied here to compute the second term of gradient about the expected feature values. Source.*

Authors: Xin, L., Li, S. E., Wang, P., Cao, W., Nie, B., Chan, C., & Cheng, B.

Reminder: Goal of IRL = Recover the reward function of an expert from demonstrations (here trajectories).
Motivations, here:
- 1- Improve the efficiency of "weights updating" in the iterative routine of IRL.
  - More precisely: generating optimal policy using model-free RL suffers from low sampling efficiency and should therefore be avoided.
  - Hence the term "accelerated" IRL.
- 2- Embed human knowledge where restricting the search space (policy space).
One idea: "Pre-designed policy subspace".
- "An intuitive idea is to randomly sample a massive of policies in advance and then to pick one of them as the optimal policy instead of finding it via RL optimisation."
How to construct the policies sub-space?
- Human knowledge about vehicle controllers is used.
- Parametrized linear controllers are implemented:
  - acc = K1∆d + K2∆v + K3*∆a, where ∆ are relative to the leading vehicle.
  - By sampling tuples of <K1, K2, K3> coefficients, 1 million (candidates) policies are generated to form the sub-space.
Section about Max-Entropy IRL (btw. very well explained, as for the section introducing IRL):
- "Ziebart et al. (2008) employed the principle of maximum entropy to resolve ambiguities in choosing trajectory distributions. This principle maximizes the uncertainty and leads to the distribution over behaviors constrained to matching feature expectations, while being no more committed to any particular trajectory than this constraint requires".
- "Maximizing the entropy of the distribution over trajectories subject to the feature constraints from expert’s trajectories implies to maximize the likelihood under the maximum entropy (exponential family) distributions. The problem is convex for MDPs and the optima can be obtained using gradient-based optimization methods".
- "The gradient [of the Lagrangian] is the difference between empirical feature expectations and the learners expected feature expectations."
How to compute the second term of this gradient?
- It implies integrating over all possible trajectories, which is infeasible.
- As Kuderer et al. (2015), one can compute the feature values of the most likely trajectory as an approximation of the feature expectation.
- "With this approximation, only the optimal trajectory associated to the optimal policy is needed, in contrast to regarding the generated trajectories as a probability distribution."
About the features.
- As noted in my experiments about IRL, they serve two purposes (in feature-matching-based IRL methods):
  - 1- In the reward function: they should represent "things we want" and "things we do not want".
  - 2- In the feature-match: to compare two policies based on their sampled trajectories, they should capture relevant properties of driving behaviours.
- Three features for this longitudinal acceleration task:
  - front-veh time headway.
  - long. acc.
  - deviation to speed limit.
Who was the expert?
- Expert followed a modified linear car-following (MLCF) model.
Results.
- Iterations are stopped after 11 loops.
- It would have been interesting for comparison to test a "classic" IRL method where RL optimizations are applied.

"Jointly Learnable Behavior and Trajectory Planning for Self-Driving Vehicles"

[ 2019 ] [📝] [ 🚗 Uber ]
[ max-margin ]

Click to expand


Both `behavioural planner` and `trajectory optimizer` share the same cost function, whose weigth parameters are learnt from demonstration. Source.

Authors: Sadat, A., Ren, M., Pokrovsky, A., Lin, Y., Yumer, E., & Urtasun, R.

Main motivation:
- Design a decision module where both the behavioural planner and the trajectory optimizer share the same objective (i.e. cost function).
- Therefore "joint".
- "[In approaches not-joint approaches] the final trajectory outputted by the trajectory planner might differ significantly from the one generated by the behavior planner, as they do not share the same objective".
Requirements:
- 1- Avoid time-consuming, error-prone, and iterative hand-tuning of cost parameters.
  - E.g. Learning-based approaches (BC).
- 2- Offer interpretability about the costs jointly imposed on these modules.
  - E.g. Traditional modular 2-stage approaches.
About the structure:
- The driving scene is described in W (desired route, ego-state, map, and detected objects). Probably W for "World"?
- The behavioural planner (BP) decides two things based on W:
  - 1- A high-level behaviour b.
    - The path to converge to, based on one chosen manoeuvre: keep-lane, left-lane-change, or right-lane-change.
    - The left and right lane boundaries.
    - The obstacle side assignment: whether an obstacle should stay in the front, back, left, or right to the ego-car.
  - 2- A coarse-level trajectory τ.
  - The loss has also a regularization term.
  - This decision is "simply" the argmin of the shared cost-function, obtained by sampling + selecting the best.
- The "trajectory optimizer" refines τ based on the constraints imposed by b.
  - E.g. an overlap cost will be incurred if the side assignment of b is violated.
- A cost function parametrized by w assesses the quality of the selected <b, τ> pair:
  - cost = w^T . sub-costs-vec(τ, b, W).
  - Sub-costs relate to safety, comfort, feasibility, mission completion, and traffic rules.
Why "learnable"?
- Because the weight vector w that captures the importance of each sub-cost is learnt based on human demonstrations.
  - "Our planner can be trained jointly end-to-end without requiring manual tuning of the costs functions".
- They are two losses for that objective:
  - 1- Imitation loss (with MSE).
    - It applies on the <b, τ> produced by the BP.
  - 2- Max-margin loss to penalize trajectories that have small cost and are different from the human driving trajectory.
    - It applies on the <τ> produced by the trajectory optimizer.
    - "This encourages the human driving trajectory to have smaller cost than other trajectories".
    - It reminds me the max-margin method in IRL where the weights of the reward function should make the expert demonstration better than any other policy candidate.

"Adversarial Inverse Reinforcement Learning for Decision Making in Autonomous Driving"

[ 2019 ] [📝] [ 🎓 Berkeley, Chalmers University, Peking University ] [ 🚗 Zenuity ]
[ GAIL, AIRL, action-masking, augmented reward function ]

Click to expand

Author: Wang, P., Liu, D., Chen, J., & Chan, C.-Y.

In Adversarial IRL (AIRL), the discriminator tries to distinguish learnt actions from demonstrated expert actions. Action-masking is applied, removing some action combinations that are not preferable, in order to reduce the unnecessary exploration. Finally, the reward function of the discriminator is extended with some manually-designed semantic reward to help the agent successfully complete the lane change and not to collide with other objects. Source.

One related concept (detailed further on this page): Generative Adversarial Imitation Learning (GAIL).
- An imitation learning method where the goal is to learn a policy against a discriminator that tries to distinguish learnt actions from expert actions.
Another concept used here: Guided Cost Learning (GCL).
- A Max-Entropy IRL method that makes use of importance sampling (IS) to approximate the partition function (the term in the gradient of the log-likelihood function that is hard to compute since it involves an integral of over all possible trajectories).
One concept introduced: Adversarial Inverse Reinforcement Learning (AIRL).
- It combines GAIL with GCL formulation.
  - "It uses a special form of the discriminator different from that used in GAIL, and recovers a cost function and a policy simultaneously as that in GCL but in an adversarial way."
- Another difference is the use of a model-free RL method to compute the new optimal policy, instead of model-based guided policy search (GPS) used in GCL:
  - "As the dynamic driving environment is too complicated to learn for the driving task, we instead use a model-free policy optimization method."
- One motivation of AIRL is therefore to cope with changes in the dynamics of environment and make the learnt policy more robust to system noises.
One idea: Augment the learned reward with some "semantic reward" term to improve learning efficiency.
- The motivation is to manually embed some domain knowledge, in the generator reward function.
- "This should provide the agent some informative guidance and assist it to learn fast."
About the task:
- "The task of our focus includes a longitudinal decision – the selection of a target gap - and a lateral decision – whether to commit the lane change right now."
- It is a rather "high-level" decision:
  - A low-level controller, consisting of a PID for lateral control and sliding-mode for longitudinal control, is the use to execute the decision.
- The authors use some action-masking technics where only valid action pairs are allowed to reduce the agent’s unnecessary exploration.

"Predicting vehicle trajectories with inverse reinforcement learning"

[ 2019 ] [📝] [ 🎓 KTH ]
[ max-margin ]

Click to expand

Author: Hjaltason, B.


About the features: The `φ` are distances read from the origin of a vision field and are represented by red dotted lines. They take value in [`0`, `1`], where `φi` = `1` means the dotted line does not hit any object and `φi` = `0` means it hits an object at origin. In this case, two objects are inside the front vision field. Hence `φ1` = `0.4` and `φ2` = `0.6`. Source.


Example of `max-margin`* `IRL`. Source.*

A good example of max-margin IRL:
- "There are two classes: The expert behaviour from data gets a label of 1, and the "learnt" behaviours a label of -1. The framework performs a max-margin optimization step to maximise the difference between both classes. The result is an orthogonal vector wi from the max margin hyperplane, orthogonal to the estimated expert feature vector µ(πE)".
- From this new R=w*f, an optimal policy is derived using DDPG.
- Rollouts are performed to get an estimated feature vector that is added to the set of "learnt" behaviours.
- The process is repeated until convergence (when the estimated values w*µ(π) are close enough).
Note about the reward function:
- Here, r(s, a, s') is also function of the action and the next state.
- Here a post about different forms of reward functions.

"A Survey of Inverse Reinforcement Learning: Challenges, Methods and Progress"

[ 2019 ] [📝] [ 🎓 University of Georgia ]
[ reward engineering ]

Click to expand

Authors: Arora, S., & Doshi, P.


Trying to generalize and classify `IRL` methods. Source.


I learnt about state visitation frequency: `ψ`(`π`)(`s`) and the feature count expectation: `µ`(`π`)(`φ`). Source.

This large review does not focus on AD applications, but it provides a good picture of IRL and can give ideas. Here are my take-aways.
Definition:
- "Inverse reinforcement learning (IRL) is the problem of modeling the preferences of another agent using its observed behavior [hence class of IL], thereby avoiding a manual specification of its reward function."
Potential AD applications of IRL:
- Decision-making: If I find your underlying reward function, and I consider you as an expert, I can imitate you.
- Prediction: If I find your underlying reward function, I can imagine what you are going to do
I start rethinking Imitation Learning. The goal of IL is to derive a policy based on some (expert) demonstrations.
- Two branches emerge, depending on what structure is used to model the expert behaviour. Where is that model captured?
  - 1- In a policy.
    - This is a "direct approach". It includes BC and its variants.
    - The task is to learn that state -> action mapping.
  - 2- In a reward function.
    - Core assumption: Each driver has an internal reward function and acts optimally w.r.t. it.
    - The main task it to learn that reward function (IRL), which captures the expert's preferences.
    - The second step consists in deriving the optimal policy for this derived reward function.
      
      As Ng and Russell put it: "The reward function, rather than the policy, is the most succinct, robust, and transferable definition of the task"
- What happens if some states are missing in the demonstration?
  - 1- Direct methods will not know what to do. And will try to interpolate from similar states. This could be risky. (c.f. distributional shift problem and DAgger).
    - "If a policy is used to describe a task, it will be less succinct since for each state we have to give a description of what the behaviour should look like". From this post
  - 2- IRL methods acts optimally w.r.t. the underlying reward function, which could be better, since it is more robust.
    - This is particularly useful if we have an expert policy that is only approximately optimal.
    - In other words, a policy that is better than the "expert" can be derived, while having very little exploration. This "minimal exploration" property is useful for tasks such as AD.
    - This is sometimes refers to as Apprenticeship learning.
One new concept I learnt: State-visitation frequency (it reminds me some concepts of Markov chains).
- Take a policy π. Let run the agent with it. Count how often it sees each state. This is called the state-visitation frequency (note it is for a specific π).
- Two ideas from there:
  - Iterating until this state-visitation frequency stops changing yields the converged frequency function.
  - Multiplying that converged state-visitation frequency with reward gives another perspective to the value function.
    - The value function can now be seen as a linear combination of the expected feature count µ(φk)(π) (also called successor feature).
One common assumption: -> "The solution is a weighted linear combination of a set of reward features".
- This greatly reduces the search space.
- "It allowed the use of feature expectations as a sufficient statistic for representing the value of trajectories or the value of an expert’s policy."
Known IRL issues (and solutions):
- 1- This is an under-constrained learning problem.
  - "Many reward functions could explain the observations".
  - Among them, they are highly "degenerate" functions with all reward values zero.
  - One solution is to impose constraints in the optimization.
    - For instance try to maximize the sum of "value-margins", i.e. the difference between the value functions of the best and the second-best actions.
    - "mmp makes the solution policy have state-action visitations that align with those in the expert’s demonstration."
    - "maxent distributes probability mass based on entropy but under the constraint of feature expectation matching."
  - Another common constraint is to encourage the reward function to be as simple as possible, similar to L1 regularization in supervised learning.
- 2- Two incomplete models:
  - 2.1- How to deal with incomplete/absent model of transition probabilities?
  - 2.2- How to select the reward features?
    - "[One could] use neural networks as function approximators that avoid the cumbersome hand-engineering of appropriate reward features".
  - "These extensions share similarity with model-free RL where the transition model and reward function features are also unknown".
- 3- How to deal with noisy demonstrations?
  - Most approaches assume a Gaussian noise and therefore apply Gaussian filters.
How to classify IRL methods?
- It can be useful to ask yourself two questions:
  - 1- What are the parameters of the Hypothesis R function`?
    - Most approaches use the "linear approximation" and try to estimate the weights of the linear combination of features.
  - 2- What for "Divergence Metric", i.e. how to evaluate the discrepancy to the expert demonstrations?
    - "[it boils down to] a search in reward function space that terminates when the behavior derived from the current solution aligns with the observed behavior."
    - How to measure the closeness or the similarity to the expert?
      - 1- Compare the policies (i.e. the behaviour).
        
        E.g. how many <state, action> pairs are matching?
        
        "A difference between the two policies in just one state could still have a significant impact."
      - 2- Compare the value functions (they are defined over all states).
        
        The authors mention the inverse learning error (ILE) = || V(expert policy) - V(learnt policy) || and the value loss (use as a margin).
- Classification:
  - Margin-based optimization: Learn a reward function that explains the demonstrated policy better than alternative policies by a margin (address IRL's "solution ambiguity").
    - The intuition here is that we want a reward function that clearly distinguishes the optimal policy from other possible policies.
  - Entropy-based optimization: Apply the "maximum entropy principle" (together with the "feature expectations matching" constraint) to obtain a distribution over potential reward functions.
  - Bayesian inference to derive P(^R|demonstration).
    - What for the likelihood P(<s, a> | ˆR)? This probability is proportional to the exponentiated value function: exp(Q[s, a]).
  - Regression and classification.

"Learning Reward Functions for Optimal Highway Merging"

[ 2019 ] [📝] [ 🎓 Stanford ] [🎞️]
[ reward engineering ]

Click to expand

Author: Weiss, E.

The assumption-free reward function that uses a simple polynomial form based on state and action values at each time step does better at minimizing both safety and mobility objectives, even though it does not incorporate human knowledge of typical reward function structures. About Pareto optimum: at these points, it becomes impossible to improve in the minimization of one objective without worsening our minimization of the other objective). Source.

What?
- A Project from the Stanford course (AA228/CS238 - Decision Making under Uncertainty). Examples of student projects can be found here.
My main takeaway:
- A simple problem that illustrates the need for (learning more about) IRL.
The merging task is formulated as a simple MDP:
- The state space has size 3 and is discretized: lat + long ego position and long position of the other car.
- The other vehicle transitions stochastically (T) according to three simple behavioural models: fast, slow, average speed driving.
- The main contribution concerns the reward design: how to shape the reward function for this multi-objective (trade-off safety / efficiency) optimization problem?
Two reward functions (R) are compared:
- 1- "The first formulation models rewards based on our prior knowledge of how we would expect autonomous vehicles to operate, directly encoding human values such as safety and mobility into this problem as a positive reward for merging, a penalty for merging close to the other vehicle, and a penalty for staying in the on-ramp."
- 2- "The second reward function formulation assumes no prior knowledge of human values and instead comprises a simple degree-one polynomial expression for the components of the state and the action."
  - The parameters are tuned using a sort of grid search (no proper IRL).
How to compare them?
- Since both T and R are known, a planning (as opposed to learning) algorithm can be used to find the optimal policy. Here value iteration is implemented.
- The resulting agents are then evaluated based on two conflicting objectives:
  - "Minimizing the distance along the road at which point merging occurs and maximizing the gap between the two vehicles when merging."
- Next step will be proper IRL:
  - "We can therefore conclude that there may exist better reward functions for capturing optimal driving policies than either the intuitive prior knowledge reward function or the polynomial reward function, which doesn’t incorporate any human understanding of costs associated with safety and efficiency."

"Game-theoretic Modeling of Traffic in Unsignalized Intersection Network for Autonomous Vehicle Control Verification and Validation"

[ 2019 ] [📝] [ 🎓 University of Michigan and Bilkent University, Ankara ]
[ DAgger, level-k control policy ]

Click to expand

Authors: Tian, R., Li, N., Kolmanovsky, I., Yildiz, Y., & Girard, A.

This paper builds on several works (also analysed further below):
- "Adaptive Game-Theoretic Decision Making for Autonomous Vehicle Control at Roundabouts" - (Tian, Li, Li, et al., 2019).
- "Game Theoretic Modeling of Vehicle Interactions at Unsignalized Intersections and Application to Autonomous Vehicle Control" - (N. Li, Kolmanovsky, Girard, & Yildiz, 2018).
- "Game-theoretic modeling of driver and vehicle interactions for verification and validation of autonomous vehicle control systems" - (N. Li et al., 2016).
Addressed problem: unsignalized intersections with heterogenous driving styles (k in [0, 1, 2])
- The problem is formulated using the level-k game-theory formalism (See analysed related works for more details).
One idea: use imitation learning (IL) to obtain an explicit level-k control policy.
- A level-k policy is a mapping pi: <ego state, other's states, ego k> -> <sequence of ego actions>.
- The ego-agent maintains belief over the level k of other participants. These estimates are updated using maximum likelihood and Bayes rule.
- A first attempt with supervised learning on a fix dataset (behavioural cloning) was not satisfying enough due to its drift shortcomings:
  - "A small error may cause the vehicle to reach a state that is not exactly included in the dataset and, consequently, a large error may occur at the next time step."
- The solution is to also aggregate experience sampled from the currently learnt policy.
  - The DAgger algorithm (Dataset Aggregation) is used in this work.
  - One point I did not understand: I am surprised that no initial "off-policy" demonstrations is used. The dataset D is initialized as empty.
  - The policy is represented by a neural network.

"Interactive Decision Making for Autonomous Vehicles in Dense Traffic"

[ 2019 ] [📝] [ 🚗 Honda ]
[ game tree search, interaction-aware decision making ]

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In the rule-based stochastic driver model describing the other agents, `2` thresholds are introduced: The `reaction threshold`, sampled from the range {`−1.5m`, `0.4m`}, describes whether or not the agent reacts to the ego car. The `aggression threshold`, uniformly sampled {`−2.2`, `1.1m`}, describes how the agent reacts. Source.

Two tree searches are performed: The first step is to identify a target merging gap based on the probability of a successful merge for each of them. The second search involves forward simulation and collision checking for multiple ego and traffic intentions. In practice the author found that ''the coarse tree - i.e. with intention only - was sufficient for long term planning and only one intention depth needed to be considered for the fine-grained search''. This reduces this second tree to a matrix game. Source.

Author: Isele, D.

Three motivations when working on decision-making for merging in dense traffic:
- 1- Prefer game theory approaches over rule-based planners.
  - To avoid the frozen robot issue, especially in dense traffic.
  - "If the ego car were to wait for an opening, it may have to wait indefinitely, greatly frustrating drivers behind it".
- 2- Prefer the stochastic game formulation over MDP.
  - Merging in dense traffic involves interacting with self-interested agents ("self-interested" in the sense that they want to travel as fast as possible without crashing).
  - "MDPs assume agents follow a set distribution which limits an autonomous agent’s ability to handle non-stationary agents which change their behaviour over time."
  - "Stochastic games are an extension to MDPs that generalize to multiple agents, each of which has its own policy and own reward function."
  - In other words, stochastic games seen more appropriate to model interactive behaviours, especially in the forward rollout of tree search:
    - An interactive prediction model based on the concept of counterfactual reasoning is proposed.
    - It describes how behaviour might change in response to ego agent intervention.
- 3- Prefer tree search over neural networks.
  - "Working with the game trees directly produces interpretable decisions which are better suited to safety guarantees, and ease the debugging of undesirable behaviour."
  - In addition, it is possible to include stochasticity for the tree search.
    - More precisely, the probability of a successful merge is computed for each potential gap based on:
      - The traffic participant’s willingness to yield.
      - The size of the gap.
      - The distance to the gap (from our current position).
How to model other participants, so that they act "intelligently"?
- "In order to validate our behaviour we need interactive agents to test against. This produces a chicken and egg problem, where we need to have an intelligent agent to develop and test our agent. To address this problem, we develop a stochastic rule-based merge behaviour which can give the appearance that agents are changing their mind."
- This merging-response driver model builds on the ideas of IDM, introducing two thresholds (c.f. figure):
  - One threshold governs whether or not the agent reacts to the ego car,
  - The second threshold determines how the agent reacts.
  - "This process can be viewed as a rule-based variant of negotiation strategies: an agent proposes he/she go first by making it more dangerous for the other, the other agent accepts by backing off."
How to reduce the computational complexity of the probabilistic game tree search, while keeping safely considerations ?
- The forward simulation and the collision checking are costly operations. Especially when the depth of the tree increases.
- Some approximations include reducing the number of actions (for both the ego- and the other agents), reducing the number of interacting participants and reducing the branching factor, as can been seen in the steps of the presented approach:
  - 1- Select an intention class based on a coarse search. - the ego-actions are decomposed into a sub-goal selection task and a within-sub-goal set of actions.
  - 2- Identify the interactive traffic participant. - it is assumed that at any given time, the ego-agent interacts with only one other agent.
  - 3- Predict other agents’ intentions. - working with intentions, the continuous action space can be discretized. It reminds me the concept of temporal abstraction which reduces the depth of the search.
  - 4- Sample and evaluate the ego intentions. - a set of safe (absence of collision) ego-intentions can be generated and assessed.
  - 5- Act, observe, and update our probability models. - the probability of safe successful merge.

"Adaptive Robust Game-Theoretic Decision Making for Autonomous Vehicles"

[ 2019 ] [📝] [ 🎓 University of Michigan ] []
[ k-level strategy, MPC, interaction-aware prediction ]

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The agent maintain belief on the `k` parameter for other vehicles and updates it at each step. Source.

Authors: Sankar, G. S., & Han, K.

One related work (described further below): Decision making in dynamic and interactive environments based on cognitive hierarchy theory: Formulation, solution, and application to autonomous driving by (Li, S., Li, N., Girard, A., & Kolmanovsky, I. 2019).
One framework: "level-k game-theoretic framework".
- It is used to model the interactions between vehicles, taking into account the rationality of the other agents.
- The agents are categorized into hierarchical structure of their cognitive abilities, parametrized with a reasoning depth k in [0, 1, 2].
  - A level-0 vehicle considers the other vehicles in the traffic scenario as stationary obstacles, hence being "aggressive".
  - A level-1 agent assumes other agents are at level-0. ...
- This parameter k is what the agent must estimate to model the interaction with the other vehicles.
One term: "disturbance set".
- This set, denoted W, describe the uncertainty in the position estimate of other vehicle (with some delta, similar to the variance in Kalman filters).
- It should capture both the uncertainty about the transition model and the uncertainty about the driver models.
- This set is considered when taking action using a "feedback min-max strategy".
  - I must admit I did not fully understand the concept. Here is a quote:
  - "The min-max strategy considers the worst-case disturbance affecting the behaviour/performance of the system and provides control actions to mitigate the effect of the worst-case disturbance."
- The important idea is to adapt the size of this W set in order to avoid over-conservative behaviours (compared to reachable-set methods).
  - This is done based on the confidence in the estimated driver model (probability distribution of the estimated k) for the other vehicles.
    - If the agent is sure that the other car follows model 0, then it should be "fully" conservative.
    - If the agent is sure it follows level 1, then it could relax its conservatism (i.e. reduce the size of the disturbance set) since it is taken into consideration.
I would like to draw some parallels:
- With (PO)MDP formulation: for the use of a transition model (or transition function) that is hard to define.
- With POMDP formulation: for the tracking of believes about the driver model (or intention) of other vehicles.
  - The estimate of the probability distribution (for k) is updated at every step.
- With IRL: where the agent can predict the reaction of other vehicles assuming they act optimally w.r.t a reward function it is estimating.
- With MPC: the choice of the optimal control following a receding horizon strategy.

"Towards Human-Like Prediction and Decision-Making for Automated Vehicles in Highway Scenarios"

[ 2019 ] [📝] [] [🎞️] [ 🎓 INRIA ] [ 🚗 Toyota ]
[ maximum entropy IRL ]

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Note:
- this 190-page thesis is also referenced in the sections for prediction and planning.
- I really like how the author organizes synergies between three modules that are split and made independent in most modular architectures:
  - (1) driver model
  - (2) behaviour prediction
  - (3) decision-making

Author: Sierra Gonzalez, D.

Related work: there are close concepts to the approach of (Kuderer et al., 2015) referenced below.
One idea: encode the driving preferences of a human driver with a reward function (or cost function), mentioning a quote from Abbeel, Ng and Russell:

“The reward function, rather than the policy or the value function, is the most succinct, robust, and transferable definition of a task”.

Other ideas:
- Use IRL to avoid the manual tuning of the parameters of the reward model. Hence learn a cost/reward function from demonstrations.
- Include dynamic features, such as the time-headway, in the linear combination of the cost function, to take the interactions between traffic participants into account.
- Combine IRL with a trajectory planner based on "conformal spatiotemporal state lattices".
  - The motivation is to deal with continuous state and action spaces and handle the presence of dynamic obstacles.
  - Several advantages (I honestly did not understand that point): the ability to exploit the structure of the environment, to consider time as part of the state-space and respect the non-holonomic motion constraints of the vehicle.
One term: "planning-based motion prediction".
- The resulting reward function can be used to generate trajectory (for prediction), using optimal control.
- Simply put, it can be assumed that each vehicle in the scene behaves in the "risk-averse" manner encoded by the model, i.e. choosing actions leading to the lowest cost / highest reward.
- This method is also called "model-based prediction" since it relies on a reward function or on the models of an MDP.
- This prediction tool is not used alone but rather coupled with some DBN-based manoeuvre estimation (detailed in the section on prediction).

"Car-following method based on inverse reinforcement learning for autonomous vehicle decision-making"

[ 2018 ] [📝] [ 🎓 Tsinghua University, California Institute of Technology, Hunan University ]
[ maximum-margin IRL ]

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Kernel functions are used on the continuous state space to obtain a smooth reward function using linear function approximation. Source.


As often, the divergence metric (to measure the `gap` between one candidate and the expert) is the expected value function estimated on sampled trajectories. Example of how to use `2` `other candidate` policies. I am still confused that each of their decision is based on a state seen by the expert, i.e. they are not building their own full trajectory. Source.

Authors: Gao, H., Shi, G., Xie, G., & Cheng, B.

One idea: A simple and "educationally relevant" application to IRL and a good implementation of the algorithm of (Ng A. & Russell S., 2000): Algorithms for Inverse Reinforcement Learning.
- Observe human behaviours during a "car following" task, assume his/her behaviour is optimal w.r.t. an hidden reward function, and try to estimate that function.
- Strong assumption: no lane-change, no overtaking, no traffic-light. In other worlds, just concerned about the longitudinal control.
Which IRL method?
- Maximum-margin. Prediction aim at learning a reward function that explains the demonstrated policy better than alternative policies by a margin.
- The "margin" is there to address IRL's solution ambiguity.
Steps:
- 1- Define a simple 2d continuous state space s = (s0, s1).
  - s0 = ego-speed divided into 15 intervals (each centre will serve to build means for Gaussian kernel functions).
  - s1 = dist-to-leader divided into 36 intervals (same remark).
  - A normalization is additionally applied.
- 2- Feature transformation: Map the 2d continuous state to a finite number of features using kernel functions.
  - I recommend this short video about feature transformation using kernel functions.
  - Here, Gaussian radial kernel functions are used:
    - Why "radial"? The closer the state to the centre of the kernel, the higher the response of the function. And the further you go, the larger the response "falls".
    - Why "Gaussian"? Because the standard deviation describes how sharp that "fall" is.
    - Note that this functions are 2d: mean = (the centre of one speed interval, the centre of one dist interval).
  - The distance of the continuous state s = (s0, s1) to each of the 15*36=540 means s(i, j) can be computed.
  - This gives 540 kernel features f(i, j) = K(s, s(i, j)).
- 3- The one-step reward is assumed to be linear combination of that features.
  - Given a policy, a trajectory can be constructed.
    - This is a list of states. This list can be mapped to a list of rewards.
    - The discounted sum of this list leads to the trajectory return, seen as expected Value function.
  - One could also form 540 lists for this trajectory (one per kernel feature). Then reduce them by discounted_sum(), leading to 540 V_f(i, j) per trajectory.
    - The trajectory return is then a simple the linear combination: theta(i, j) * V_f(i, j).
  - This can be computed for the demonstrating expert, as well as for many other policies.
  - Again, the task it to tune the weights so that the expert results in the largest values, against all possible other policies.
- 4- The goal is now to find the 540 theta(i, j) weights parameters solution of the max-margin objective:
  - One goal: costly single-step deviation.
    - Try to maximize the smallest difference one could find.
      - I.e. select the best non-expert-policy action and try to maximize the difference to the expert-policy action in each state.
      - max[over theta] min[over π] of the sum[over i, j] of theta(i, j) * [f_candidate(i, j) - f_expert(i, j)].
    - As often the value function serves as "divergence metric".
  - One side heuristic to remove degenerate solutions:
    - "The reward functions with many small rewards are more natural and should be preferred". from here.
    - Hence a regularization constraint (a constraint, not a loss like L1!) on the theta(i, j).
  - The optimization problem with strict constraint is transformed into an optimization problem with "inequality" constraint.
    - Violating constraints is allowed by penalized.
    - As I understood from my readings, that relaxes the linear assumption in the case the true reward function cannot be expressed as a linear combination of the fixed basis functions.
  - The resulting system of equations is solved here with Lagrange multipliers (linear programming was recommended in the orginal max-margin paper).
- 5- Once the theta(i, j) are estimated, the R can be expressed.
About the other policy "candidates":
- "For each optimal car-following state, one of the other car-following actions is randomly selected for the solution".
- In other words, in V(expert) > V(other_candidates) goal, "other_candidates" refers to random policies.
- It would have been interesting to have "better" competitors, for instance policies that are optional w.r.t. the current estimate of R function. E.g. learnt with RL algorithms.
  - That would lead to an iterative process that stops when R converges.

"A Human-like Trajectory Planning Method by Learning from Naturalistic Driving Data"

[ 2018 ] [📝] [ 🎓 Peking University ] [ 🚗 Groupe PSA ]
[ sampling-based trajectory planning ]

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Source.

Authors: He, X., Xu, D., Zhao, H., Moze, M., Aioun, F., & Franck, G.

One idea: couple learning and sampling for motion planning.
- More precisely, learn from human demonstrations (offline) how to weight different contributions in a cost function (as opposed to hand-crafted approaches).
- This cost function is then used for trajectory planning (online) to evaluate and select one trajectory to follow, among a set of candidates generated by sampling methods.
One detail: the weights of the optimal cost function minimise the sum of [prob(candidate) * similarities(demonstration, candidate)].
- It is clear to me how a cost can be converted to some probability, using softmax().
- But for the similarity measure of a trajectory candidate, how to compute "its distance to the human driven one at the same driving situation"?
- Should the expert car have driven exactly on the same track before or is there any abstraction in the representation of the situation?
- How can it generalize at all if the similarity is estimated only on the location and velocity? The traffic situation will be different between two drives.
One quote:

"The more similarity (i.e. less distance) the trajectory has with the human driven one, the higher probability it has to be selected."

"Learning driving styles for autonomous vehicles from demonstration"

[ 2015 ] [📝] [ 🎓 University of Freiburg ] [ 🚗 Bosch ]
[ MaxEnt IRL ]

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Source.

Authors: Kuderer, M., Gulati, S., & Burgard, W.

One important contribution: Deal with continuous features such as integral of jerk over the trajectory.
One motivation: Derive a cost function from observed trajectories.
- The trajectory object is first mapped to some feature vector (speed, acceleration ...).
One Q&A: How to then derive a cost (or reward) from these features?
- The authors assume the cost function to be a linear combination of the features.
- The goal is then about learning the weights.
- They acknowledge in the conclusion that it may be a too simple model. Maybe neural nets could help to capture some more complex relations.
One concept: "Feature matching":
- "Our goal is to find a generative model p(traj| weights) that yields trajectories that are similar to the observations."
- How to define the "Similarity"?
  - The "features" serve as a measure of similarity.
Another concept: "ME-IRL" = Maximum Entropy IRL.
- One issue: This "feature matching" formulation is ambiguous.
  - There are potentially many (degenerated) solutions p(traj| weights). For instance weights = zeros.
- One idea is to introduce an additional goal:
  - In this case: "Among all the distributions that match features, they to select the one that maximizes the entropy."
- The probability distribution over trajectories is in the form exp(-cost[features(traj), θ]), to model that agents are exponentially more likely to select trajectories with lower cost.
About the maximum likelihood approximation in MaxEnt-IRL:
- The gradient of the Lagrangian cost function turns to be the difference between two terms:
  - 1- The empirical feature values (easy to compute from the recorded).
  - 2- The expected feature values (hard to compute: it requires integrating over all possible trajectories).
    - An approximation is made to estimate the expected feature values: the authors compute the feature values of the "most" likely trajectory, instead of computing the expectations by sampling.
  - Interpretation:
    - "We assume that the demonstrations are in fact generated by minimizing a cost function (IOC), in contrast to the assumption that demonstrations are samples from a probability distribution (IRL)".
One related work:
- "Learning to Predict Trajectories of Cooperatively Navigating Agents" by (Kretzschmar, H., Kuderer, M., & Burgard, W., 2014).

`Prediction` and `Manoeuvre Recognition`

"Learning Predictive Models From Observation and Interaction"

[ 2019 ] [📝] [🎞️] [ 🎓 Honda Research Institute ] [ 🚗 University of Pennsylvania, Stanford University, UC Berkeley ]
[ visual prediction, domain transfer, nuScenes, BDD100K ]

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The idea is to learn a latent representation z that corresponds to the true action. The model can then perform joint training on the two kinds of data: it optimizes the likelihood of the interaction data, for which the actions are available, and observation data, for which the actions are missing. Hence the visual predictive model can predict the next frame xt+1 conditioned on the current frame xt and action learnt representation zt. Source.


The visual prediction model is trained using two driving sets: `action`-conditioned videos from Boston and `action`-free videos from the Singapore. Frames from both subsets come from `BDD100K` and `nuScenes` datasets. Source.

Authors: Schmeckpeper, K., Xie, A., Rybkin, O., Tian, S., Daniilidis, K., Levine, S., & Finn, C.

On concrete industrial use-case:
- "Imagine that a self-driving car company has data from a fleet of cars with sensors that record both video and the driver’s actions in one city, and a second fleet of cars that only record dashboard video, without actions, in a second city."
- "If the goal is to train an action-conditioned model that can be utilized to predict the outcomes of steering actions, our method allows us to train such a model using data from both cities, even though only one of them has actions."
Motivations (mainly for robotics, but also AD):
- Generate predictions for complex tasks and new environments, without costly expert demonstrations.
- More precisely, learn an action-conditioned video predictive model from two kinds of data:
  - 1- passive observations: [x0, a1, x1 ... aN, xN].
    - Videos of another agent, e.g. a human, might show the robot how to use a tool.
    - Observations represent a powerful source of information about the world and how actions lead to outcomes.
    - A learnt model could also be used for planning and control, i.e. to plan coordinated sequences of actions to bring about desired outcomes.
    - But may suffer from large domain shifts.
  - 2- active interactions: [x0, x1 ... xN].
    - Usually more expensive.
Two challenges:
- 1- Observations are not annotated with suitable actions: e.g. only access to the dashcam, not the throttle for instance.
  - In other words, actions are only observed in a subset of the data.
  - The goal is to learn from videos without actions, allowing it to leverage videos of agents for which the actions are unknown (unsupervised manner).
- 2- Shift in the "embodiment" of the agent: e.g. robots' arms and humans' ones have physical differences.
  - The goal is to bridge the gap between the two domains (e.g., human arms vs. robot arms).
What is learnt?
- p(xc+1:T|x1:c, a1:T)
- I.e. prediction of future frames conditioned on a set of c context frames and sequence of actions.
What tests?
- 1- Different environment within the same underlying dataset: driving in Boston and Singapore.
- 2- Same environment but different embodiment: humans and robots manipulate objects with different arms.
What is assessed?
- 1- Prediction quality (AD test).
- 2- Control performance (robotics test).

"Deep Learning-based Vehicle Behaviour Prediction For Autonomous Driving Applications: A Review"

[ 2019 ] [📝] [ 🎓 University of Warwick ] [ 🚗 Jaguar Land Rover ]
[ multi-modality prediction ]

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The author propose new classification of behavioural prediction methods. Only deep learning approaches are considered and `physics`-based approaches are excluded. The criteria are about the `input`, `ouput` and deep learning `method`. Source.


First criterion is about the input: What is the prediction based on? Important is to capture road structure and interactions while staying flexible in the representation (e.g. describe different types of intersections and work with varying numbers of `target vehicles` and `surrounding vehicles`). Partial observability should be considered by design. Source.


Second criterion is about the output: What is predicted? Important is to propagate the uncertainty from the input and consider multiple options (multi-modality). Therefore to reason with probabilities. Bottom - why `multi-modality` is important. Source.

Authors: Mozaffari, S., Al-Jarrah, O. Y., Dianati, M., Jennings, P., & Mouzakitis, A.

One mentioned review: (Lefèvre et al.) classifies vehicle (behaviour) prediction models to three groups:
- 1- physics-based
  - Use dynamic or kinematic models of vehicles, e.g. a constant velocity (CV) Kalman Filter model.
- 2- manoeuvre-based
  - Predict vehicles' manoeuvres, i.e. a classification problem from a defined set.
- 3- interaction-aware
  - Consider interaction of vehicles in the input.
About the terminology:
- "Target Vehicles" (TV) are vehicles whose behaviour we are interested in predicting.
- The other are "Surrounding Vehicles" (SV).
- The "Ego Vehicle" (EV) can be also considered as an SV, if it is close enough to TVs.
Here, the authors ignore the physics-based methods and propose three criteria for comparison:
- 1- Input.
  - Track history of TV only.
  - Track history of TV and SVs.
  - Simplified bird’s eye view.
  - Raw sensor data.
- 2- Output.
  - Intention class: From a set of pre-defined discrete classes, e.g. go straight, turn left, and turn right.
  - Unimodal trajectory: Usually the one with highest likelihood or the average).
  - Intention-based trajectory: Predict the trajectory that corresponds to the most probable intention (first case).
  - Multimodal trajectory: Combine the previous ones. Two options, depending if the intention set is fixed or dynamically learnt:
    - static intention set: predict for each member of the set (an extension to intention-based trajectory prediction approaches).
    - dynamic intention set: due to dynamic definition of manoeuvres, they are prone to converge to a single manoeuvre or not being able to explore all the existing manoeuvres.
- 3- In-between (deep learning method).
  - RNN are used because of their temporal feature extracting power.
  - CNN are used for their **spatial feature extracting ability (especially with bird’s eye views).
Important considerations for behavioural prediction:
- Traffic rules.
- Road geometry.
- Multimodality: there may exist more than one possible future behaviour.
- Interaction.
- Uncertainty: both aleatoric (measurement noise) and epistemic (partial observability). Hence the prediction should be probabilistic.
- Prediction horizon: approaches can serve different purposes based on how far in the future they predict (short-term or long-term future motion).
Two methods I would like to learn more about:
- social pooling layers, e.g. used by (Deo & Trivedi, 2019):
  - "A social tensor is a spatial grid around the target vehicle that the occupied cells are filled with the processed temporal data (e.g., LSTM hidden state value) of the corresponding vehicle. It contains both the temporal dynamic of vehicles represented and spatial inter-dependencies among them."
- graph neural networks, e.g. (Diehl et al., 2019) or (Li et al., 2019):
  - Graph Convolutional Network (GCN).
  - Graph Attention Network (GAT).
Comments:
- Contrary to the object detection task, there is no benchmark for systematically evaluating previous studies on vehicle behaviour prediction.
  - Urban scenarios are excluded in the comparison since NGSIM I-80 and US-101 highway driving datasets are used.
  - Maybe the INTERACTION Dataset could be used.
- The authors suggest embedding domain knowledge in the prediction, and call for practical considerations (industry-supported research).
  - "Factors such as environment conditions and set of traffic rules are not directly inputted to the prediction model."
  - "Practical limitations such as sensor impairments and limited computational resources have not been fully taken into account."

"Multi-Modal Simultaneous Forecasting of Vehicle Position Sequences using Social Attention"

[ 2019 ] [📝] [ 🎓 Ecole CentraleSupelec ] [ 🚗 Renault ]
[ multi-modality prediction, attention mechanism ]

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Two multi-head `attention` layers are used to account for `social` interactions between all vehicles. They are combined with `LSTM` layers to offer joint, long-range and multi-modal forecasts. Source.


Source.

Authors: Mercat, J., Gilles, T., Zoghby, N. El, Sandou, G., Beauvois, D., & Gil, G. P.

Previous work: "Social Attention for Autonomous Decision-Making in Dense Traffic" by (Leurent, & Mercat, 2019), detailed on this page as well.
Motivations:
- 1- joint - Considering interactions between all vehicles.
- 2- flexible - Independant of the number/order of vehicles.
- 3- multi-modal - Considering uncertainty.
- 4- long-horizon - Predicting over a long range. Here 5s on simple highway scenarios.
- 5- interpretable - E.g. using the social attention coefficients.
- 6- long distance interdependencies - The authors decide to exclude the spatial grid representations that "limit the zone of interest to a predefined fixed size and the spatial relation precision to the grid cell size".
Main idea: Stack LSTM layers with social multi-head attention layers.
- More precisely, the model is broken into four parts:
  - 1- An Encoder processes the sequences of all vehicle positions (no information about speed, orientation, size or blinker).
  - 2- A Self-attention layer captures interactions between all vehicles using "dot product attention". It has "multiple head", each specializing on different interaction patterns, e.g. "closest front vehicle in any lane".
  - 3- A Predictor, using LSTM cells, forecasts the positions.
  - A second multi-head self-attention layer is placed here.
  - 4- A final Decoder produces sequences of Gaussians mixtures for each vehicle.
    - "What is forecast is not a mixture of trajectory density functions but a sequence of position mixture density functions. There is a dependency between forecasts at time tk and at time tk+1 but no explicit link between the modes at those times."
Two quotes about multi-modality prediction:
- "When considering multiple modes, there is a challenging trade-off to find between anticipating a wide diversity of modes and focusing on realistic ones".
- "VAE and GANs are only able to generate an output distribution with sampling and do not express a PDF".
Baselines used to compare the presented "Social Attention Multi-Modal Prediction" approach:
- Constant velocity (CV), that uses Kalman filters (hence single modality).
- Convolutional Social Pooling (CSP), that uses convolutional social pooling on a coarse spatial grid. Six mixture components are used.
- Graph-based Interaction-aware Trajectory Prediction (GRIP), that uses a spatial and temporal graph representation of the scene.

"MultiPath : Multiple Probabilistic Anchor Trajectory Hypotheses for Behavior Prediction"

[ 2019 ] [📝] [ 🚗 Waymo ]
[ anchor, multi-modality prediction, weighted prediction ]

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Source.


A discrete set of intents is modelled as a set of `K=3` anchor trajectories. Uncertainty is assumed to be unimodal given `intent` (here `3` intents are considered) while `control uncertainty` is modelled with a Gaussian distribution dependent on each waypoint state of an anchor trajectory. Such an example shows that modelling multiple intents is important. Source.

Authors: Chai, Y., Sapp, B., Bansal, M., & Anguelov, D.

One idea: "Anchor Trajectories".
- "Anchor" is a common idea in ML. Concrete applications of "anchor" methods for AD include Faster-RCNN and YOLO for object detections.
  - Instead of directly predicting the size of a bounding box, the NN predicts offsets from a predetermined set of boxes with particular height-width ratios. Those predetermined set of boxes are the anchor boxes. (explanation from this page).
- One could therefore draw a parallel between the sizes of bounding boxes in Yolo and the shape of trajectories: they could be approximated with some static predetermined patterns and refined to the current context (the actual task of the NN here).
  - "After doing some clustering studies on ground truth labels, it turns out that most bounding boxes have certain height-width ratios." [explanation about Yolo from this page]
  - "Our trajectory anchors are modes found in our training data in state-sequence space via unsupervised learning. These anchors provide templates for coarse-granularity futures for an agent and might correspond to semantic concepts like change lanes, or slow down." [from the presented paper]
- This idea reminds also me the concept of pre-defined templates used for path planning.
One motivation: model multiple intents.
- This contrasts with the numerous approaches which predict one single most-likely trajectory per agent, usually via supervised regression.
- The multi-modality is important since prediction is inherently stochastic.
  - The authors distinguish between intent uncertainty and control uncertainty (conditioned on intent).
- A Gaussian Mixture Model (GMM) distribution is used to model both types of uncertainty.
  - "At inference, our model predicts a discrete distribution over the anchors and, for each anchor, regresses offsets from anchor waypoints along with uncertainties, yielding a Gaussian mixture at each time step."
One risk when working with multi-modality: directly learning a mixture suffers from issues of "mode collapse".
- This issue is common in GAN where the generator starts producing limited varieties of samples.
- The solution implemented here is to estimate the anchors a priori before fixing them to learn the rest of our parameters (as for Faster-RCNN and Yolo for instance).
Second motivation: weight the several trajectory predictions.
- This contrasts with methods that randomly sample from a generative model (e.g. CVAE and GAN), leading to an unweighted set of trajectory samples (not to mention the problem of reproducibility and analysis).
- Here, a parametric probability distribution is directly predicted: p(trajectory|observation), together with a compact weighted set of explicit trajectories which summarizes this distribution well.
  - This contrasts with methods that outputs a probabilistic occupancy grid.
About the "top-down" representation, structured in a 3d array:
- The first 2 dimensions represent spatial locations in the top-down image
- "The channels in the depth dimension hold static and time-varying (dynamic) content of a fixed number of previous time steps."
  - Static context includes lane connectivity, lane type, stop lines, speed limit.
  - Dynamic context includes traffic light states over the past 5 time-steps.
  - The previous positions of the different dynamic objects are also encoded in some depth channels.
One word about the training dataset.
- The model is trained via imitation learning by fitting the parameters to maximize the log-likelihood of recorded driving trajectories.
- "The balanced dataset totals 3.85 million examples, contains 5.75 million agent trajectories and constitutes approximately 200 hours of (real-world) driving."

"SafeCritic: Collision-Aware Trajectory Prediction"

[ 2019 ] [📝] [ 🎓 University of Amsterdam ] [ 🚗 BMW ]
[ Conditional GAN ]

Click to expand


The Generator predicts trajectories that are scored against two criteria: The Discriminator (as in `GAN`) for `accuracy` (i.e. consistent with the observed inputs) and the Critic (the generator acts as an Actor) for `safety`. The random noise vector variable `z` in the Generator can be sampled from `N`(`0`, `1`) to sample novel trajectories. Source.


Several features offered by the predictions of `SafeCritic`: accuracy, diversity, attention* and safety. Source.*

Authors: van der Heiden, T., Nagaraja, N. S., Weiss, C., & Gavves, E.

Main motivation:
- "We argue that one should take into account safety, when designing a model to predict future trajectories. Our focus is to generate trajectories that are not just accurate but also lead to minimum collisions and thus are safe. Safe trajectories are different from trajectories that try to imitate the ground truth, as the latter may lead to implausible paths, e.g, pedestrians going through walls."
- Hence the trajectory predictions of the Generator are evaluated against multiple criteria:
  - Accuracy: The Discriminator checks if the prediction is coherent / plausible with the observation.
  - Safety: Some Critic predicts the likelihood of a future dynamic and static collision.
- A third loss term is introduced:
  - "Training the generator is harder than training the discriminator, leading to slow convergence or even failure."
  - An additional auto-encoding loss to the ground truth is introduced.
  - It should encourage the model to avoid trivial solutions and mode collapse, and should increase the diversity of future generated trajectories.
  - The term mode collapse means that instead of suggesting multiple trajectory candidates (multi-modal), the model restricts its prediction to only one instance.
About RL:
- The authors mentioned several terms related to RL, in particular they try to dray a parallel with Inverse RL:
  - "GANs resemble IRL in that the discriminator learns the cost function and the generator represents the policy."
- I got the feeling of that idea, but I was honestly did not understand where it was implemented here. In particular no MDP formulation is given.
About attention mechanism:
- "We rely on attention mechanism for spatial relations in the scene to propose a compact representation for modelling interaction among all agents [...] We employ an attention mechanism to prioritize certain elements in the latent state representations."
- The grid-like scene representation is shared by both the Generator and the Critic.
About the baselines:
- I like the "related work" section which shortly introduces the state-of-the-art trajectory prediction models based on deep learning. SafeCritic takes inspiration from some of their ideas, such as:
  - Aggregation of past information about multiple agents in a recurrent model.
  - Use of Conditional GAN to offer the possibility to also generate novel trajectory given observation via sampling (standard GANs have not encoder).
  - Generation of multi-modal future trajectories.
  - Incorporation of semantic visual features (extracted by deep networks) combined with an attention mechanism.
- SocialGAN, SocialLSTM, Car-Net, SoPhie and DESIRE are used as baselines.
- R2P2 and SocialAttention are also mentioned.

"A Review of Tracking, Prediction and Decision Making Methods for Autonomous Driving"

[ 2019 ] [📝] [ 🎓 University of Iasi ]

Click to expand

One figure:


Classification of motion models* based on three increasingly abstract levels - adapted from (Lefèvre, S., Vasquez. D. & Laugier C. - 2014). Source.*

Authors: Leon, F., & Gavrilescu, M.

A reference to one white paper: "Safety first for automated driving" 2019 - from Aptiv, Audi, Baidu, BMW, Continental, Daimler, Fiat Chrysler Automobiles, HERE, Infineon, Intel and Volkswagen (alphabetical order). The authors quote some of the good practices about Interpretation and Prediction:
- Predict only a short time into the future (the further the predicted state is in the future, the less likely it is that the prediction is correct).
- Rely on physics where possible (a vehicle driving in front of the automated vehicle will not stop in zero time on its own).
- Consider the compliance of other road users with traffic rules.
Miscellaneous notes about prediction:
- The authors point the need of high-level reasoning (the more abstract the feature, the more reliable it is long term), mentioning both "affinity" and "attention" mechanisms.
- They also call for jointly addressing vehicle motion modelling and risk estimation (criticality assessment).
- Gaussian Processed is found to be a flexible tool for modelling motion patterns and is compared to Markov Models for prediction.
  - In particular, GP regressions have the ability to quantify uncertainty (e.g. occlusion).
- "CNNs can be superior to LSTMs for temporal modelling since trajectories are continuous in nature, do not have complicated "state", and have high spatial and temporal correlations".

"Deep Predictive Autonomous Driving Using Multi-Agent Joint Trajectory Prediction and Traffic Rules"

[ 2019 ] [📝] [🎞️] [ 🎓 Seoul National University ]

Click to expand

One figure:


The framework consists of four modules: encoder module, interaction module, prediction module* and control module. Source.*

Authors: Cho, K., Ha, T., Lee, G., & Oh, S.

One previous work: "Learning-Based Model Predictive Control under Signal Temporal Logic Specifications" by (Cho & Ho, 2018).
One term: "robustness slackness" for STL-formula.
- The motivation is to solve dilemma situations (inherent to strict compliance when all rules cannot be satisfied) by disobeying certain rules based on their predicted degree of satisfaction.
- The idea is to filter out non-plausible trajectories in the prediction step to only consider valid prediction candidates during planning.
- The filter considers some "rules" such as Lane keeping and Collision avoidance of front vehicle or Speed limit (I did not understand why they are equally considered).
- These rules are represented by Signal Temporal Logic (STL) formulas.
  - Note: STL is an extension of Linear Temporal Logic (with boolean predicates and discrete-time) with real-time and real-valued constraints.
- A metric can be introduced to measure how well a given signal (here, a trajectory candidate) satisfies a STL formula.
  - This is called "robustness slackness" and acts as a margin to satisfaction of STL-formula.
- This enables a "control under temporal logic specification" as mentioned by the authors.
Architecture
- Encoder module: The observed trajectories are fed to some LSTM whose internal state is used by the two subsequent modules.
- Interaction module: To consider interaction, all LSTM states are concatenated (joint state) together with a feature vector of relative distances. In addition, a CVAE is used for multi-modality (several possible trajectories are generated) and capture interactions (I did not fully understand that point), as stated by the authors:
  - "The latent variable z models inherent structure in the interaction of multiple vehicles, and it also helps to describe underlying ambiguity of future behaviours of other vehicles."
- Prediction module: Based on the LSTM states, the concatenated vector and the latent variable, both future trajectories and margins to the satisfaction of each rule are predicted.
- Control module: An MPC optimizes the control of the ego car, deciding which rules should be prioritized based on the two predicted objects (trajectories and robustness slackness).

"An Online Evolving Framework for Modeling the Safe Autonomous Vehicle Control System via Online Recognition of Latent Risks"

[ 2019 ] [📝] [ 🎓 Ohio State University ] [ 🚗 Ford ]
[ MDP, action-state transitions matrix, SUMO, risk assessment ]

Click to expand

One figure:


Both the state space* and the transition model are adapted online, offering two features: prediction about the next state and detection of unknown (i.e. `risky`) situations. Source.*

Authors: Han, T., Filev, D., & Ozguner, U.

Motivation
- "Rule-based and supervised-learning methods cannot recognize unexpected situations so that the AV controller cannot react appropriately under unknown circumstances."
- Based on their previous work on RL “Highway Traffic Modeling and Decision Making for Autonomous Vehicle Using Reinforcement Learning” by (You, Lu, Filev, & Tsiotras, 2018).
Main ideas: Both the state space and the transition model (here discrete state space so transition matrices) of an MDP are adapted online.
- I understand it as trying to learn the transition model (experience is generated using SUMO), hence to some extent going toward model-based RL.
- The motivation is to assist any AV control framework with a so-called "evolving Finite State Machine" (e-FSM).
  - By identifying state-transitions precisely, the future states can be predicted.
  - By determining states uniquely (using online-clustering methods) and recognizing the state consistently (expressed by a probability distribution), initially unexpected dangerous situations can be detected.
  - It reminds some ideas about risk assessment discussed during IV19: the discrepancy between expected outcome and observed outcome is used to quantify risk, i.e. the surprise or misinterpretation of the current situation).
Some concerns:
- "The dimension of transition matrices should be expanded to represent state-transitions between all existing states"
  - What when the scenario gets more complex than the presented "simple car-following" and that the state space (treated as discrete) becomes huge?
- In addition, "the total number of transition matrices is identical to the total number of actions".
  - Alone for the simple example, the acceleration command was sampled into 17 bins. Continuous action spaces are not an option.

"A Driving Intention Prediction Method Based on Hidden Markov Model for Autonomous Driving"

[ 2019 ] [📝] [ 🎓 IEEE ]
[ HMM, Baum-Welch algorithm, forward algorithm ]

Click to expand

One figure:


Source.

Authors: Liu, S., Zheng, K., Member, S., Zhao, L., & Fan, P.

One term: "mobility feature matrix"
- The recorded data (e.g. absolute positions, timestamps ...) are processed to form the mobility feature matrix (e.g. speed, relative position, lateral gap in lane ...).
- Its size is T × L × N: T time steps, L vehicles, N types of mobility features.
- In the discrete characterization, this matrix is then turned into a set of observations using K-means clustering.
- In the continuous case, mobility features are modelled as Gaussian mixture models (GMMs).
This work implements HMM concepts presented in my project Educational application of Hidden Markov Model to Autonomous Driving.

"Online Risk-Bounded Motion Planning for Autonomous Vehicles in Dynamic Environments"

[ 2019 ] [📝] [ 🎓 MIT ] [ 🚗 Toyota ]
[ intention-aware planning, manoeuvre-based motion prediction, POMDP, probabilistic risk assessment, CARLA ]

Click to expand

One figure:


Source.

Authors: Huang, X., Hong, S., Hofmann, A., & Williams, B.

One term: "Probabilistic Flow Tubes" (PFT)
- A motion representation used in the "Motion Model Generator".
- Instead of using hand-crafted rules for the transition model, the idea is to learns human behaviours from demonstration.
- The inferred models are encoded with PFTs and are used to generate probabilistic predictions for both manoeuvre (long-term reasoning) and motion of the other vehicles.
- The advantage of belief-based probabilistic planning is that it can avoid over-conservative behaviours while offering probabilistic safety guarantees.
Another term: "Risk-bounded POMDP Planner"
- The uncertainty in the intention estimation is then propagated to the decision module.
- Some notion of risk, defined as the probability of collision, is evaluated and considered when taking actions, leading to the introduction of a "chance-constrained POMDP" (CC-POMDP).
- The online solver uses a heuristic-search algorithm, Risk-Bounded AO* (RAO*), takes advantage of the risk estimation to prune the over-risky branches that violate the risk constraints and eventually outputs a plan with a guarantee over the probability of success.
One quote (this could apply to many other works):

"One possible future work is to test our work in real systems".

"Towards Human-Like Prediction and Decision-Making for Automated Vehicles in Highway Scenarios"

[ 2019 ] [📝] [] [🎞️] [ 🎓 INRIA ] [ 🚗 Toyota ]
[ planning-based motion prediction, manoeuvre-based motion prediction ]

Click to expand

Author: Sierra Gonzalez, D.

Prediction techniques are often classified into three types:
- physics-based
- manoeuvre-based (and goal-based).
- interaction-aware
As I understood, the main idea here is to combine prediction techniques (and their advantages).
- The driver-models (i.e. the reward functions previously learnt with IRL) can be used to identify the most likely, risk-aversive, anticipatory manoeuvres. This is called the model-based prediction by the author since it relies on one model.
  - But relying only on driver models to predict the behaviour of surrounding traffic might fail to predict dangerous manoeuvres.
  - As stated, "the model-based method is not a reliable alternative for the short-term estimation of behaviour, since it cannot predict dangerous actions that deviate from what is encoded in the model".
  - One solution is to add a term that represents how the observed movement of the target matches a given maneuver.
  - In other words, to consider the noisy observation of the dynamics of the targets and include these so-called dynamic evidence into the prediction.
Usage:
- The resulting approach is used in the probabilistic filtering framework to update the belief in the POMDP and in its rollout (to bias the construction of the history tree towards likely situations given the state and intention estimations of the surrounding vehicles).
- It improves the inference of manoeuvres, reducing rate of false positives in the detection of lane change manoeuvres and enables the exploration of situations in which the surrounding vehicles behave dangerously (not possible if relying on safe generative models such as IDM).
One quote about this combination:

"This model mimics the reasoning process of human drivers: they can guess what a given vehicle is likely to do given the situation (the model-based prediction), but they closely monitor its dynamics to detect deviations from the expected behaviour".

One idea: use this combination for risk assessment.
- As stated, "if the intended and expected maneuver of a vehicle do not match, the situation is classified as dangerous and an alert is triggered".
- This is an important concept of risk assessment I could identify at IV19: a situation is dangerous if there is a discrepancy between what is expected (given the context) and what is observed.
One term: "Interacting Multiple Model" (IMM), used as baseline in the comparison.
- The idea is to consider a group of motion models (e.g. lane keeping with CV, lane change with CV) and continuously estimate which of them captures more accurately the dynamics exhibited by the target.
- The final predictions are produced as a weighted combination of the individual predictions of each filter.
- IMM belongs to the physics-based predictions approaches and could be extended for manoeuvre inference (called dynamics matching). It is often used to maintain the beliefs and guide the observation sampling in POMDP.
- But the issue is that IMM completely disregards the interactions between vehicles.

"Decision making in dynamic and interactive environments based on cognitive hierarchy theory: Formulation, solution, and application to autonomous driving"

[ 2019 ] [📝] [ 🎓 University of Michigan ]
[ level-k game theory, cognitive hierarchy theory, interaction modelling, interaction-aware decision making ]

Click to expand

Authors: Li, S., Li, N., Girard, A., & Kolmanovsky, I.

One concept: cognitive hierarchy.
- Other drivers are assumed to follow some "cognitive behavioural models", parametrized with a so called "cognitive level" σ.
- The goal is to obtain and maintain belief about σ based on observation in order to optimally respond (using an MPC).
- Three levels are considered:
  - level-0: driver that treats other vehicles on road as stationary obstacles.
  - level-1: cautious/conservative driver.
  - level-2: aggressive driver.
One quote about the "cognitive level" of human drivers:

"Humans are most commonly level-1 and level-2 reasoners".

Related works:

Li, S., Li, N., Girard, A. & Kolmanovsky, I. [2019]. "Decision making in dynamic and interactive environments based on cognitive hierarchy theory, Bayesian inference, and predictive control" [pdf]
Li, N., Oyler, D., Zhang, M., Yildiz, Y., Kolmanovsky, I., & Girard, A. [2016]. "Game-theoretic modeling of driver and vehicle interactions for verification and validation of autonomous vehicle control systems" [pdf]
- "If a driver assumes that the other drivers are level-1 and takes an action accordingly, this driver is a level-2 driver".
- Use RL with hierarchical assignment to learn the policy:
  - First, the π-0 (for level-0) is learnt for the ego-agent.
  - Then π-1 with all the other participants following π-0.
  - Then π-2 ...
- Action masking: "If a car in the left lane is in a parallel position, the controlled car cannot change lane to the left".
  - "The use of these hard constrains eliminates the clearly undesirable behaviours better than through penalizing them in the reward function, and also increases the learning speed during training"
Ren, Y., Elliott, S., Wang, Y., Yang, Y., & Zhang, W. [2019]. "How Shall I Drive ? Interaction Modeling and Motion Planning towards Empathetic and Socially-Graceful Driving" [pdf] [code]


Source.


Source.

`Rule-based` `Decision Making`

"Trajectory Optimization and Situational Analysis Framework for Autonomous Overtaking with Visibility Maximization"

[ 2019 ] [📝] [🎞️] [ 🎓 National University of Singapore, Delft University, MIT ]
[ FSM, occlusion, partial observability ]

Click to expand


Left: previous work Source. Right: The `BP` `FSM` consists in `5` states and `11` transitions. Each transition from one state to the other is triggered by specific alphabet unique to the state. For instance, `1` is `Obstacle to be overtaken in ego lane detected`. Together with the `MPC` set of parameters, a guidance path is passed to the trajectory optimizer. Source.

Authors: Andersen, H., Alonso-mora, J., Eng, Y. H., Rus, D., & Ang Jr, M. H.

Main motivation:
- Deal with occlusions, i.e. partial observability.
- Use case: a car is illegally parked on the vehicle’s ego lane. It may fully occlude the visibility. But has to be overtaken.
One related works:
- "Trajectory Optimization for Autonomous Overtaking with Visibility Maximization" - (Andersen et al., 2017)
- [🎞️].
- [🎞️].
- [🎞️].
About the hierarchical structure.
- 1- A high-level behaviour planner (BP).
  - It is structured as a deterministic finite state machine (FSM).
  - States include:
    - Follow ego-lane
    - Visibility Maximization
    - Overtake
    - Merge back
    - Wait
  - Transition are based on some deterministic risk assessment.
    - The authors argue that the deterministic methods (e.g. formal verification of trajectory using reachability analysis) are simpler and computationally more efficient than probabilistic versions, while being very adapted for this information maximization:
    - This is due to the fact that the designed behaviour planner explicitly breaks the traffic rule in order to progress along the vehicle’s course.
- Interface 1- > 2-:
  - Each state correspond to specific set of parameters that is used in the trajectory optimizer.
  - "In case of Overtake, a suggested guidance path is given to both the MPC and `backup trajectory generator".
- 2- A trajectory optimizer.
  - The problem is formulated as receding horizon planner and the task is to solve, in real-time, the non-linear constrained optimization.
    - Cost include guidance path deviation, progress, speed deviation, size of blind spot (visible area) and control inputs.
    - Constraints include, among other, obstacle avoidance.
    - The prediction horizon of this MPC is 5s.
  - Again (I really like this idea), MPC parameters are set by the BP.
    - For instance, the cost for path deviation is high for Follow ego-lane, while it can be reduced for Visibility Maximization.
    - "Increasing the visibility maximization cost resulted in the vehicle deviating from the path earlier and more abrupt, leading to frequent wait or merge back cases when an oncoming car comes into the vehicle’s sensor range. Reducing visibility maximization resulted in later and less abrupt deviation, leading to overtaking trajectories that are too late to be aborted. We tune the costs for a good trade-off in performance."
    - Hence, depending on the state, the task might be to maximize the amount of information that the autonomous vehicle gains along its trajectory.
- "Our method considers visibility as a part of both decision-making and trajectory generation".

"Jointly Learnable Behavior and Trajectory Planning for Self-Driving Vehicles"

[ 2019 ] [📝] [ 🚗 Uber ]
[ max-margin ]

Click to expand


Source.

Authors: Sadat, A., Ren, M., Pokrovsky, A., Lin, Y., Yumer, E., & Urtasun, R.

Main motivation:
- Design a decision module where both the behavioural planner and the trajectory optimizer share the same objective (i.e. cost function).
- Therefore "joint".
- "[In approaches not-joint approaches] the final trajectory outputted by the trajectory planner might differ significantly from the one generated by the behavior planner, as they do not share the same objective".
Requirements:
- 1- Avoid time-consuming, error-prone, and iterative hand-tuning of cost parameters.
  - E.g. Learning-based approaches (BC).
- 2- Offer interpretability about the costs jointly imposed on these modules.
  - E.g. Traditional modular 2-stage approaches.
About the structure:
- The driving scene is described in W (desired route, ego-state, map, and detected objects). Probably W for "World"?
- The behavioural planner (BP) decides two things based on W:
  - 1- An high-level behaviour b.
    - The path to converge to based on one chosen manoeuvre: keep-lane, left-lane-change, or right-lane-change.
    - The left and right lane boundaries.
    - The obstacle side assignment: whether an obstacle should stay in the front, back, left, or right to the ego-car.
  - 2- A coarse-level trajectory τ.
  - The loss has also a regularization term.
  - This decision is "simply" the argmin of the shared cost-function, obtained by sampling+selecting the best.
- The "trajectory optimizer" refines τ based on the constraints imposed by b.
  - For instance an overlap cost will be incurred if the side assignment of b is violated.
- A cost function parametrized by w assesses the quality of the selected <b, τ> pair:
  - cost = w^T . sub-costs-vec(τ, b, W).
  - Sub-costs relate to safety, comfort, feasibility, mission completion, and traffic rules.
Why "learnable"?
- Because the weight vector w that captures the importance of each sub-cost is learnt based on human demonstrations.
  - "Our planner can be trained jointly end-to-end without requiring manual tuning of the costs functions".
- They are two losses for that objective:
  - 1- Imitation loss (with MSE).
    - It applies on the <b, τ> produced by the BP.
  - 2- Max-margin loss to penalize trajectories that have small cost and are different from the human driving trajectory.
    - It applies on the <τ> produced by the trajectory optimizer.
    - "This encourages the human driving trajectory to have smaller cost than other trajectories".
    - It reminds me the max-margin method in IRL where the weights of the reward function should make the expert demonstration better than any other policy candidate.

"Liability, Ethics, and Culture-Aware Behavior Specification using Rulebooks"

[ 2019 ] [📝] [] [🎞️] [🎞️] [ 🎓 ETH Zurich ] [ 🚗 nuTonomy, Aptiv ]
[ sampling-based planning, safety validation, reward function, RSS ]

Click to expand

Some figures:


Defining the `rulebook`. Source.


The `rulebook` is associated to an operator `=<` to prioritize between rules. Source.


The `rulebook` serves for deciding which trajectory to take* and can be adapted using a series of operations. Source.*

Authors: Censi, A., Slutsky, K., Wongpiromsarn, T., Yershov, D., Pendleton, S., Fu, J., & Frazzoli, E.

Allegedly how nuTonomy (an Aptiv company) cars work.
One main concept: "rulebook".
- It contains multiple rules, that specify the desired behaviour of the self-driving cars.
- A rule is simply a scoring function, or “violation metric”, on the realizations (= trajectories).
- The degree of violation acts like some penalty term: here some examples of evaluation of a realization x evaluated by a rule r:
  - For speed limit: r(x) = interval for which the car was above 45 km/h.
  - For minimizing harm: r(x) = kinetic energy transferred to human bodies.
- Based on Use as a comparison operator to rank candidate trajectories.
One idea: Hierarchy of rules.
- With many rules being defined, it may be impossible to find a realization (e.g. trajectory) that satisfies all.
- But even in critical situation, the algorithm must make a choice - the least catastrophic option (hence no concept of infeasibility.)
- To deal with this concept of "Unfeasibility", priorities between conflicting rules which are therefore hierarchically ordered.
- Hence a rulebook R comes with some operator <: <R, <>.
- This leads to some concepts:
- Safety vs. infractions.
  - Ex.: a rule "not to collide with other objects" will have a higher priority than the rule "not crossing the double line".
- Liability-aware specification.
  - Ex.: (edge-case): Instruct the agent to collide with the object on its lane, rather than collide with the object on the opposite lane, since changing lane will provoke an accident for which it would be at fault.
  - This is close to the RSS ("responsibility-sensitive safety" model) of Mobileye.
- Hierarchy between rules:
  - Top: Guarantee safety of humans.
    - This is written analytically (e.g. a precise expression for the kinetic energy to minimize harm to people).
  - Bottom: Comfort constraints and progress goals.
    - Can be learnt based on observed behaviour (and also tend to be platform- and implementation- specific).
  - Middle: All the other priorities among rule groups
    - There are somehow open for discussion.
How to build a rulebook:
- Rules can be defined analytically (e.g. LTL formalism) or learnt from data (for non-safety-critical rules).
- Violation functions can be learned from data (e.g. IRL).
- Priorities between rules can also be learnt.
One idea: manipulation of rulebooks.
- Regulations and cultures differ depending on the country and the state.
- A rulebook <R, <> can easily be adapted using three operations (priority refinement, rule augmentation, rule aggregation).
Related work: Several topics raised in this paper reminds me subjects addressed in Emilio Frazzoli, CTO, nuTonomy - 09.03.2018
- 1- Decision making with FSM:
  - Too complex to code. Easy to make mistake. Difficult to adjust. Impossible to debug (:cry:).
- 2- Decision making with E2E learning:
  - Appealing since there are too many possible scenarios.
  - But how to prove that and justify it to the authorities?
    - One solution is to revert such imitation strategy: start by defining the rules.
- 3- Decision making "cost-function-based" methods
  - 3-1- RL / MCTS: not addressed here.
  - 3-2- Rule-based (not the if-else-then logic but rather traffic/behaviour rules).
- First note:
  - Number of rules: small (15 are enough for level-4).
  - Number of possible scenarios: huge (combinational).
- Second note:
  - Driving baheviours are hard to code.
  - Driving baheviours are hard to learn.
  - But driving baheviours are easy to assess.
- Strategy:
  - 1- Generate candidate trajectories
    - Not only in time and space.
    - Also in term of semantic (logical trajectories in Kripke structure).
  - 2- Check if they satisfy the constraints and pick the best.
    - This involves linear operations.
- Conclusion:
  - "Rules and rules priorities, especially those that concern safety and liability, must be part of nation-wide regulations to be developed after an informed public discourse; it should not be up to engineers to choose these important aspects."
  - This reminds me the discussion about social-acceptance I had at IV19.^
  - As E. Frazzoli concluded during his talk, the remaining question is:
    - "We do not know how we want human-driven vehicle to behave?"
    - Once we have the answer, that is easy.

Some figures from this related presentation:


Candidate trajectories are not just spatio-temporal* but also semantic. Source.*


Define priorities* between rules, as Asimov did for his laws. Source.*


As raised here by the main author of the paper, I am still wondering how the presented framework deals with the different sources of uncertainties. Source.

"Provably Safe and Smooth Lane Changes in Mixed Traffic"

[ 2019 ] [📝] [] [🎞️] [ 🎓 FZI & KIT ]
[ path-velocity decomposition, IDM, RSS ]

Click to expand

Some figures:


The first `safe?` check might lead to conservative behaviours (huge gaps would be needed for safe lane changes). Hence it is relaxed with some `Probably Safe?` condition. Source.


Source.


Formulation by Pek, Zahn, & Althoff, 2017. Source.

Authors: Naumann, M., Königshof, H., & Stiller, C.

Main ideas:
- The notion of safety is based on the responsibility sensitive safety (RSS) definition.
  - As stated by the authors, "A safe lane change is guaranteed not to cause a collision according to the previously defined rules, while a single vehicle cannot ensure that it will never be involved in a collision."
- Use set-based reachability analysis to prove the "RSS-safety" of lane change manoeuvre based on gap evaluation.
  - In other words, it is the responsibility of the ego vehicle to maintain safe distances during the lane change manoeuvre.
Related works: A couple of safe distances are defined, building on
- RSS principles (after IV19, I tried to summarize some of the RSS concepts here).
- "Verifying the Safety of Lane Change Maneuvers of Self-driving Vehicles Based on Formalized Traffic Rules", (Pek, Zahn, & Althoff, 2017)

"Decision-Making Framework for Autonomous Driving at Road Intersections: Safeguarding Against Collision, Overly Conservative Behavior, and Violation Vehicles"

[ 2018 ] [📝] [🎞️] [ 🎓 Daejeon Research Institute, South Korea ]
[ probabilistic risk assessment, rule-based probabilistic decision making ]

Click to expand

One figure:


Source.

Author: Noh, S.

Many ML-based works criticize rule-based approaches (over-conservative, no generalization capability and painful parameter tuning).
- True, the presented framework contains many parameters whose tuning may be tedious.
- But this approach just works! At least they go out of the simulator and show some experiments on a real car.
- I really like their video, especially the multiple camera views together with the RViz representation.
- It can be seen that probabilistic reasoning and uncertainty-aware decision making are essential for robustness.
One term: "Time-to-Enter" (tte).
- It represents the time it takes a relevant vehicle to reach the potential collision area (CA), from its current position at its current speed.
- To deal with uncertainty in the measurements, a variant of this heuristic is coupled with a Bayesian network for probabilistic threat-assessment.
One Q&A: What is the difference between situation awareness and situation assessment?
- In situation awareness, all possible routes are considered for the detected vehicles using a map. The vehicles whose potential route intersect with the ego-path are classified as relevant vehicles.
- In situation assessment, a threat level in {Dangerous, Attentive, Safe} is inferred for each relevant vehicle.
One quote:

"The existing literature mostly focuses on motion prediction, threat assessment, or decision-making problems, but not all three in combination."

`Model-Free` `Reinforcement Learning`

"End-to-end Reinforcement Learning for Autonomous Longitudinal Control Using Advantage Actor Critic with Temporal Context"

[ 2019 ] [📝] [ 🎓 University of Surrey ] [ 🚗 Jaguar Land Rover ]
[ sampling efficiency, switching actions ]

Click to expand


In the reward function, the `time headway` term encourages the agent to maintain a `headway` close to `2s`, while the `headway-derivative` term rewards the agent for taking actions which bring it closer to the ideal headway. Source.


Using recurrent units* in the actor net leads to a smoother driving style and maintains a closer headway to the `2s` target. Source.*

Authors: Kuutti, S., Bowden, R., Joshi, H., Temple, R. De, & Fallah, S.

Motivations for "headway-keeping", i.e. longitudinal control, using model-free RL:
- 1- Address inherent sampling inefficiency.
- 2- Address common jerky driving behaviours, i.e. aim at smoother longitudinal trajectories.
  - "Without any temporal context given, the agent has to decide the current action without any consideration for the previous actions, sometimes leading to rapid switching between the throttle and brake pedals."
The task: keep a 2s time headway from the lead vehicle in IPG CarMaker.
- The state consists in:
  - ego-speed
  - ego-acc
  - delta speed to leader
  - time headway to leader
- Personal note: Since the longitudinal control of the ego-car has no influence on the speed of the leading vehicle, the transition function of this MDP should be stochastic.
One idea for sampling efficiency: "Proxy Network for Vehicle Dynamics".
- For training, the simulator was constrained to running at real-time (timestep = 40ms). At the same time, model-free methods require many samples.
- One idea is to learn the "ego-dynamics", i.e. one part of the transition function.
  - input = [ego-speed, ego-acc, previous pedal action, current pedal action, road coefficient of friction]
  - output = ego-speed at the next time-step.
- The authors claim that this model (derived with supervised learning), can be used to replace the simulator when training the RL agent:
  - "The training was completed using the proxy network in under 19 hours."
  - As noted above, this proxy net does not capture the full transition. Therefore, I do not understand how it can substitute the simulator and "self-generate" samples, except if assuming constant speed of the leading vehicle - which would boil down to some target-speed-tracking task instead."
    - In addition, one could apply planning techniques instead of learning.
One idea again jerky action switch:
- The authors add a 16 LSTM units to the actor network.
- Having a recurrent cell provides a temporal context and leads to smoother predictions.
How to deal with continuous actions?
- 1- The actor network estimates the action value mean µ and the estimated variance σ.
- 2- This transformed into a Gaussian probability distribution, from which the control action is then sampled.
About the "Sequenced" experience replay:
- The RL agent is train off-policy, i.e. the experience tuples used for updating the policy were not collected from the currently-updated policy, but rather drawn from a replay buffer.
- Because the LSTM has an internal state, experience tuples <s, a, r, s'> cannot be sampled individually.
- "Since the network uses LSTMs for temporal context, these minibatches are sampled as sequenced trajectories of experiences."
About the "Trauma memory":
- A second set of experiences is maintained and used during training.
- This "Trauma memory" stores trajectories which lead to collisions.
- The ratio of trauma memory samples to experience replay samples is set to 1/64.

"Multi-lane Cruising Using Hierarchical Planning and Reinforcement Learning"

[ 2019 ] [📝] [ 🚗 Huawei ]
[ hierarchical planning, SUMO ]

Click to expand


The high-level behavioural planner (`BP`) selects a `lane` while the underlying motion planner (`MoP`) select a `corridor`. Source.


The decision-making is hierarchically divided into three levels. The first two (`BP` and `MoP`) are learning-based. The `keep-lane` and `switch-lane` tasks are achieved using a shared `MoP` agent. The last module that decides of low-level commands such as `throttle` and `steering` is left rule-based since it is car-specific. Source.

Authors: Rezaee, K., Yadmellat, P., Nosrati, M. S., Abolfathi, E. A., Elmahgiubi, M., & Luo, J.

Motivation:
- Split the decision-making process based on different levels of abstraction. I.e. hierarchical planning.
About the hierarchical nature of "driving" and the concept of "symbolic punctuation":
- "Different from regular robotic problems, driving is heavily symbolically punctuated by signs and rules (e.g. lane markings, speed limit signs, fire truck sirens, turning signals, traffic lights) on top of what is largely a continuous control task."
- In other words, higher level decisions on discrete state transitions should be coordinated with lower level motion planning and control in continuous state space.
About the use of learning-based methods for low-level control:
- The authors mentioned, among other, the work of Mobileye, where RL is used in the planning phase to model the vehicle’s acceleration given results from the prediction module.
  - "Given well-established controllers such as PID and MPC, we believe that learning-based methods are more effective in the high and mid-level decision making (e.g. BP and Motion Planning) rather than low-level controllers."
Another concept: "skill-based" planning:
- It means that planning submodules are specialized for a driving sub-task (e.g. lane keeping, lane switching).
- The authors introduce a road abstraction: a lane (selected by the BP) is divided into (5) corridors (selected by the MoP).
  - Corridor selection is equivalent to selecting a path among a set of predefined paths.
Proposed structure:
- 1- The Behavioural planner (BP) outputs a high-level decisions in {keep lane, switch to the left lane, switch to the right lane}.
  - Also, some target speed??? No, apparently a separated module sets some target speed set-point based on the BP desire and the physical boundaries, such as heading cars, or any interfering objects on the road - Not clear to me.
- 2- The Motion planners (MoP) outputs a target corridor and a target speed.
- 3- A separated and non-learnt trajectory controller converts that into low-level commands (throttle and acceleration)
About the high-level Option: How to define termination?
- "The typical solution is to assign a fixed expiration time to each option and penalize the agent if execution time is expired."
- According the authors, BP should not wait until its command gets executed (since any fixed lifetime for BP commands is dangerous).
  - "BP should be able to update its earlier decisions, at every time step, according to the new states."
How to train the two modules?
- "We design a coarse-grained reward function and avoid any fine-grained rules in our reward feedback."
- The reward function of the MoP depends on the BP objective:
  - Reward +1 is given to the MoP agent if being in the middle corridor (ok, but is the target lane considered?? Not clear to me), AND:
    - EITHER the speed of the ego vehicle is within a threshold of the BP target speed,
    - OR the minimum front gap is within a threshold of the safe distance d (computed based on TTC).
- The MoP is first trained with random BP commands (target lanes are sampled every 20s). Then the BP agent is trained: its gets positive rewards for driving above a threshold while being penalized at each lane change.
- Main limitation:
  - Training is done separately (both BP and MoP agents cannot adapt to each other, hence the final decision might be sub-optimal).
Another advantage: alleged easy sim-to-real transfer.
- "In practice, low-level policies may result in oscillatory or undesirable behaviours when deployed on real-world vehicles due to imperfect sensory inputs or unmodeled kinematic and dynamic effects."
  - "Our state-action space abstraction allows transferring of the trained models from a simulated environment with virtually no dynamics to the one with significantly more realistic dynamics without a need for retraining."

"Learning to Drive using Waypoints"

[ 2019 ] [📝] [ 🎓 Carnegie Mellon University ]
[ PPO, waypoint-based navigation, CARLA ]

Click to expand


The communication of navigation goals at intersection is not done using high-level commands (c.f. `conditional RL`), but rather by giving the `PPO` agent a list of predefined waypoints to follow. Source.

Authors: Agarwal, T., Arora, H., Parhar, T., Deshpande, S., & Schneider, J.

One idea (alternative to conditional learning):
- "Past approaches have used a higher-level planner that directs the agent using high-level commands on turning. Instead of this, we propose to use trajectory waypoints to guide navigation, which are readily available in real world autonomous vehicles."
- Using such a predefined path probably constrains applications to single-lane scenarios, and relies on an up-to-date HD map.
- "We acknowledge that both the baseline methods use higher level navigation features and RGB images in contrast to richer low-level waypoint features and simpler semantically segmented images used in our approach."
The agent policy network (PPO) takes two inputs:
- 1- The bottleneck embedding of an auto-encoder applied on semantically segmented images.
- 2- The coming n waypoints to follow. They are 2m-spaced and are extracted from an offline map.
- Both are independant of the realistism of the simulator it has trained on. One could therefore expect the approach to transfer well to real-world.

"Social Attention for Autonomous Decision-Making in Dense Traffic"

[ 2019 ] [📝] [🎞️] [ 🎓 Inria ] [ 🚗 Renault ]
[ attention mechanism ]

Click to expand

Weights in the encoding linear are shared between all vehicles. Each encoding contains individual features and has size dx. For each head in the stack, different linear projections (Lq, Lk, Lv) are applied on them. Results of projections are key and values (plus a query for the ego-agent). Based on the similarity between the ego-query q0 and the keys, an attention matrix is built. This matrix should select a subset of vehicles that are important, depending on the context. It is multiplied with the concatenation of the individual values features, and then passed to a decoder where results from all heads are combined. The output are the estimated q-values.. Source.

Example with a stack of two heads. Both direct their attention to incoming vehicles that are likely to collide with the ego-vehicle. Visualization of the attention matrix: The ego-vehicle is connected to every vehicle by a line whose width is proportional to the corresponding attention weight. The green head is only watching the vehicles coming from the left, while the blue head restricts itself to vehicles in the front and right directions.. Source.

Authors: Leurent, E., & Mercat, J.

Question: About the MDP state (or representation of driving scene): how surrounding vehicles can be represented?
Motivations / requirements:
- 1- Deal with a varying number of surrounding vehicles (problematic with function approximation which often expects constant-sized inputs).
- 2- The driving policy should be permutation-invariant (invariant to the ordering chosen to describe them).
- 3- Stay accurate and compact.
Current approaches:
- List of features representation (fails at 1 and 2).
  - Zero-padding can help for varying-size inputs.
- Spatial grid representation (suffers from accuracy-size trade-off).
One concept: "Multi-head social attention mechanism".
- The state may contain many types of information. But the agent should only pay attention to vehicles that are close or conflict with the planned route.
  - "Out of a complex scene description, the model should be able to filter information and consider only what is relevant for decision."
- About "attention mechanism":
  - "The attention architecture was introduced to enable neural networks to discover interdependencies within a variable number of inputs".
  - For each head, a stochastic matrix called the attention matrix is derived.
  - The visualisation of this attention matrix brings interpretability.

"End-to-End Model-Free Reinforcement Learning for Urban Driving using Implicit Affordances"

[ 2019 ] [📝] [🎞️] [ 🎓 Mines ParisTech ] [ 🚗 Valeo ]
[ affordance learning, CARLA ]

Click to expand

An encoder is trained to predict high-level information (called affordances). The RL agent does not use directly them as input state but rather one layer before (hence implicit affordances). This compact representation offers benefits for interpretability, and for training efficiency (lighter to save in the replay buffer). A command {follow lane, turn left/right/straight, change lane left/right} for direction at intersection and lane changes is passed to the agent via a conditional branch. Source.


Augmentation is needed for robustness and generalization (to address the distribution mismatch - also in `IL`) (left). Here, the camera is moved around the autopilot. One main finding is the benefit of using adaptive target speed in the reward function (right). Source.

Authors: Toromanoff, M., Wirbel, E., & Moutarde, F.

One quote:
- "A promising way to solve both the data efficiency (particularly for DRL) and the black box problem is to use privileged information as auxiliary losses, also coined "affordances" in some recent papers."
One idea: The state of the RL agent are affordance predictions produced by a separated network.
- 1- An encoder in trained in a supervised way to predict high-level information, using a stack of images as input.
  - Two main losses for this supervised phase:
    - 1- Traffic light state (binary classification).
    - 2- semantic segmentation.
  - Infordances inclues:
    - Semantic segmentation maps (ground-truth available directly in CARLA).
    - Distance and state of the incoming traffic light.
    - Whether the agent is at an intersection or not.
    - Distance from the middle of the lane.
    - Relative rotation to the road.
  - Why "implicit" affordance?
    - "We coined this scheme as "implicit affordances" because the RL agent implicit affordances because the RL agent do not use the explicit predictions but have only access to the implicit features (i.e the features from which our initial supervised network predicts the explicit affordances)."
    - Hence multiple affordances are predicted in order to help the supervised training (similar to auxiliary learning).
  - This feature extractor is frozen while training the RL agent.
- 2- These predicted affordances serve as input to a model-free RL agent.
  - The dueling network of the Rainbow- IQN Ape-X architecture was removed (very big in size and no clear improvement).
  - Training is distributed:
    - "CARLA is too slow for RL and cannot generate enough data if only one instance is used."
  - Despite being limited to discrete actions (4 for throttle/brake and 9-24 for steering), it shows very good results.
    - "We also use a really simple yet effective trick: we can reach more fine-grained discrete actions by using a bagging of multiple [here 3 consecutive] predictions and average them."
  - Frequency of decision is not mentioned.
- Benefits of this architecture:
  - Affordances features are way lighter compared to images, which enables the use of a replay buffer (off-policy).
    - All the more, since the images are bigger than usual (4 288x288 frames are concatenated, compared to "classical" single 84x84 frames) in order to capture the state of traffic lights.
  - This decomposition also brings some interpretable feedback on how the decision was taken (affordances could also be used as input of a rule-based controller).
- This 2-stage approach reminds me the concept "learn to see" / "learn to act" concept of (Chen et al. 2019) in "Learning by Cheating".
  - "As expected, these experiments prove that training a large network using only RL signal is hard".
About "model-free" RL.
- As opposed to "model-based", e.g. (Pan et al. 2019) where a network is also trained to predict high-level information.
- But these affordances rather relate to the transition model, such as probability of collision or being off-road in the near futures from a sequence of observations and actions.
About the reward function:
- It relies on 3 components:
  - 1- Desired lateral position.
    - To stay in the middle of the lane.
  - 2- Desired rotation.
    - To prevent oscillations near the center of lane.
  - 3- Desired speed, which is adaptive.
    - "When the agent arrives near a red traffic light, the desired speed goes linearly to 0 (the closest the agent is from the traffic light), and goes back to maximum allowed speed when it turns green. The same principle is used when arriving behind an obstacle, pedestrian, bicycle or vehicle."
    - The authors find that without this adaptation, the agent fails totally at braking for both cases of red traffic light or pedestrian crossing.

"Hierarchical Reinforcement Learning Method for Autonomous Vehicle Behavior Planning"

[ 2019 ] [📝] [ 🎓 Carnegie Mellon ] [ 🚗 General Motors ]
[ Hierarchical RL ]

Click to expand


`Driving` is treated as a task with multiple sub-goals. A `meta` controller choose an option among `STOP AT STOP-LINE`, `FOLLOW FRONT VEHICLE`. Based on this sub-goal, a `low-level` controller decides the `THROTTLE` and `BRAKE` action. Source.


The `state`* is shared across different hierarchical layers (`option` and `action`). An attention model extracts the relevant information for the current `sub-goal`. Source.*

Authors: Qiao, Z., Tyree, Z., Mudalige, P., Schneider, J., & Dolan, J. M.

Motivations:
- 1- Have an unique learning-based method (RL) for behavioural decision-making in an environment where the agent must pursue multiple sub-goals.
- 2- Improve convergence speed and sampling efficiency of traditional RL approaches.
- These are achieved with hierarchical RL, by reusing the different learnt policies across similar tasks.
About the hierarchical structure: concepts of "Actions" and "Options".
- 1- First, a high-level option, seen as a sub-goal, is selected by some "meta-controller", depending on the scenario:
  - Option_1 = STOP AT STOP-LINE.
  - Option_2 = FOLLOW FRONT VEHICLE.
- 2- Based on the selected sub-goal, an action network generates low-level longitudinal "actions" about throttle and brake.
  - The continues until a next sub-goal is generated by the meta controller.
About the state:
- The authors decide to share one state set for the whole hierarchical structure.
- For each sub-goal, only relevant information is extracted using an attention mechanism:
- "An attention model is applied to define the importance of each state element I(state, option) with respect to each hierarchical level and sub-goal".
- For instance:
  - "When the ego car is approaching the front vehicle, the attention is mainly focused on dfc/dfs." (1 - distance_to_the_front_vehicle / safety_distance)
  - "When the front vehicle leaves without stopping at the stop-line, the ego car transfers more and more attentions to ddc/dds during the process of approaching the stop-line." (1 - distance_to_stop_sign / safety_distance)
About the reward function:
- The authors call that "hybrid reward mechanism":
  - An "extrinsic meta reward" for the option-level.
  - An "intrinsic reward" for the action-level, conditioned on the selected option.
- For instance, the terms related to the target_speed in the intrinsic reward will be adapted depending if the meta controller defines a highway driving or an urban driving sub-goal.
- At first sight, this reward function embeds a lot of parameters.
  - One could say that the effort parameter engineering in rule-based approaches has been transferred to the reward-shaping.
One idea: "Hierarchical Prioritized Experience Replay" (HPER).
- When updating the weights of the two networks, transitions {s, o, a, ro, ra, s'} are considered.
- But "If the output of the option-value network o is chosen wrongly, then the success or failure of the corresponding action-value network is inconsequential to the current transition."
- In other words, for a fair evaluation of the action, it would be nice to have the correct option.
- One solution consists in setting priorities when sampling from the replay buffer:
  - In the option-level, priorities are based on the error directly (the TD-error for the optionNet, similar to the traditional PER).
  - In the lower level, priorities are based on the difference between errors coming from two levels, i.e. the difference between (TD-error for the actionNet) and (TD-error for the optionNet).
About the simulator: VIRES VTD.

"DeepRacer: Educational Autonomous Racing Platform for Experimentation with Sim2Real Reinforcement Learning"

[ 2019 ] [📝] [] [🎞️] [ 🚗 Amazon ]
[ sim2real, end-to-end, sampling efficiency, distributed RL ]

Click to expand


`real2sim` experiment for `model-free RL` `end-to-end` (from monocular camera* to low-level controls) track following using a `1/18th` scale car. Source.*


Comparison of tools to perform `RL` `sim2real`* applications. Source.*

Authors: Balaji, B., Mallya, S., Genc, S., Gupta, S., Dirac, L., Khare, V., Roy, G., Sun, T., Tao, Y., Townsend, B., Calleja, E., Muralidhara, S. & Karuppasamy, D.

What?
- "DeepRacer is an experimentation and educational platform for sim2real RL."
About the POMDP and the algorithm:
- Observation: 160x120 grayscale front view.
- Action: 10 discretized values: 2 levels for throttle and 5 for steering.
- Timestep: One action per observation. At 15 fps (average speed 1.6 m/s).
- model-free (that is important) PPO to learn the policy.
Tools:
- Amazon RoboMaker: An extension of the ROS framework with cloud services, to develop and test deploy this robot software.
- Amazon SageMaker: The Amazon cloud platform to train and deploy machine learning models at scale using the Jupyter Notebook.
- Amazon S3 (Simple Storage Service) to save and store the neural network models.
- [OpenAI Gym] interface between the agent and the env.
- Intel Coach RL framework for easy experimentation.
- Intel OpenVINO, a toolkit for quick development of vision-based applications, to convert the Tensorflow models to an optimized binary.
- Gazebo robotics simulator.
- ROS for the communication between the agent and the simulation.
- ODE (Open Dynamics Engine) to simulate the laws of physics using the robot model.
- Ogre as graphics rendering engine.
- Redis, an in-memory database, as a buffer to store the experience tuples <obs, action, reward, next_obs>.
About sim2real:
- The policy is learnt (+tested) from simulations ("replica track") and then transferred (+tested) to real world.
- "The entire process from training a policy to testing in the real car takes < 30 minutes."
- The authors mention several approaches for sim2real:
  - Mixing sim and real experiences during training:
    - E.g. learn features from a combination of simulation and real data.
    - Mix expert demonstrations with simulations.
  - Model-based dynamics transfer:
    - Assess simulation bias.
    - Learn model ensembles.
    - Perform calibration: here, they have to match the robot model to the measured dimensions of the car.
  - Domain randomization: simulation parameters are perturbed during training to gain robustness.
    - Add adversarial noise.
    - Observation noise, such as random colouring.
    - Action noise (up to 10% uniform random noise to steering and throttle).
    - Add "reverse direction of travel" each episode (I did not understand).
  - Privileged learning:
    - E.g. learn semantic segmentation at the same time, as an auxiliary task.
- They note that (I thought they were using grayscale?):
  - "Random colour was the most effective method for sim2real transfer."
About the distributed rollout mechanism, to improve the sampling efficiency:
- "We introduce a training mechanism that decouples RL policy updates with the rollouts, which enables independent scaling of the simulation cluster."
- As I understand, multiple simulations are run in parallel, with the same dynamics model.
- Each worker sends its experience tuples (not the gradient) to the central Redis buffer where updates of the shared model are performed.

"Autonomous Driving using Safe Reinforcement Learning by Incorporating a Regret-based Human Lane-Changing Decision Model"

[ 2019 ] [📝] [ 🎓 Michigan State University, Clemson University ]
[ safe RL, action masking, motivational decision model, regret theory, CARLA ]

Click to expand


Every time after the `RL` agent chooses an action, the supervisor checks whether this action is `safe` or not. It does that by predicting the trajectories of other cars using a regret decision model. Source.


The two actions (`keep lane` and `change lane`) have different probabilities of occurrence, and different harms (costs) (`I`) that can be formulated as utilities (`II`), expressed with physical variables (`III`). Affordance indicators are used in the world representation. Source.

Authors: Chen, D., Jiang, L., Wang, Y., & Li, Z.

Main motivation for safeRL for two-lane highway scenarios:
- "We found that when using conventional RL, about 14.5% training epochs ended with collisions. [...] Collisions cause training unstable as the RL algorithm needs to reset the simulation whenever collisions happen."
One idea: A "safety supervisor" evaluates the consequences of an ego-action and decides whether an it is "safe" using a prediction model.
- "What is predicted?"
  - The lane-changing decisions of other cars.
- "How?"
  - The approach belongs to the motivational methods, hence explainable (as opposed to the data-driven methods).
  - A "regret decision model" is developed:
    - "Risks in driving have two dimensions: harm (=costs) and probabilities."
    - A probability (relating to the time-to-collision), a weighting effect function (non-linear function of the probability), a cost function, a utility (cost normalized by collision cost) and a regret (non-linear function of the utility) are defined.
    - The (deterministic) decision is based on the resulting the net advantage (~weighted regret) of option C (change) over option K (keep lane).
- "Can we draw a parallel with how human drivers reason?"
  - "Not all the drivers have experienced collisions, but all of them can perceive the threat of a potential collision. It is plausible to assume that the surrounding driver perceives the threat of the approaching vehicle to be proportional to its kinematic energy." Hence somehow proportional to speed².
- "What does safe means here?"
  - Short-horizon rollouts are performed.
    - 1- The ego trajectory is anticipated based on the action candidate.
    - 2- Trajectories are obtained for other all cars using the predictor and their current speeds.
  - The action is considered safe if the ego agent stays on the road and if his trajectories does not intersect (in spatial-temporal domain) with any other trajectory.
- What if the action is deemed unsafe?
  - 1- The supervisor replaces the identified risky action with a safe option (probably based on the model).
    - This avoids terminating the episode and starting a new one during training and obviously it improves safety during testing.
  - 2- An experience tuple <state, action, r_collision, * (no next-action)> is created and added to the experience replay buffer (together with safe tuples). This will be sampled during the update phase.
    - It reminds me the idea of another work by the authors ("Deep Q-Learning with Dynamically-Learned Safety Module : A Case Study in Autonomous Driving" - detailed below) where two distinct buffers (safe and collision) are maintained and equally sampled.
- "What values for the model parameters?"
  - Ideally, these values should be inferred for each observed car (they are parts of the hidden state). For instance, within a POMDP formulation, using a belief tracker.
  - Here, a single set of parameters is derived via max-likelihood based on human demonstration.
- "What is the impact on training?"
  - It is faster and more stable (almost constant improving rate) compared to conventional RL.
Another idea: hierarchical decision-making:
- The RL agent first selects "manoeuvre-like" decisions among {decelerating, cruising, accelerating} and {lane change, lane keeping}.
- Its decision is then implemented by a low-level controller (PID).
  - "This hierarchical design greatly reduces the training time compared to methods using agents to output control signals directly."
One related previous work (about regret decision model):
- "A human-computer interface design for quantitative measure of regret theory" - (Jiang L. & Wang Y., 2019).

"Learning Resilient Behaviors for Navigation Under Uncertainty Environments"

[ 2019 ] [📝] [🎞️] [🎞️] [ 🎓 Fuzhou University, University of Maryland, University of Hong Kong ] [ 🚗 Baidu ]
[ uncertainty estimation, uncertainty-aware policy, SAC ]

Click to expand


The confidence in the prediction* (left) is used as an `uncertainty` estimate. This estimate impacts the decision (`μ` = `mean` of the steering action distribution) of the agent. Source.*


The `variance` of the steering action distribution (`behavioural uncertainty`) is not estimated by the agent itself, but rather built by a simple mapping-function from the `environmental uncertainty` estimated by the prediction module. Source.

Authors: Fan, T., Long, P., Liu, W., Pan, J., Yang, R., & Manocha, D.

One motivation: derive an uncertainty-aware + model-free + RL policy.
- "Uncertainty-aware":
  - The core idea is to forward the observation uncertainty to the behaviour uncertainty (i.e. uncertainty of the action distribution), in order to boost exploration during training.
- Model-free as opposed to model-based RL methods.
  - In model-based methods, the collision probability and uncertainty prediction can [explicitly] be formulated into the risk term for an MPC to minimize.
  - Here, the action selection is directly output by a net, based on raw laser data (i.e. not from some MPC that would require challenging parameter tuning).
- RL as opposed to IL (where uncertainty estimation has already been applied, by recycling techniques of supervised learning).
  - Besides, in IL, it is difficult to learn policies to actively avoid uncertain regions.
The proposed framework consists in 3 parts:
- 1- A prediction net:
  - Inputs: A sequence of laser scans and velocity data.
  - Outputs:
    - 1-1. The motion prediction of surrounding environments (this seems to be trajectories).
    - 1-2. Some associated uncertainty: "we expect to measure the environmental uncertainty by computing the confidence of the prediction results."
  - It is trained with supervised learning with recorded trajectories. The loss function therefore discourages large uncertainty while minimizing prediction errors.
- 2- A policy net:
  - Inputs:
    - 2-1. The two estimates (uncertainty and motion information) of the predictor.
    - 2-2. The current laser scan, the current velocity and the relative goal position.
  - Outputs: The mean* of the action distribution (steering).
  - It is trained with model-free RL, using the ROS-based Stage simulator.
- 3- A parallel NON-LEARNABLE mapping function estimates the variance of that action distribution directly from the environmental uncertainty.
  - Input: The "predicted" environmental uncertainty. (I would have called it observation uncertainty).
  - Output: The variance of the action distribution (steering).
  - About the mapping: The uncertainty predictions are weighted according to the distance of each laser point:
    - "We consider that the closer the laser point, the higher the impact on the action."
  - Again, the variance of the action distribution is not learnt!
    - It reminds me the work of (Galias, Jakubowski, Michalewski, Osinski, & Ziecina, 2019) where best results are achieved when the policy outputs both the mean and variance.
    - Here, the policy network should learn to adaptively generate the mean value according to the variance value (capturing the environment uncertainty), e.g. exhibit more conservative behaviours in the case of high environmental uncertainty.
About the RL method: SAC = Soft Actor-Critic.
- The above defined mapping forwards environmental uncertainties to the action variance.
  - The idea is then to encourage the agent to reduce this action variance (distribution entropy) in order to obtain some "uncertainty-averse" behaviour.
  - SAC is appropriate for that:
- "The key idea of SAC is to maximize the expected return and action entropy together instead of the expected return itself to balance the exploration and exploitation."
  - SAC trains a stochastic policy with entropy regularization, and explores in an on-policy way.
  - My interpretation:
    - 1- The agent is given (non-learnable) an action variance from the uncertainty mapping.
    - 2- This impacts its objective function.
    - 3- It will therefore try to decrease this uncertainty of action distribution and by doing so will try to minimize the environmental uncertainty.
    - 4- Hence more exploration during the training phase.
- Similar to the ε-greedy annealing process in DQNs, the temperature parameter is decayed during training to weight between the two objectives (entropy of policy distribution and expected return).
  - SAC incorporates the entropy measure of the policy into the reward to encourage exploration, i.e. the agent should act as randomly as possible [encourage uniform action probability] while it is still able to succeed at the task.
Bonus (not directly connected to their contributions): How to model uncertainties in DNNs?
- "The aleatoric uncertainty (data uncertainty) can be modelled by a specific loss function for the uncertainty term in the network output".
- "The epistemic uncertainty (i.e. model uncertainty) can be captured by the Monte-Carlo Dropout (MC-Dropout) technique" - dropout can be seen as a Bayesian approximation.

"Deep Q-Learning with Dynamically-Learned Safety Module : A Case Study in Autonomous Driving"

[ 2019 ] [📝] [ 🎓 University of Michigan and West Virginia University ] [ 🚗 Ford ]
[ DQN ]

Click to expand


The `affordance indicator` refers to the `MDP` `state`. It has length `20` and contains and represents the spatio-temporal information of the nearest traffic vehicles. The agent controls the discrete acceleration (`maintain`, `accelerate`, `brake`, and `hard brake`) and selects its lane (`keep lane`, `change to right`, and `change to left`). Source.


Two purposes: `1-` Accelerates the learning process without inhibiting meaningful exploration. `2-` Learn to avoid accident-prone states. Note that collisions still occur, but less often. Source.

Authors: Baheri, A., Nageshrao, S., Kolmanovsky, I., Girard, A., Tseng, E., & Filev, D.

Main motivation: guide the exploration process when learning a DQN-based driving policy by combining two safety modules (rule-based and learning-based):
- A handcrafted safety module.
  - A heuristic rule ensures a minimum relative gap to a traffic vehicle.
- A dynamically-learned safety module.
  - 1- It first predicts future states within a finite horizon.
  - 2- And then determines if one of the future sates violates the handcrafted safety rule.
One idea for DQN: Two buffers are used to store experience (off-policy learning). They are equally sampled in the update stage.
- 1- Safe buffer: As in classic DQN.
- 2- Collision buffer: The TD-target only consists in the observed reward (i.e. the s is a terminal state).
  - An experience tuple ends in this collision buffer:
    - 1- If the handcrafted safety module considers the action is not safe or if the action leads to a static collision in the next observed state.
    - 2- If the dynamically-learned safety module detects a dynamic collision for any future predicted states. In that case, an additional negative term is assigned to the reward.
- Although the sampling is uniform in each buffer, the use of two buffer can been seen as some variant of prioritized experience replay (PER) where the sampling is biased to expose the agent to critical situations.
- Contrary to action masking, the "bad behaviours" are not discarded (half the batch is sampled from the collision buffer).
  - The agent must therefore generalize the states that could lead to undesirable states.
Relation to model-based RL:
- The transition function of the MDP is explicitly learnt by the RNN (mapping <a, s> to s').
- The dynamically-learned safety module incorporates a model lookahead. But prediction is not used for explicit planning.
- Instead, it determines whether the future states lead to undesirable behaviours and consequently adapts on-the-fly the reward function.
Related works:
- "Autonomous Highway Driving using Deep Reinforcement Learning" - (Nageshrao, Tseng, & Filev, 2019).
  - Basically, the same approach is presented.
  - "Training a standard DDQN agent without explicit safety check could not learn a decent policy and always resulted in collision. [...] Even with continuous adaptation the mean number safety trigger never converges to zero."
  - Since the NN function approximation can potentially chose a non-safe action, the agent should be augmented with some checker that detects safety violation.
  - The authors introduce the idea of a "short horizon safety checker".
  - If the original DDQN action choice is deemed unsafe, then the safety check replaces it with an alternate "safe" action, in this case relying on an IDM controller.
  - This technic enhances learning efficiency and without inhibiting meaningful exploration
- For additional contents on "Safe-RL in the context of autonomous vehicles", the first author @AliBaheri has written this github project.

"Simulation-based reinforcement learning for autonomous driving"

[ 2019 ] [📝] [🎞️] [🎞️] [ 🎓 Universities of Warsaw and Jagiellonian ] [ 🚗 deepsense.ai ]
[ sim-to-real, end-to-end, CARLA ]

Click to expand


Neural architecture of the policy function trained with `PPO`: the `RGB` image is concatenated with its semantic segmentation. Randomisation is performed to prevent over-fitting and increase sampling-efficiency. It is also worth mentioning the high-level navigation command that is provided to guide the agent when approaching intersections. Source.

Several option for producing the std of the steering distribution. Best results are achieved when the policy outputs both mean and std. The left screenshot illustrates that shaped rewards (as opposed to sparse rewards where rewards are only five at the goal state) can bias learning and lead to un-intended behaviours: to make the agent stay close to the centre line, the authors originally penalized the gap in X, Y but also Z coordinates. ''Due to technical reasons our list of lane-centre positions was actually placed above the road in the Z axis. This resulted in a policy that drives with two right side wheels placed on a high curb, so its elevation is increased and distance to the centre-line point above the ground is decreased''. Source-1 Source-2.

Authors: Galias, C., Jakubowski, A., Michalewski, H., Osinski, B., & Ziecina, P.

One goal: learn the continuous steering control of the car to stay on its lane (no consideration of traffic rules) in an end-to-end fashion using RL.
- The throttle is controlled by a PID controller with constant speed target.
- In addition, the simulated environment is static, without any moving cars or pedestrians.
- The authors want to test how good a policy learnt in simulation can transfer to real-world. This is sometimes called sim-to-real.
  - For this second motivation, this reminds me the impressive Wayve's work "Simulation Training, Real Driving".
How to model the standard deviation parameter of the continuous steering action distribution?
- It could be set it to a constant value or treated as an external learnable variable (detached from the policy).
- But the authors found that letting the policy control it, as for the mean, gave the best results.
  - It allows the policy to adjust the degree of exploration on a per-observation basis.
  - "An important implementation detail was to enforce an upper boundary for the standard deviation. Without such a boundary the standard deviation would sometime explode and never go down below a certain point (the entropy of the policy climbs up), performing poorly when deployed on real-world cars."
An interesting variant: end-to-mid, i.e. do not directly predict raw control commands.
- In another work, the task was not to directly predict the CARLA steering command, but rather some target waypoint on the road, and "outsource" the steering control task to some external controller.
- "Given a target waypoint, low-level steering of the driving wheel is executed in order to reach this point. In simulation, it is realized by a PID controller while in the case of the real car, we use a proprietary control system. The action space is discrete - potential waypoints are located every 5 degrees between −30 and 30, where 0 is the current orientation of the vehicle."
Two ideas: The use of 3 sources of observations. And the inclusion of segmentation mask to the state (input) of the RL net:
- 1- A RGB image.
  - It is concatenated by its semantic segmentation: it passes through a previous-learnt segmentation network.
  - This can be thought as an intermediate human-designed or learned representations of the real world.
  - As said, the seg-net has been learnt before with supervised learning.
    - But it could also be (further) trained online, at the same time as the policy, leading to the promising concept of auxiliary learning.
    - This has been done for instance by (Tan, Xu, & Kong, 2018), where a framework of RL with image semantic segmentation network is developped to make the whole model adaptable to reality.
- 2- A high-level navigation command to guide the agent when approaching an intersection.
  - In {LANE FOLLOW, GO STRAIGHT, TURN RIGHT, TURN LEFT}.
  - This is inspired by (Codevilla et al., 2017).
- 3- The current speed and acceleration.
- The authors tried to also include the information about the last action.
  - Without success: the car was rapidly switching between extreme left and extreme right.
  - In other words, the steering was controlling in a pulse width modulation-like manner.
One idea to promote generalisation and robustness: Randomisation.
- It should also prevent overfitting and improve sample efficiency.
- Some randomizations are applied to visual camera input.
  - An additional loss term is introduce to check if the policy outputs a similar distribution for the perturbed and unperturbed images.
- Some perturbations are also applied to the car dynamics.
One idea for debug: generate saliency map.
- The idea is to find which region of the image has the most impact in the prediction of the steering command.
- This can be done by blurring different patches of the input image, i.e. removing information from that patch, and measuring the output difference.
Some good ideas mentioned for future works:
- To improve the driving stability, try to focus the training on fragments of scenarios with the highest uncertainty. c.f. concepts of Bayesian RL.
- Go to mid-to-end, using an intermediate representation layer, for example a 2D-map or a bird's-eye view. e.g. ChauffeurNet - also detailed on this page.
- To further improve the sampling efficiency, model-based methods could be integrated. c.f. "Wayve Simulation Training, Real Driving"

"Dynamic Interaction-Aware Scene Understanding Reinforcement Learning in Autonomous Driving"

[ 2019 ] [📝] [ 🎓 University of Freiburg ] [ 🚗 BMW ]
[ feature engineering, graph neural networks, interaction-aware networks, SUMO ]

Click to expand

Some figures:

In the proposed DeepSet approach, embeddings are first created depending on the object type (using φ1 for vehicles and φ2 for lanes), forming the encoded scene. They are 'merged' only in a second stage to create a fixed vector representation. Deep Set can be extended with Graph Convolutional Networks when combining the set of node features to capture the relations - interaction - between vehicles. Source.

Authors: Huegle, M., Kalweit, B., Werling, M., & Boedecker, J.

Two motivations:
- 1- Deal with an arbitrary number of objects or lanes.
  - The authors acknowledge that a fix-size state will be enough for scenarios like highways driving where interactions with the direct neighbours of the agent are most important.
    - But they also note that a variable-length list can be very important in certain situations such as urban driving.
  - To deal with the variable-length dynamic input set X-dyn, there encodings are just summed.
    - This makes the Q-function permutation invariant w.r.t. the order of the dynamic input and independent of its size.
- 2- Model interactions between objects in the scene representations.
  - The structure of Graph Convolutional Networks (GCN) is used for that. All node features are combined by the sum.
  - "Graph Networks are a class of neural networks that can learn functions on graphs as input and can reason about how objects in complex systems interact."
Baselines:
- "Graph-Q is compared to two other interaction-aware Q-learning algorithms, that use input modules originally proposed for supervised vehicle trajectory prediction."
  - Convolutional Social Pooling (SocialCNN) is using a grid-map: "a social tensor is created by learning latent vectors of all cars by an encoder network and projecting them to a grid map in order to learn spatial dependencies".
  - Vehicle Behaviour Interaction Networks (VBIN) imposes working with a fixed number of cars since the embedding vectors are just concatenated, i.e. not summarizing as in the Deep Sets approach.
- A built-in SUMO rule-based controller is also used for comparison.
Previous works:
- Dynamic input for deep reinforcement learning in autonomous driving - detailed below.
  - Introducing the idea of Deep Set.
- High-level Decision Making for Safe and Reasonable Autonomous Lane Changing using Reinforcement Learning - detailed below.
  - How to ensure safety when working with a DQN?
  - The concept of action masking is applied, i.e. the technique of not exposing to the agent dangerous or non-applicable actions during the action selection.

"Driving in Dense Traffic with Model-Free Reinforcement Learning"

[ 2019 ] [📝] [ simulator ] [ MPC ] [ ML ] [🎞️] [ 🎓 Berkeley & Carnegie Mellon ] [ 🚗 Honda ]
[ SISL, PPO, MPC, merging scenarios ]

Click to expand

Some figures:


Source.


The occupancy-grid-like observation space is divided into `4` channels, each containing `3` lanes. An ego-vehicle specific feature vector is also considered. The authors use policy-gradient Proximal Policy Optimisation - `PPO` - method and decided not to share parameters between the actor and the critic. Source.


In another work, the authors try to incorporate an `RNN` as a prediction model into an `MPC` controller, leading to a reliable, interpretable, and tunable framework which also contains a data-driven model that captures interactive motions between drivers. Source.

Authors: Saxena, D. M., Bae, S., Nakhaei, A., Fujimura, K., & Likhachev, M.

One motivation: learn to perform comfortable merge into dense traffic using model-free RL.
- Dense traffic situations are difficult: traditional rule-based models fail entirely.
  - One reason is that in heavy traffic situations vehicles cannot merge into a lane without cooperating with other drivers.
- Model-free means it does not rely on driver models of other vehicles, or even on predictions about their motions. No explicit model of inter-vehicle interactions is therefore needed.
- Model-free also means that natively, safety cannot be guaranteed. Some masking mechanisms (called "overseer") are contemplated for future work.
One idea for merging scenarios:
- Many other works "only accommodate a fixed merge point as opposed to the more realistic case of a finite distance for the task (lane change or merge) as in our work."
One idea to adapt IDM to dense scenarios:
- "IDM is modified to include a stop-and-go behaviour that cycles between a non-zero and zero desired velocity in regular time intervals. This behaviour is intended to simulate real-world driving behaviours seen in heavy-traffic during rush-hour."
One idea about action space:
- "Learning a policy over the acceleration and steering angle of a vehicle might lead to jerky or oscillatory behaviour which is undesirable. Instead, we train our network to predict the time derivatives of these quantities, i.e. jerk j and steering rate δ˙. This helps us maintain a smooth signal over the true low-level control variables."
- The policy for jerk and steering rate is parameterised as Beta distributions (allegedly "this makes training more stable as the policy gradients are unbiased with respect to the finite support of β-distributions").
One of their related works used as baseline:
- "Cooperation-Aware Lane Change Maneuver in Dense Traffic based on Model Predictive Control with Recurrent Neural Network" from (Bae et al., 2019).
- "A RNN generates predictions for the motions of neighbouring vehicles based on a history of their observations. These predictions are then used to create safety constraints for an MPC optimisation."
  - 1- For the prediction part:
    - A Social-GAN (socially acceptable trajectories) is used to predict the other vehicles’ interactive motions that are reactive to the ego vehicle’s actions.
    - Other prediction modules are tested, such as a constant velocity model.
    - "When all drivers are cooperative, all three prediction models can lead to successful lane change. That is, the imprecision of predictions on drivers’ interactive motions is not critical when the drivers are very cooperative, since the drivers easily submit space to other vehicles, even with rough control inputs resulting from inaccurate motion predictions. This, however, is no longer valid if the drivers are aggressive."
  - 2- For the MPC part:
    - A heuristic algorithm based on Monte Carlo simulation along with a roll-out is used to deal with the non-convexity of the problem (some constraints are non-linear).
    - To reduce the search complexity during the sampling phase, the action space is adapted. For instance only steering angles to the left are considered when turning left.
- "This SGAN-enabled controller out-performs the learning-based controller in the success rate, (arguably) safety as measured by minimum distances, and reliability as measured by variances of performance metrics, while taking more time to merge".

"Behavior Planning at Roundabouts"

[ 2019 ] [📝] [ 🎓 Carnegie Mellon ]
[ POMDP, SUMO, generalization ]

Click to expand

Some figures:


Using recurrent units in a `DQN` and considering the action history. Source.


`Hidden modes` framework: decomposing the non-stationary* environment into multiple stationary environments, where each `mode` is an `MDP` with distinct dynamics. Source.*

Author: Khurana, A.

One idea: using recurrent nets (hence DRDQN) to integrate history in order to cope with partial observability.
- Two LSTM layers, (considering 15 past observations) was added after DQN layers.
- They also try to include the action history of the agent, leading to ADRQN. But this does not modify the score.
Another idea: several context-based action spaces:
- Decision in roundabouts consists in:
  - Merging - action_space = {go, no go}
  - Traversal - action_space = {acc, decelerate, change-left, change-right , cv}
  - Exit - action_space = {acc, decelerate, change-left, change-right , cv}
- Justification for using discrete action space: behavioural planning happens on a slower time scale than motion planning or trajectory control.
- This reminds me some works on hierarchical RL (e.g. Options framework).
Another idea: Curriculum learning
- "Each model is first trained without any other interacting vehicle so that it learns the most optimal policy and later with other vehicles with random initialization. In later stages, an additional bonus reward is given to merging and traversal if they lead to successful exit to enable long-term consistent behaviours."
One note about the POMDP formulation:
- "This also enables us to integrate planning and prediction into a single problem, as the agent learns to reason about its future."
- I am a little bit confused by their formulation of the POMDP.
  - I would have expected some hidden parameters to be defined and some belief on them to be tracked, as often done for AD, e.g. the intention of other participants.
  - Instead, the partial observability refers here to the uncertainty in perception: "there is a 0.2 probability that a car present in the agent’s perception field is dropped in the observation".
  - This imperfect state estimation encourages the robustness.
One note about model-free RL:
- Using RL seems relevant to offer generalization in complex scenarios.
- But as noted by the authors: "the rule-based planner outperforms others in the case of a single-lane roundabout as there’s no scope for lane change."_
One addressed problem: "non-stationary" environments.
- A single policy learned on a specific traffic density may perform badly on another density (the dynamic of the world modelled by the MDP changes over time).
- The goal is to generalize across different traffic scenarios, especially across different traffic densities.
- One idea is to decompose the non-stationary environment into multiple stationary environments, where each mode is an MDP with distinct dynamics: this method is called Hidden modes.
  - How to then switch between modes? The authors proposed to use external information (Google Maps could for instance tell ahead if traffic jams occur on your planned route).
  - But as the number of discrete modes increases, the hidden-mode method suffers from oscillations at the boundaries of the mode transitions.
- Thus the second idea is to consider one single model: this method is called Traffic-Conditioned.
  - Some continuously varying feature (ratio of velocity of other vehicles to target speed) is used. It should be representative of the non-stationary environment.
- One quote about the relation of hidden-mode formulation to hierarchical RL:
  - "For generalization, the hidden-mode formulation can also be viewed as a hierarchical learning problem where one MDP/POMDP framework selects the mode while the other learns the driving behaviour given the mode".

"Reinforcement Learning Approach to Design Practical Adaptive Control for a Small-Scale Intelligent Vehicle"

[ 2019 ] [📝] [ 🎓 Chongqing University ]
[ non-deep RL, online learning, model-based RL ]

Click to expand

Authors: Hu, B., Li, J., Yang, J., Bai, H., Li, S., & Sun, Y.

What I like in this paper:
- The authors make sure they understand tabular RL methods, e.g. the difference between on-policy SARSA(1), SARSA(λ) and off-policy Q-learning, before going to deep RL.
  - "Compared with Q-learning, which can be described as greedy, bold, and brave, SARSA(1) is a conservative algorithm that is sensitive to control errors."
- They include a model-based algorithm (Dyna-Q) in their study. This seems promising when training directly in real world, where the sampling efficiency is crucial.
- They claim RL methods bring advantages compared to PID controllers in term of adaptability (generalization - i.e. some PID parameters appropriate for driving on a straight road may cause issues in sharp turns) and burden of parameter tuning.
- They consider the sampling efficiency (better for model-based) and computational time per step (better for 1-step TD methods than for SARSA(λ)).
  - "Q-learning algorithm has a poorer converging speed than the SARSA(λ) and Dyna-Q algorithm, it balances the performance between the converging speed, the final control behaviour, and the computational complexity."
- Obviously, this remains far from real and serious AD. But this paper gives a good example of application of basic RL methods.

"Learning When to Drive in Intersections by Combining Reinforcement Learning and Model Predictive Control"

[ 2019 ] [📝] [ 🎓 Chalmers University ] [ 🚗 Zenuity ]
[ model-free RL, MPC, Q-Masking, POMDP ]

Click to expand

Two figures:


Source Left - Source Right.


This work applies the path-velocity decomposition and focuses on the longitudinal control. Three intentions are considered: aggressive `take-way`, `cautious` (slows down without stopping), and passive `give-way`. Source.

Authors: Tram, T., Batkovic, I., Ali, M., & Sjöberg, J.

Main idea: hierarchy in learnt/optimized decision-making.
- A high-level decision module based on RL uses the feedback from the MPC controller in the reward function.
- The MPC controller is also responsible for handling the comfort of passengers in the car by generating a smooth acceleration profile.
Previous works:
- "Learning Negotiating Behavior Between Cars in Intersections using Deep Q-Learning" - (Tram, Batkovic, Ali, & Sjöberg, 2019)
- "Autonomous Driving in Crossings using Reinforcement Learning" - (Jansson & Grönberg, 2017)
- In particular they reused the concept of "actions as Short Term Goals (STG)". e.g. keep set speed or yield for crossing car instead of some numerical acceleration outputs.
  - This allows for comfort on actuation and safety to be tuned separately, reducing the policy selection to a classification problem.
  - The use of such abstracted / high-level decisions could be a first step toward hierarchical RL techniques (macro-action and option framework).
- Another contribution consists in replacing the Sliding Mode (SM) controller used previously by an MPC, allegedly to "achieve safer actuation by using constraints".
  - The intention of all agents is implemented with a SM controller with various target values.
I find it valuable to have details about the training phase (no all papers do that). In particular:
- The normalization of the input features in [-1, 1].
- The normalization of the reward term in [-2, 1].
- The use of equal weights for inputs that describe the state of interchangeable objects.
- Use of a LSTM, as an alternative to a DQN with stacked observations. (Findings from (Jansson & Grönberg, 2017)).
Additional notes:
- The main benefits of the combination seem to be about avoiding over conservative behaviours while improving the "sampling-efficient" of the model-free RL approach.
  - Such approach looks to be particularly relevant (in term of success rate and collision-to-timeout ratio [CTR]) for complex scenarios, e.g. 2-crossing scenarios.
  - For simple cases, the performance stays close to the baseline.
- The reliance (and the burden) on an appropriate parametrisation inherent to rule-based has not disappeared and the generalisation seems limited:
  - "Since MPC uses predefined models , e.g. vehicle models and other obstacle prediction models, the performance relies on their accuracy and assumptions ."
- The problem is formulated as a POMDP.
  - Honestly, from a first read, I did not find how belief tracking is performed. Maybe something related to the internal memory state of the LSTM cells?
- Sadly the simulator seems to be home-made, which makes reproducibility tricky.
One quote about Q-masking, i.e. the technique of not exposing to the agent dangerous or non-applicable actions during the action selection.
- "Q-masking helps the learning process by reducing the exploration space by disabling actions the agent does not need to explore."
- Hence the agent does not have to explore these options, while ensuring a certain level of safety (but this requires another rule-based module 😊 ).

"Cooperation-Aware Reinforcement Learning for Merging in Dense Traffic"

[ 2019 ] [📝] [] [ 🎓 Stanford ] [ 🚗 Honda ]
[ POMDP, offline RL, value-based RL, interaction-aware decision making, belief state planning ]

Click to expand

One figure:


Source.

Authors: Bouton, M., Nakhaei, A., Fujimura, K., & Kochenderfer, M. J.

One idea: offline belief state RL to solve dense merging scenarios modelled as a POMDP.
- A belief updater explicitly maintains a probability distribution over the driver cooperation levels of other cars.
Another idea: projection of the merging vehicle on the main lane. This reduces the problem to one dimension, allowing for IDM. Similar to the abstraction presented by R. Regele.
One term: "C-IDM": Cooperative Intelligent Driver Model.
- It builds on IDM to control the longitudinal acceleration of the other vehicle and adds a cooperation level c ∈ [0, 1].
- Non-cooperative vehicles (c=0) are blind to the merging ego car and follow the vanilla IDM, while cooperative vehicles will yield (c=1).
Another term: "Burn-in time"
- When creating a new configuration, vehicles and their parameters are drawn from distributions and then move according to driver models.
- The idea is between 10s and 20s before spawning the ego car, to allows the initial state to converge to a more realistic situation.
- It should help for generalization.
Another term: "Curriculum Learning": The idea is to train the agent by gradually increasing the difficulty of the problem. In this case the traffic density.
Two take-aways (similar to what I identified at IV19)

"Previous works has shown that only relying on deep RL is not sufficient to achieve safety. The deployment of those policies would require the addition of a safety mechanism."

"Using deep reinforcement learning policies to guide the search of a classical planner (MCTS) may be a promising direction."

"Interaction-aware Decision Making with Adaptive Behaviors under Merging Scenarios"

[ 2019 ] [📝] [🎞️] [ 🎓 Berkeley ] [ 🚗 Honda ]
[ multi agent RL, interaction-aware decision making, curriculum learning, action masking ]

Click to expand

One figure:


Note: the visibility of each agent is assumed to be `100m` in front and back, with `0.5m`/`cell` resolution, for both its current lane (`obs_cl`) and the other lane (`obs_ol`). Source.

Authors: Hu, Y., Nakhaei, A., Tomizuka, M., & Fujimura, K.

One term: "IDAS": interaction-aware decision making with adaptive strategies.
- The main goal is to generate manoeuvres which are safe but less conservative than rule-based approaches such as IDM and/or FSM.
- The idea is to learn how to negotiate with other drivers, or at least consider interactions in the decision process.
One idea: use multi-agent RL (MARL) to consider interactions between the multiple road entities.
- In particular, the agent receives rewards for its personal objective as well as for its contribution to the team’s "success" (multi-agent credit assignment).
One idea: a masking mechanism prevents the agent from exploring states that violate common sense of human judgment (c.f. RSS) and increase the learning efficiency.
- This idea of adding rule-based constraints to a RL policy has been applied in many works. Recently in Wang, J. et al. for instance where prediction is also considered.
- Here, masking is not only based on safety, but also on considers vehicle kinematics and traffic rules.
- A remaining question is where to apply the masking: either before the action selection (exposing only a subset of feasible actions), or after (penalizing the agent if it takes a forbidden action).
One quote (on the way to transfer to the real world):

"A motion control module will convert the discretized acceleration of the behaviour planner into continuous acceleration by applying algorithms like MPC at a higher frequency (100Hz)".

"Microscopic Traffic Simulation by Cooperative Multi-agent Deep Reinforcement Learning"

[ 2019 ] [📝] [ 🎓 University of Parma ] [ 🚗 VisLab ]
[ multi-agent A3C, off-policy learning ]

Click to expand

One figure:


Source.

Authors: Bacchiani, G., Molinari, D., & Patander, M.

One idea: "parallel actor-learners": to decorrelate the sequence of experiences used to update the policy network, a common approach is to sample <s, a, r, s'> tuples from a memory buffer (experience replay).
- Here, a multiple-agent setting is used instead: each agent acts in a different instance of the environment (hence diverse experiences) and sends its updates asynchronously to a central network.
Another idea: "hybrid state representation": coupling some grid-like representation (path to follow, obstacles, navigable space) with a vector of explicit features (e.g. target speed, distance to goal, elapsed time ratio).
- This combination offers a generic scene representation (i.e. independent of the number of vehicles) while allowing for tuning explicit goals (target speeds) in the state.
- Such hybrid representation seems popular, as identified at IV19).
Other ideas:
- Stacking the n=4 most recent views to capture the evolution of the scene (e.g. relative speeds).
- Action repeat technique for temporal abstraction to stabilize the learning process (c.f. "frame skip").
One concept: "Aggressiveness tuning". Together with the target speed, the elapsed time ratio (ETR) feature is used to tune the aggressiveness of the car:

"ETR Values close to 1 will induce the agent to drive faster, in order to avoid the predicted negative return for running out of time. Values close to 0 will tell the driver that it still has much time, and it is not a problem to yield to other vehicles."

"Dynamic Input for Deep Reinforcement Learning in Autonomous Driving"

[ 2019 ] [📝] [🎞️] [ 🎓 University of Freiburg ] [ 🚗 BMW ]
[ feature engineering, off-policy learning, DQN, SUMO ]

Click to expand

One diagram is better than 100 words:


By summing all the dynamic terms (one per surrounding vehicle), the input keeps a constant size. Source.

Authors: Huegle, M., Kalweit, G., Mirchevska, B., Werling, M., & Boedecker, J.

One goal: find a flexible and permutation-invariant representation to deal with variable sized inputs (variable number of surrounding vehicles) in high-level decision making in autonomous lane changes, using model-free RL.
- Four representations are considered:
  - Relational Grid (fixed-size vector)
  - Occupancy grid (fixed-size grid processed by some CNN)
  - Set2Set-Q (some RNN combined with an attention mechanism to create a set representation which is permutation invariant w.r.t. the input elements)
  - DeepSet-Q (proposed approach)
- They are used as inputs to train and evaluate DQN agents.
One finding:
- "Deep Sets were able to outperform CNN and recurrent attention approaches and demonstrated better generalization to unseen scenarios".
One quote: about the use of policy-based (as opposed to value-based), on-policy (as opposed to off-policy), model-free RL (here PPO).

"Due to the higher demand for training and the non-trivial application to autonomous driving tasks of on-policy algorithms, we switched to off-policy Q-learning."

"Seeking for Robustness in Reinforcement Learning : Application on Carla Simulator"

[ 2019 ] [📝] [ 🎓 Université de Strasbourg & ENIT Tunis ] [ 🚗 Segula ]
[ CARLA, A2C ]

Click to expand

Some background:


`n`-step TD learning. Source.

Authors: Jaâfra, Y., Laurent, J.-L., Deruyver, A., & Naceur, M. S.

One related work: reading this paper reminded me of one conclusion of the 2019 CARLA AD Challenge:

"Robust open-source AV stacks are not a commodity yet: No public AV stack has solved the challenge yet."

One idea: use an actor-critic architecture with multi-step returns (n-step A2C) to "achieve a better robustness".
- The introduction of a critic aims at reducing the variance of the gradient of policy-based methods.
- As illustrated in the above figure, in value-based methods, the TD-target of a critic can be computed in several ways:
  - With bootsrapped, using the current estimate for the next state s': 1-step TD - low variance but biased estimate ...
  - ... Up to considering all the steps in the trajectory until termination: Monte Carlo - high variance but unbiased estimate.
  - In between are multi-step returns critics (MSRC). Obviously a trade-off between bias/variance.
Some limitations:
- The MDP is not very detailed, making reproduction and comparison impossible.
  - For instance, the action space allegedly contains 3 discrete driving instructions [steering, throttle, and brake], but not concrete number is given.
  - The same applies to the state and reward function: no quantitative description.
  - Based on their text, I can assume the authors were participating to the 2019 CARLA AD challenge. Maybe track 1. But again, information about the town/scenario is missing.
- No real comparison is performed: it should be feasible to use the built-it rule-based agent present in CARLA as a baseline.
- Why not also supplying a video of the resulting agent?
One additional work: Meta-Reinforcement Learning for Adaptive Autonomous Driving, (Jaafra, Luc, Aline, & Mohamed, 2019) [pdf] [poster].
- This idea is to use multi-task learning to improve generalization capabilities for an AD controller.
- As detailed, "Meta-learning refers to learn-to-learn approaches that aim at training a model on a set of different but linked tasks and subsequently generalize to new cases using few additional examples".
- In other words, the goal is to find an optimal initialization of parameters, to then quickly adapt to a new task through a few standard gradient descents(few-shot generalization).
- A gradient-based meta-learner inspired from Model-Agnostic Meta-Learning (MAML - Finn et al., 2017) is used.
- RL performance in non-stationary environments and generalisation in AD are interesting topics. But no clear benefit is demonstrated, and the above limitations apply also here.

"High-level Decision Making for Safe and Reasonable Autonomous Lane Changing using Reinforcement Learning"

[ 2018 ] [📝] [ 🎓 University of Freiburg ] [ 🚗 BMW ]
[ Q-Masking, RSS ]

Click to expand

One figure:


Source.

Authors: Mirchevska, B., Pek, C., Werling, M., Althoff, M., & Boedecker, J.

It relates to the RSS and Q-masking principles.
- The learning-based algorithm (DQN) is combined with a rule-based checker to ensure that only safe actions are chosen at any time.
- A Safe Free Space is introduced.
  - For instance, the agent must keep a safe distance from other vehicles so that it can stop without colliding.
What if the Safe Free Space is empty?
- "If the action is considered safe, it is executed; if not, we take the second-best action. If that one is also unsafe, we stay in the current lane."
About the PELOPS simulator:
- It has been developed between [ 🚗 fka (ZF) ] and [ 🚗 BMW ].
- In future works (see above), they switch to an open source simulator: SUMO.

"Safe Reinforcement Learning on Autonomous Vehicles"

[ 2018 ] [📝] [ 🚗 Honda ]
[ action masking, risk assessment, reachability set, SUMO ]

Click to expand


The `prediction` module masks* undesired actions at each time step. Source.*


Here a related patent from the authors 🔒 😉. Source.

Authors: Isele, D., Nakhaei, A., Fujimura, K.

Previous works about learning control with DQNs at diverse intersections:
- "Navigating occluded intersections with autonomous vehicles using deep reinforcement learning" - (Isele et al., 2017)
- "Transferring Autonomous Driving Knowledge on Simulated and Real Intersections" - (Isele et al., 2017)
How to make model-free RL "safe"? Two options are mentioned (both required expert knowledge):
- 1- Modifying the reward function (requires careful tuning).
- 2- Constraining exploration (e.g. action masking, action shielding).
  - "The methods can completely forbid undesirable states and are usually accompanied by formal guarantees".
  - In addition, the learning efficiency can be increased (fewer states to explore).
- For additional contents on "Safe-RL in the context of autonomous vehicles", one could read this github project by @AliBaheri.
Here: The DQN is augmented with some action-masking mechanism.
- More precisely, a prediction model is used:
  - The predicted position of the ego car is compared against the predicted position of all other traffic cars (called forward predictions).
  - If an overlap of the regions is detected, the action is marked as unsafe.
  - Else, the agent is allowed to freely explore the safe state space, using traditional model-free RL techniques.
- Note: this illustrates the strong relation between prediction and risk assessment.
One challenge: Ensuring "safety" not just for the next step, but for the whole trajectory.
- "To ensure that the agent never takes an unsafe action, we must check not only that a given action will not cause the agent to transition to an unsafe state in the next time step, but also that the action will not force the agent into an unsafe state at some point in the future."
- "Note that this is closely related to the credit assignment problem, but the risk must be assigned prior to acting".
- This made me think of tree search techniques, where a path is explored until its terminal node.
- To cope with the exponential complexity, the authors proposed some approximation to restrict the exploration space.
  - One of them being the temporal abstraction for actions (see this video series for a quick introduction).
  - The idea of this so-called option or intentions framework, is to distinguish between low-level and high-level actions
    - "This can be thought of as selecting an open-loop high-level decision followed by subsequent bounded closed-loop low-level corrections."
    - For a given time horizon, the trajectories described with these options are way shorter, hence reducing the size of the state set that is to be checked.
  - This leads to the definition of a functional local-state (I did not understand all the details) including some variance term:
    - "The variance acts as a bound that encompasses the variety of low-level actions that produce similar high-level actions. Additionally, we will use the variance to create safety bounds".
One remark: similar to the "collision checker" for path planning, I can imagine that this prediction module becomes the computational bottleneck of the framework.

"Automating Vehicles by Deep Reinforcement Learning Using Task Separation with Hill Climbing"

[ 2017 ] [📝] [ 🎓 IMT Lucca ]
[ stochastic policy search, gradient-free RL, policy-gradient RL, reward shaping ]

Click to expand


Source.

Author: Plessen, M. G.

One remark: to be honest, I find this publication not very easy to understand. But it raises important questions. Here are some take-aways.
One term: (TSHC) = Task Separation with Hill Climbing
- Hill Climbing has nothing to do with the gym MountainCar env.
  - It rather refers to as a gradient-free optimization method: the parameters are updated based on greedy local search.
  - For several reasons, the author claims derivative-free optimization methods are simpler and more appropriate for his problem, compared to policy-gradient RL optimization methods such as PPO and DDPG where tuned-parameters are numerous and sparse rewards are propagating very slowly.
- The idea of Task Separation concerns the main objective of the training phase: "encode many desired motion primitives (training tasks) in a neural network", hoping for generalisation when exposed to new tasks.
  - It is said to serve for exploration in optimization: each task leads to a possible region with locally optimal solution, and the best solution among all identified locally optimal solutions is selected.
One concept: sparse reward.
- Reward shaping is an important problem when formulation the decision-making problem for autonomous driving using a (PO)MDP.
- The reward signal is the main signal used for the agent to update its policy. But if it only receives positive reward when reaching the goal state (i.e. sparse reward), two issues appear:
  - First, it will take random actions until, by chance, it gets some non-zero reward. Depending on how long it takes to get these non-zero rewards, it might take the agent extremely long to learn anything.
  - Secondly, because nonzero rewards are seen so rarely, the sequence of actions that resulted in the reward might be very long, and it is not clear which of those actions were really useful in getting the reward. This problem is known as credit assignment in RL. (Explanations are from here).
- Two options are considered in this work:
  - "Rich reward signals", where a feedback is provided at every time step (r becomes function of t).
  - "Curriculum learning", where the learning agent is first provided with simpler examples before gradually increasing complexity.
- After trials, the author claims that no consistent improvement could be observed with these two techniques, adding that the design of both rich reward signals and "simple examples" for curriculum learning are problematic.
  - He rather kept working with sparse rewards (maximal sparse rewards), but introduced some "virtual velocity constraints" to speed up the training.
I like the points he made concerning feature selection for the state, i.e. how to design the state s(t) of the MDP.
- He notes that s(t) must always relate the current vehicle state with reference to a goal state.
  - In other words, one should use relative features for the description of the position and velocity, relative to their targets.
- In addition, s(t) should also consider the past and embed a collection of multiple past time measurements.
  - It seems sounds. But this would indicate that the "Markov property" in the MDP formulation does not hold.

`Model Based` `Reinforcement Learning`

"Automatic learning of cyclist’s compliance for speed advice at intersections - a reinforcement learning-based approach"

[ 2019 ] [📝] [ 🎓 Delft University ]
[ Dyna-2, Dyna-Q ]

Click to expand


The proposed algorithm learns the cyclist’s behaviour in reaction to the advised `speed`. It is used to make prediction about the next state, allowing for a search that help to plan the best next move of the cyclist on-the-fly. A `look-up table` is used to model `F`. Source.

Authors: Dabiri, A., Hegyi, A., & Hoogendoorn, S.

Motivation:
- 1- Advise a cyclist what speed to adopt when approaching traffic lights with uncertainty in the timing.
  - To me, it looks like the opposite of numerous works that control traffic lights, assuming behaviours of vehicles, in order to optimize the traffic flow. Here, it may be worth for cyclists to speed up to catch a green light and avoid stopping.
  - Note that this is not a global optimization for a group of cyclists (e.g. on crossing lanes). Only one single cyclist is considered.
  - Note that the so-called "agent" is not the cyclist, but rather the module that provides the cyclist a speed advice.
- 2- Do not assume full compliance of the cyclist to the given advice, i.e. take into account the effect of disregarding the advice.
Challenges:
- 1- There is no advanced knowledge on how the cyclist may react to the advice he/she receives.
  - The other dynamics (or transition) models (deterministic kinematics of the bike and stochastic evolution of the traffic light state) are assumed to be known.
- 2- The computation time available at each decision step is limited: we cannot afford to wait for next-state to be known before starting to "search".
Main ideas:
- Learn a model of the reaction of cyclist to the advice (using a look-up table), on real-time (it seems continuous learning to me).
- Use a second search procedure to obtain a local approximation of the action-value function, i.e. to help the agent to select its next action.
- Hence:
  - "Combine learning and planning to decide of the speed of a cyclist at an intersection".
One strong inspiration: Dyna-2 (Silver & Sutton, 2007).
- "The value function is a linear combination of the transient and permanent memories, such that the transient memory tracks a local correction to the permanent memory".
- Without transient memory, it reduces to linear Sarsa.
- Without permanent memory, it reduces to a sample-based search algorithm.
One idea: use 2 search procedures:
- "Similar to Dyna-2, Dyna-c [c for cyclist], learns from the past and the future:"
- 1- Search I: The long-term action-value is updated from what has happened in real world.
  - Q(s,a), which is updated from real experience.
  - This long-term memory is used to represent general knowledge about the domain.
  - Search I can benefit from a local approximation provided by Search II. How? is I a real search or just argmax()?
- 2- Search II: The short-term action-value is updated from what could happen in the future.
  - Q¯(s,a), which uses simulated experience for its update and focuses on generating a local approximation of the action-value function.
  - Based on the learnt model and the selected action, the agent predicts the state in the next time step.
  - It can simulate experiences (search procedure) that start from this "imagined" state and update Q¯ accordingly.
Difference with dyna-q. Time constrain: we can neither afford to wait for the next observation nor to take too long to think after observing it (as opposed to e.g. GO).
- Search II has exactly one timestep to perform its searches:
  - "Just after the action is taken and before reaching to the next time step, the agent has Ts = ∆t seconds to perform Search II."
One take-away:
- "Proper initialisation of Q can significantly improve the performance of the algorithm [I note the logically equivalent contrapositive]; the closer the algorithm starts to the real optimal action-value, the better."
- "Here, Q is initialised with its optimal value in case of full compliance of the cyclist [next-observed speed = advised speed]. Stochastic Dynamic Programming (SDP) is used for such initialisation."

"ReQueST: Learning Human Objectives by Evaluating Hypothetical Behavior"

[ 2019 ] [📝] [🎞️] [] [ 🎓 UC Berkeley ] [ 🚗 DeepMind ]
[ safe exploration, reward learning ]

Click to expand


Right: Procedure to learn the hidden reward function: Using an offline-learnt `generative model`, query trajectories are produced for each acquisition function (`AF`). Transitions of these trajectories are labelled by the user. The reward model ensemble is retrained on the updated training data using maximum-likelihood estimation. Source.


Four acquisition functions: `Maximize predicted rewards` makes the car drive fast and far. `Maximize reward model uncertainty` makes the car drive close to the border. `Minimize predicted rewards` makes the car drives off-road. `Maximize the novelty of training data` makes the car stay still (since most training examples show cars in motion). Animated figure here.

Authors: Reddy, S., Dragan, A. D., Levine, S., Legg, S., & Leike, J.

One quote:
- "We align agent behavior with a user’s objectives by learning a model of the user’s reward function and training the agent via (model-based) RL."
One term: "reward query synthesis via trajectory optimization" (ReQueST)
- synthesis:
  - The model first learns a generative model, i.e. a transition or forward dynamics function.
  - It is trained using off-policy data and maximum-likelihood estimation, i.e. unsupervised learning.
  - It is used to produce synthetic trajectories (instead of using the default training environment).
  - Note: building a forward dynamics model for cars in interactive environments_ looks very challenging.
- reward query:
  - The user labels each transition in the synthetic trajectories based on some reward function (unknown to the agent).
  - Based on these signals, the agent learns a reward model r(s, a, s'), i.e. unsupervised learning.
  - The task can be regression or classification, for instance:
    - good - the car drives onto a new patch of road.
    - unsafe - off-road.
    - neutral - in a previously-visited road patch.
  - "We use an ensemble method to model uncertainty."
- trajectory optimization:
  - Once the reward model has converged, a model-based RL agent that optimizes the learned rewards is deployed.
  - It combines planning with model-predictive control (MPC).
One concept: "acquisition function" (AF).
- It answers the question: how to generate "useful" query trajectories?
  - One option is to sample random trajectories from the learnt generative model.
  - "The user knows the rewards and unsafe states, but querying the user is expensive." So it has to be done efficiently.
  - To generate useful queries, trajectories are synthesized so as to maximize so-called "acquisition functions" (AF).
- The authors explain (I did not understand everything) that these FA serve (but not all) as proxy for the "value of information" (VOI):
  - "The AF evaluates how useful it would be to elicit reward labels for trajectory τ".
- The maximization of each of the 4 FA is intended to produce different types of hypothetical behaviours, and get more diverse training data and a more accurate reward model:
  - 1- Maximize reward model uncertainty.
    - It is based on ensemble disagreement, i.e. generation of trajectories that maximize the disagreement between ensemble members.
    - The car is found to drive to the edge of the road and slowing down.
  - 2- Maximize predicted rewards.
    - The agent tries to act optimally when trying to maximize this term.
    - It should detect when the reward model incorrectly outputs high rewards (reward hacking).
  - 3- Minimizes predicted rewards.
    - "Reward-minimizing queries elicit labels for unsafe states, which are rare in the training environment unless you explicitly seek them out."
    - The car is going off-road as quickly as possible.
  - 4- Maximize the novelty of training data.
    - It produces novel trajectories that differ from those already in the training data, regardless of their predicted reward.
    - "The car is staying still, which makes sense since the training data tends to contain mostly trajectories of the car in motion."
- More precisely, the trajectory generation targets two objectives (balanced with some regularization constant):
  - 1- Produce informative queries, i.e. maximize the AFs.
  - 2- Produce realistic queries, i.e. maximize the probability of the generative model (staying on the distribution of states in the training environment).
About safe exploration.
- Via AF-3, the reward model learns to detect unsafe states.
- "One of the benefits of our method is that, since it learns from synthetic trajectories instead of real trajectories, it only has to imagine visiting unsafe states, instead of actually visiting them."
- In addition (to decide when the model has learnt enough), the user observes query trajectories, which reveals what the reward model has learned.

"Semantic predictive control for explainable and efficient policy learning"

[ 2019 ] [📝] [🎞️] [] [ 🎓 UC Berkeley (DeepDrive Center), Shanghai Jiao Tong University, Nanjing University ]
[ MPC, interpretability, CARLA ]

Click to expand


`SPC`, inspired from `MPC`, is composed of one semantic feature extractor, one semantic and event predictor, and one guide for action selection. Source.

Authors: Pan, X., Chen, X., Cai, Q., Canny, J., & Yu, F.

Motivations:
- 1- Sample efficiency.
- 2- Interpretability.
- Limitation of behavioural cloning methods:
  - "Direct imitative behaviors do not consider future consequences of actions explicitly. [...] These models are reactive and the methods do not incorporate reinforcement or prediction signals."
- Limitations of model-free RL methods:
  - "To train a reliable policy, an RL agent requires orders of magnitude more training data than a human does for the same task."
  - "An unexplainable RL policy is undesirable as a single bad decision can lead to a severe consequence without forewarning."
One term: "Semantic Predictive Control" (SPC).
- It is inspired by Model Predictive Control (MPC) in that it seeks an optimal action sequence over a finite horizon and only executes the first action.
- "Semantic" because the idea it to try to predict future semantic maps, conditionned on action sequences and current observation.
- SPN is trained on rollout data sampled online in the environment.
Structure:
- 1- Semantic estimation.
  - Multi-scale intermediate features are extracted from RGB observations, using "Deep Layer Aggregation" (DLA), a special type of skip connections.
  - As noted:
    - "Using semantic segmentation as a latent state representation helps to improve data efficiency."
  - This multi-scale feature representation is passed together with the planned action into the prediction module to iteratively produce future feature maps.
- 2- Representation prediction.
  - What is predicted?
    - 2.1- The future scene segmentation
    - 2.2- Some task-dependent variables (seen as "future events") conditioned on current observation and action sequence. This can include:
      - Collision signal (binary).
      - Off-road signal (binary).
      - Single-step travel distance (scalar).
      - Speed (scalar).
      - Driving angle (scalar).
      - Note: in their POC with flappy bird, authors also predicted the discounted sum of rewards.
- 3- Action sampling guidance.
  - How to select actions?
    - 3.1- One possible solution is to perform gradient descent to optimize an action sequence.
    - 3.2- Another solution is to perform a grid search on the action space, and select the one with the smallest cost.
    - 3.3- Instead, the authors propose to use the result of the SMP:
      - "SPN outputs an action guidance distribution given a state input, indicating a coarse action probability distribution".
      - Then, they sample multiple action sequences according to this action guidance distribution, then evaluates their costs, and finally pick the best one.

"Vision-Based Autonomous Driving : A Model Learning Approach"

[ 2019 ] [📝] [ 🎓 University of Michigan ] [ 🚗 Ford ]
[ VAE, stochastic policy search, CARLA ]

Click to expand

One figure:


The `perception` module, the `memory` or `prediction` module, and the `control` module. Source.

Authors: Baheri, A., Kolmanovsky, I., Girard, A., Tseng, E., & Filev, D.

The idea is to first learn a model of the environment (the transition function of the MDP) and subsequently derive a policy based on it.
Three modules are used:
- 1- A VAE is trained to encode front camera views into an abstract latent representation.
- 2- A LSTM is trained to predict the latent representation of the one time-step ahead frame, given the action taken and the current state representation. Based on this prediction (mean and std), a next state representation is sampled using the VAE.
- 3- A CMA-ES is trained to take actions (steering, acceleration, and brake) based on the LSTM hidden state (capturing history information) and the current state representation (predicted). The problem is formulated as an MDP.
One idea about the continuous action space:
- "We combine the acceleration and brake commands into a single value between −1 to +1, where the values between −1 and 0 correspond to the brake command and the values between 0 and 1 correspond to the acceleration command".
- The authors use the term "acceleration command" for one of the actions. CARLA works with throttle, as human use the gas-pedal.
- I have realized that the mapping acceleration -> throttle is very complex. Therefore I think the agent is learning the throttle and considering the single NN layer used for the controller, this may be quite challenging.
About the CMA-ES:
- ES means "Evolution Strategy", i.e. an optimization technique based on ideas of evolution, iterating between of variation (via recombination and mutation) and selection.
  - ES is easy to implement, easy to scale, very fast if parallelized and extremely simple.
- CMA means "Covariance Matrix Adaptation".
  - This means that in the variation phase, not only the mean but also the covariance matrix of the population is updated to increase the probability of previously successful steps.
  - Therefore, it can be seen as Cross-Entropy Methods (CEM) with momentum.
About sampling efficiency:
- The authors note that IL and model-free RL baselines were taking resp. 14 hours and 12 days of driving for training and were both outperformed by the presented model-based RL approach which required 5 hours of human driving.
  - This only considers the time to interact with the environment, i.e. to record images.
  - It would be interesting to consider the time needed to learn the policy afterward.
- CMA-ES, as a derivative-free method, is one of the least sample efficient approach.
  - I find interesting that an evolutionary algorithm was chosen given the motivation of increasing sampling efficiency.
About model-based RL:
- The performance really depends on the ability to learn a reliable model of the environment.
  - The low-level representation of the VAE (size 128) may not capture the most difficult situations.
  - The authors suggest looking at mid-level representations such as the affordance representation of DeepDriving instead.
- Here, the authors strictly split the two tasks: First learn a model. Then do planning.
- Why not keeping interacting from time to time with the env, in order to vary the sources of experience?
  - This should still be more sample efficient than model-free approaches while making sure the agent keep seeing "correct" transitions.

"Vision‑based control in the open racing car simulator with deep and reinforcement learning"

[ 2019 ] [📝] [ 🎓 Chinese Academy of Sciences ]
[ PILCO, TORCS ]

Click to expand

Some figures:


First extract some variables - e.g. `curvature`, `desired speed`, `lateral offset`, `offset in heading` - from images using supervised learning and then apply control learnt with model-based `RL`. Source.


The model-based* `PILCO` algorithm is used to quickly learn to predict the desired speed. Source.*

Authors: Zhu, Y., & Zhao, D.

Definitions:
- State variables: x = [lateral deviation, angle deviation, desired speed].
- Dynamical variables: y = [x, curvature].
- Cost variables z = [y, current speed].
- Control variables: u = [steering, throttle or brake].
- The variable current speed is always known: either given by TORCS or read from CAN bus.
One idea: contrary to E2E, the authors want to separate perception and control. Hence the training is divided into two steps:
- 1- Extract dynamical variables y from the simulator (assume full observation) and learn a driving controller. -> Using model-based RL.
- 2- Try to extract y from images. -> Using supervised learning.
- This step-by-step method brings advantages such as the possibility for intermediate checks and uncertainty propagation.
  - But both learning processes are isolated. And one defective block can cause the whole chain to fail.
  - In particular, the authors note that the CNN fails at predicting 0-lateral-offset, i.e. when the car is close to the centre, causing the full system to "vibrate".
  - This could be addressed on the controller side (damping factor or adding action consistency in the cost function), but it would be better to back-propagate these errors directly to the perception, as in pixel-to-control approaches.
What is learnt by the controller?
- One option would be to learn the transition function leading to the new state: x[t+1] = f(y, u, x). This is what the simulator applies internally.
- Instead, here, the distribution of the change in state is learnt: delta(x) = x[t+1] - x[t] = f(y, u, x).
- Data is collected through interactions and used to optimize the parameters of the controller:
  - Training inputs are formed by some recorded Y = [y, u].
  - Training targets are built with some recorded ΔX = [delta(x)].
Another idea: the car is expected to run at different velocities.
- Hence vary the desired speed depending on the curvature, the current velocity and the deviation in heading.
- This is what the agent must learn to predict.
- In the reward function of PILCO, the term about desired velocity play the largest role (if you do not learn to decelerate before a turn, your experiences will always be limited since you will get off-road at each sharp turn).
One algorithm: PILCO = Probabilistic Inference for Learning COntrol.
- In short, this is a model-based RL algorithm where the system dynamics is modelled using a Gaussian process (GP).
- The GP predicts outcome distribution of delta(x) with probabilities. Hence first letter P.
  - In particular, the job is to predict the mean and the standard deviation of this distribution which is assumed to be Gaussian.
  - This probabilistic nature is important since model-based RL usually suffers from model bias.
- The cost variables are also predicted and based on this z distribution, the optimal control u is derived using policy gradient search (PGS).
  - More precisely, the control variables u is assumed to be function of the expected cost z via an affine transformation followed by some saturation: u = sat(w*z + b).
  - Hence PGS aims at finding {w, b}: the predicted return and its derivatives are used to optimize the controller parameters.
  - The new controller is again used in TORCS to generate data and the learning process is repeated.
Why and how is the vanilla PILCO modified?
- The computational complexity of PILCO is linear in the size of training set.
  - Instead of using sparse GP method (e.g. FITC), the authors decide to prune the dataset instead.
  - In particular, observation data are sparsely collected and renewed at each iteration.
- Other modifications relate to the difference between input output/variable types. And the use of different scenarios to calculate the expected returns.
One quote about the difference between PILCO and MPC:
- "The concept of PILCO is quite similar to explicit MPC algorithms, but MPC controllers are usually defined piecewise affine. For PILCO, control law can be represented in any differentiable form."

`Planning` and `Monte Carlo Tree Search`

"Crossing of Road Intersections : Decision-Making Under Uncertainty for Autonomous Vehicles"

[ 2019 ] [📝] [ 🎓 INRIA ] [ 🚗 Renault ]
[ POMCP, interaction-aware, SCANeR ]

Click to expand


The author calls for a probabilistic framework to reason and make decision, due to the inherent `perception` uncertainty and `behaviour` (`interaction`) uncertainty. Also, learning-based methods are avoided due to their susceptibility to over-fit if the dataset is not balanced. Source.

The author prefers probabilistic methods, in order to deal with uncertainties while trying to offer some interpretability. The navigation module outputs a single action to be implemented. Another option would have been to deliver some policy which could be followed for several steps, limiting inconsistent transitions (especially for comfort) and favouring long-horizon reasoning. Source.


The intended manoeuvre* is inferred based on observed `speed`, `acceleration` and heading - no `position` term - and will be treated as an observation in the `POMDP`. Source.*


As noted in my report of `IV19`, risk assessment can be performed by comparing the expectated behaviour (`expectation`) to the inferred behaviour (`intention`), i.e. what should be done in the situation and what is actually observed. A discrepancy can detect some misinterpretation of the scene. Source.

Decomposition of the probabilistic transition function. Only the longitudinal control via discrete acceleration is considered. The state x consists of physical and behavioural parts. In particular, it includes the behaviour expectation for each vehicle, i.e. what should be done according to the traffic rules. It also come with a behavioural intention for which is the inferred manoeuvre followed by the observed vehicle. intention continuation is used to describe the transition about intention, while gap acceptance model are used for the transition about expected behaviour. Finally, note that the selected acceleration action only influences the physical term of the ego vehicle. Source.

One contribution is called Functional Discretisation. So-called motion patterns are stored within an HD-map as polygon delimiting the intersection entrance and crossing zones. This discrete crossing and merging zones are not manually defined but learnt based on simulated vehicle trajectories. The continuous intersection space is therefore divided into non-uniform discrete areas. Top-right: three crossing scenarios are considered, with different pairs of priorities. Source.


The `trust` KPI is based the `time gap`, i.e. the delta in predicted time of when each vehicle will reach the crossing point. This should be ''maintained'' over `4s` over all the approach. Hence the use of ''temporal'' logic. The `unsafe stop` KPI states that the vehicle should never be stand still within the unsafe area. Source.

Author: Barbier M.

What?
- A PhD thesis.
Motivation:
- Interaction-aware and uncertainty-aware handling of a signed intersection.
How to capture and deal with interaction in the decision?
- The intended manoeuvre is inferred (behavioural classification) and subsequently treated as an observation.
- By comparing it with the expected manoeuvre, the agent should determine how to interact with the other vehicle.
About the behavioural classification.
- lateral part: {Turn right, Turn left, Go straight}.
- longitudinal part: {Stop, Yield, Cross}.
- Six features are used:
  - max and min speed.
  - max and min acceleration
  - Maximum right and left deviation from the mean heading angle.
- I would be afraid that maximum and minimum values could come from outliers and would rather have worked with quantiles (e.g. 10% and 90%).
About risk assessment:
- "The intended manoeuvre represents what the driver is doing, whereas the expected manoeuvre represents what the situation requires."
- One idea to compute the difference between what the other driver IS DOING (called intention) and what one driver SHOULD DO (called expectation)
  - The expected behaviour can be derived e.g. according to priority rules or to gap acceptance models.
- This discrepancy is useful for risk assessment since it can detect some misinterpretation of the scene:
  - Situations where intention and expectation do not match could result in a risky interaction.
- By penalizing states with a large difference, the reward function incorporates feedbacks about interaction and encourages the agent to select actions that reduce this risk.
About the (large) search horizon:
- "The configuration with γ = 0.85 and C = 30 is chosen as it meets these characteristics. The discount value results in a search horizon of 12 seconds".
About the online POMDP solver:
- POMCP.
- action continuation is used as rollout policy.
- "[One could] include imitation learning to initiate V(ha) with knowledge obtained by looking at human drivers."
- "A memory could be used to initialize the value function from previous exploration, accelerating the search for the optimal policy."
How to evaluate the decision-making framework?
- The author argues that evaluation should be decorrelated from the reward crafting, hence having separated KPIs:
  - The reason is that systems that used their performances indicators in their value estimation are likely to over-fit.
  - "Goodhart's law stating that 'when a metric is used as a target, it ceases to be a good metric'"
- Another idea to avoid reward hacking: the reward function is designed with multiple objectives: trade-off between performances, risks and interactions.
How to decide the threshold in KPIs?
- Statistical Model Checking is applied to vary the bound of KPIs.
  - Bounded Linear Temporal Logic (BLTL) allows to state conditions that will eventually be true.
- The author works for instance with the probability of crossing the intersection in less than a given time.
About ENABLE-S3:
- The author uses the validation and verification architecture of this European project.
- From ENABLE-S3 website: "Pure simulation cannot cover physics in detail due to its limitations in modelling and computation. Real-world tests are too expensive, too time consuming and potentially dangerous. Thus, ENABLE-S3 aims at developing an innovative solution capable of combining both worlds in an optimized manner [...] and facilitate the market introduction of automated systems in Europe."

"DESPOT-α: Online POMDP Planning With Large State And Observation Spaces"

[ 2019 ] [📝] [ 🎓 National University Of Singapore ]
[ POMDP, online solver, DESPOT, parallelization, large observation space ]

Click to expand


Unlike standard belief tree, some `observation` branches* are removed in a `DESPOT`. Source.*

Top - Illustration of the particle divergence problem: When observation space is large, particles quickly diverge into separate belief nodes in the belief tree, each of which contains only a single particle. This causes over-optimistic behaviours. Bottom - In a DESPOT-α, each node has the same number of particles as the root of the tree and weighting is performed based on the observations. This prevents the over-optimistic evaluation of value of the belief. Source.

Authors: Garg, N. P., Hsu, D., & Lee, W. S.

Previous work: "Determinized Sparse Partially Observable Tree" (DESPOT) by (Ye, Somani, Hsu & Lee. 2017).
About DESPOT:
- Why Partially Observable ?
  - As the state is not fully observable, the agent must reason (and maintain) with beliefs, which are probability distributions over the states given history h.
  - The belief is a sufficient statistic that contains all the information from the history of actions and observations (a1, z1, a2, z2, ... , at, zt).
  - "By reasoning in belief space, POMDPs are able to maintain a balance between exploration and exploitation and hence provide a principled framework for [sequential] decision making under uncertainty."
- Why Tree ?
  - Because a search tree of histories is constructed, online.
  - The "belief tree search" aspect has to do with the online nature of the solver (as opposed to offline methods that compute an approximately optimal policy _ over the entire belief space, prior to execution)_:
    - "At each time step, it plans locally and chooses an optimal action for the current belief only, by performing lookahead search in the neighborhood of the current belief. It then executes the chosen action immediately."
  - "Many POMDP solvers do online planning by doing forward search from the current belief, constructing a tree which branches each time an action is required, and also each time an observation may be observed".
    - Each node implicitly represents a belief.
      - "Implicitly" since it contains a particle set that approximates the belief. This contrasts with other approaches that explicitly represent the belief as a probability distribution over the state space, e.g. with exact updates using Bayes' theorem.
      - Planning is only performed from the current belief, which is the root node.
    - Each node branches into |A| action edges.
    - Each action edge further branches into |Z| observation edges.
  - A DESPOT is built through trials, consisting of exploration and backup on sampled scenarios.
- Why Determinized ?
  - Because the search is focused on a set of randomly sampled "scenarios" that are sampled a priori.
    - A set of random numbers are generated in advance, as the first belief is given.
      - As I understood, they are called scenarios ("abstract simulation trajectories").
    - "A small number of sampled scenarios is sufficient to give a good estimate of the true value of any policy."
      - These determinized scenarios make DESPOT differ from POMCP which performs MCTS on a belief tree using UCT.
  - Here is my interpretation:
    - Imagine you are playing a game where your motion relies on the outcome of some dice, e.g. Monopoly or game of snakes and ladders
      - Option 1- At each timestep, you roll the dice and move accordingly.
      - Option 2- Before starting, you roll the dice x times. You then put the dice away and start playing: at each timestep, you move according to the i-th generated number.
    - Here, these generated numbers (scenarios) are used to decide the noise injected in the evaluation of the two models used for the tree expansion: measurement and transition functions.
      - That means it is known in advance, before starting building the tree, that the n-th belief-leaf will be generated from the measurement function using the n-th sampled number as noise parameter.
  - "Like DESPOT, DESPOT-α uses the "particle belief approximation" and searches a determinized sparse belief tree".
- Why Sparse ?
  - It is related to the question: How to represent a belief?
    - DESPOT represents the belief as a set of particles (particles are sampled states), as for POMCP.
      - This enables to overcome the issue of large state space.
    - "While a standard belief tree captures the execution of all policies under all possible scenarios, a DESPOT captures the execution of all policies under a set of sampled scenarios."
    - Because some observation branches are removed, a DESPOT can be viewed as a sparse approximation of the standard belief tree:
      - The tree contains all the action branches, but only the observation branches under the sampled scenarios.
      - This also implies that DESPOT does not perform belief update over the entire state space (addressing the curse of dimensionality).
  - In other words, a DESPOT is structurally similar to standard belief trees, but contains only belief nodes reachable under the K sampled scenarios.
    - Size of SparseSampling: |A|^D.C^D (sparse because only C observations are sampled for each action branch, and D is the depth).
    - Size of DESPOT: |A|^D.K (for K sampled scenarios).
Additional notes about DESPOT:
- Motivations: Address two curses.
  - 1- Curse of dimensionality: the state space, and correspondingly the dimensionality of the belief size, grows exponentially with the number of state variables.
  - 2- Curse of history: the belief tree grows exponentially with depth.
  - DESPOT (as for POMCP) breaks the two curses through sampling:
    - "It alleviates the curse of dimensionality by sampling states from a belief and alleviates the curse of history by sampling observations."
- DESPOT contains all the main ideas for online planning via belief tree search:
  - 1- Heuristic search: The tree is incrementally constructed under the guidance of a heuristic.
  - 2- Branch-and-bound pruning: Upper bounds (computed from state-based heuristics) and lower bounds (computed from default policies) on the value at each belief node are used to prune suboptimal subtrees.
    - Note that the gap between the lower bound and upper bound can represent the uncertainty at the belief node.
  - 3- Monte Carlo sampling: Only a randomly sampled subset of observation branches is explored at each node.
- Regularization.
  - Since many scenarios are not sampled, and because the chosen policy optimizes for the sampled scenarios, it can happen that the policy does not perform well.
  - Regularization can be used to address that overfitting.
More about "1- heuristic search": Search-guidance based on the value function.
- "To make sure that even the partially constructed tree is able to compute a good policy, heuristics based on upper bound and lower bound on the value of belief nodes are used to guide the search".
- Note that this requires the computation of the value of belief nodes: V(b).
- How to estimate the value? Using α-vectors.
One concept: α-vectors.
- One important property:
  - "The value function of a POMDP can be approximated arbitrarily well by a convex piece-wise linear function of the belief".
  - V(b) = max over α [∑ over s (b(s).α(s))]
- "An α-vector is associated with a conditional plan and, for each state s, captures the reward of executing the plan starting from state s."
- Note that the number of components in an α-vector correspond to the number of states and hence can be exponentially large.
- In a DESPOT-α, α-vectors will be efficiently approximated to reduce computation, to approximate the lower bound on value of belief nodes.
- Hence the name Determinized Sparse Partially Observable Tree With α-Vector Update.

Main motivation for DESPOT-α:
- Address the problem particle divergence to scale to large observation spaces.
- "When the observation space is large, particles quickly diverge into separate belief nodes in the belief tree, each of which contains only a single particle."
- The uncertainty can be underestimated by the derived policy, leading to poor and over-optimistic actions.
Main idea of DESPOT-α:
- To prevent the over-optimistic evaluation of value of the belief, the idea is to keep a constant number of particles, and weight them (as for POMCPOW and PFT-DPW that extend POMCP).
- "Instead of propagating only the particles producing the same observation to the child of a belief-action node, we propagate all the particles to the child nodes and update the weights of particles according to relative likelihood of observation p(z|s, a)."
- This is similar to particle filters.
New issue: when computing the heuristics, propagating each particle to every child belief node impacts the computational efficiency.
- "Always having C child belief nodes prevents over optimistic evaluation of value of belief but also makes the tree size (C.|A|)^D".
Solution (not in POMCPOW and PFT-DPW):
- Share the value function calculation among different (but similar) belief nodes, by grouping observations together.
  - "We can merge the observations, when the value of the resulting beliefs is maximized by the same α-vector."
  - "We can use α-vectors to share the computation done for one trial among "sibling" belief nodes for improving lower bounds".
- This leads to the concept of "sibling belief nodes": Nodes which differ from each other only in last observation.
  - "We are implicitly grouping beliefs whose values are maximised by same α-vector by sharing α-vectors between sibling belief nodes."
  - "As sibling belief nodes share the same set of scenarios with different weights, α-vector calculated for one belief node can be used to calculate approximate lower bound for the sibling belief nodes by simply doing an inner product of weights of the particles and the α-vector".

To sum up - Contributions:
- 1- Sample a fixed number of observations for each action branch like in sparse sampling, while still using determinized scenarios like DESPOT (it still contains only the observation branches reachable by sampled scenarios).
- 2- Introduce a particle approximation of the α-vector to improve the efficiency of online policy search.
- 3- Further speed-up the search by leveraging CPU and GPU parallelization introduced in HyP-DESPOT.
  - Here K particles can be expanded in parallel, which is efficient since each node contains all the particles.

"Risk-Aware Reasoning for Autonomous Vehicles"

[ 2019 ] [📝] [ 🎓 Khalifa University, Abu Dhabi ]
[ risk-bounded planning, chance constraint, POMDP, hierachical planning ]

Click to expand

Architecture to deal with uncertainty and produce risk-aware decisions. Probabilistic vehicle motions are modelled using Probabilistic Flow Tube (PFT). These PFTs learnt from demonstrating trajectories represent a sequence of probabilistic reachable sets, and are used to calculate the risk of collision. This risk quantification serves in the CC-POMDP formulation of the short-horizon planner, where the ego-agent should plan the best sequence of actions while respecting a bound on the probability of collision. Uncertainty is also propagated in the higher modules of the hierarchical planning where Temporal Plan Networks with Uncertainty (STNUs) are used to derive short-term objectives. Source.

Authors: Khonji, M., Dias, J., & Seneviratne, L.

One remark: Not too many details are given about the implementation, but it is interesting to read reformulation of concepts met in other works.
One related work:
- Several ideas (RAO*, PFT, CC-POMDP) reminded me the work of (Huang, Hong, Hofmann, & Williams, 2019) - Online Risk-Bounded Motion Planning for Autonomous Vehicles in Dynamic Environments - detailed further above.
- The first author has been actually collaborating with this research group.
One idea: hierarchical planning.
- The uncertainty-aware decision-making task is decomposed between a high-level planner, a short-horizon planner and some MPC-based precomputed and learned manoeuvre trajectories.
- Three levels of actions are distinguished for short-horizon planner:
  - Micro Actions are primitive actions, e.g. accelerate, decelerate, maintain.
  - Manoeuvre Actions are sequences of micro actions, e.g. merge left. merge right.
  - Macro Actions are sequences of manoeuvre actions, e.g. pass the front vehicle, go straight until next intersection.
One concept: "chance constraint" optimization.
- Some measure of uncertainty (e.g. about perception, about unknown intention, about control) is available to the short-horizon planner.
- To goal is to solve the optimization problem (as for vanilla POMDP formulations) i.e. find the optimal sequence of ego-vehicle actions, while ensuring that the probability of meeting a certain constraint (e.g. too small gap or collision) is above a certain level.
  - In other words, and contrary to strict constrained optimization, here there is a bound on the probability of violating constraints.
  - The policymaker can set the desired level of conservatism in the plan.
- The authors mention RAO*. This is solver for "chance-constrained POMDP" (CC-POMDP).
  - During the search, it uses heuristics to quickly detect and prune overly-risky policy branches.

"Tactical decision-making for autonomous driving: A reinforcement learning approach"

[ 2019 ] [📝] [ 🎓 Chalmers University ] [ 🚗 Volvo ]
[ POMDP, MCTS ]

Click to expand

Some figures:


`MCTS` is especially beneficial when it is necessary to plan relatively far into the future. Source.


The `RL`-learnt neural network predicts two values used to guide the search. Source.


Treat surrounding vehicles as interchangeable objects* using `CNN` layers. Source.*


Comparison of sampling efficiency* - need for `domain knowledge` and `computational speed` should also be considered. Source.*

Author: Hoel, C.-J.

Three related works corresponding to three proposed approaches (all RL-related):
- 1- Genetic algorithm (policy based RL) to train a rule-based driver model (each chromosome encodes a rule-based driver model via a set of instructions).
  - "An evolutionary approach to general-purpose automated speed and lane change behavior" - (Hoel et al. 2018).
- 2- DQN.
  - "Automated Speed and Lane Change Decision Making using Deep Reinforcement Learning" - (Hoel et al. 2018).
- 3- Combining planning (MCTS) and learning (model-free RL).
  - The neural network predicts two values used to guide the search:
    - The value function V(state).
    - The policy p(state, action) for each possible action.
  - This time, a transition function (or generative model) G(s, a) is considered and used during the SIMULATION phase of the search.
  - "Combining Planning and Deep Reinforcement Learning in Tactical Decision Making for Autonomous Driving" - (Hoel et al. 2019).
    - I have analysed this paper in this section of my summary of IV19.
One remark about the POMDP formulation:
- Only the physical parts of the state (position and speed) are observed.
- The parameters of the surrounding drivers, which are assumed to behave according to the IDM/MOBIL models, is not directly accessible by the ego-agent.
- A particle filter is used to estimate them (belief state estimation).
One idea: Treat surrounding vehicles as interchangeable objects using CNN layers.
- Using CNN layers with max-pooling creates a translational invariance between the vehicles.
- "The output is independent on the ordering of the vehicles in the input vector, and it also removes the problem of specifying a fixed input vector size, which instead can be made larger than necessary and padded with dummy values for the extra slots"
About "sampling efficiency", "domain knowledge" and trade-off of speed vs. generality:
- The GA agent requires much domain knowledge in the form of handcrafted features (form of the instructions).
- The DQN agent requires between 2 and 3 orders of magnitude less driving time than the GA agent.
- "The MCTS/NN agent requires the most domain knowledge, since it needs a generative model G of the environment, a belief state estimator, and possibly knowledge on how to prune actions that lead to collisions."
Results:
- The baseline is a rule-based approach built with IDM and MOBIL driver models (also used in the generative model and to simulate other vehicles).
- "All methods outperform the baseline IDM/MOBIL model by taking decisions that allows the vehicle to navigate through traffic between 5% and 10% faster."
- MCTS is especially beneficial when it is necessary to plan relatively far into the future (e.g. highway exit case).

"WiseMove: A Framework for Safe Deep Reinforcement Learning for Autonomous Driving"

[ 2019 ] [📝] [] [ 🎓 University of Waterloo ]
[ MCTS, options framework, LTL, hierarchical decision making, POMDP ]

Click to expand

One figure:


Source.

Authors: Lee, J., Balakrishnan, A., Gaurav, A., & Feb, L. G.

One related work: The presented approach reminds me the work of Paxton, C., Raman, V., Hager, G. D., & Kobilarov, M..
One term: "WiseMove": the presented options-based modular safe DRL framework.
- The modular, or hierarchical, aspect comes from the option framework. Sometimes called macro-actions.
- For more on Hierarchical RL, check out this thegradient.pub post.
- The idea is to decompose the decision by working with temporal abstracted actions (e.g. slow down, turn left) on a high-level (like a behaviour planner).
- Each of these so called options rely on low-level primitive policies that implement their manoeuvres (similar to a geometrical trajectory optimizer).
One idea: LTL formalism is used to check the validity of high-level decisions.
- An option is defined by (1) a underlying primitive policy, but also by (2) an initial condition and (3) a terminal condition.
- For instance, the option take-over is available only if a vehicle is on my lane and a second lane exists. The manoeuvre is finished when I arrive in front of the other vehicle.
- I like to think of it as another sort of masking mechanism.
- Here, these conditions are expressed as hand-crafted rules in an LTL-like syntax.
One remark: I think we are currently missing open-source simulators that offers OpenAI gym-like APIs for training and testing RL approaches for decision making.
- Several interfaces to SUMO have been developed.
- For instance @LucasAlegre, @bstriner, @SaloniDash7, @sycdlcrain or FLOW which looks promising since it keeps being developed.
- Here, the author of WiseMove release an env python module (together with verifier, options and backends) that should fulfil this function.
Another remark: Combining learning [RL] and planning [(MC) tree search] is an idea I find very promising.
- Here, the safest next option is selected based on the stochastic look-aheads performed by the MCTS (safety check).
- In return, the options effectively reduce the number of decisions needed to reach any depth in the tree (sampling efficiency).

"A Simulation-Based Reinforcement Learning Approach for Long-Term Maneuver Planning in Highway Traffic Scenarios"

[ 2019 ] [📝] [ 🎓 Technische Universität Darmstadt ] [ 🚗 Opel ]
[ combining learning/planning, hierarchical/modular decision making, POMDP, SUMO ]

Click to expand

One diagram is better than 100 words:


The term `action` comprises a lateral manoeuvre decision and a set speed request. Source.

Authors: Augustin, D., Schucker, J., Tschirner, J., Hofmann, M., & Konigorski, L.

One remark: I like the hierarchy and modularity of the approach.
- Especially the fact that the action stays high-level (speed desire and high-level manoeuvre), as opposed to steering angle and throttle commands that are often used in RL.
One promising tool: FLOW
- FLOW is a Python library that interfaces the RL libraries RLlib and rllab with SUMO. It has been developed and is supported by UC Berkeley.
- It has not been used many times (because of the lack of Windows support?). Instead, many research using SUMO develop their own interface, which makes comparison and reproduction difficult.
- A few recent FLOW-based works can be mentioned though:
  - "Simulation to scaled city: zero-shot policy transfer for traffic control via autonomous vehicles" by (Jang et al., 2018)
  - "Benchmarks for reinforcement learning in mixed-autonomy traffic" by (Vinitsky et al., 2018)

"Towards Human-Like Prediction and Decision-Making for Automated Vehicles in Highway Scenarios"

[ 2019 ] [📝] [] [🎞️] [ 🎓 INRIA ] [ 🚗 Toyota ]
[ MCTS, online POMDP, POMCP, progressive widening, SUMO ]

Click to expand

Note: the planning part of this thesis takes advantage of the prediction approaches and the driver models referenced in previous sections.

In particular, the derived predictive model is used for both the belief update (instead of often-used particle filters and IMM filters) and as a generative model (for forward simulations) of the POMDP.
The value estimations in the tree search are based on the learnt driver model and the long-term prediction methods previously referenced.

Some figures:


Comparison of recent POMDP-based planning modules. Source.


Construction of the tree search with belief updates and model-based rollouts. Source.


Source.

Author: Sierra Gonzalez, D.

The author targets some "human-like tactical planning".
- The POMDP formulation is ideal since it considers uncertainty in the controls, states, and the intentions of the traffic participants.
- The idea is to include estimation of intentions for long-term anticipation.
One idea: about the rollout policy used for the construction of the search tree.
- One option is to use a random rollout policy.
- Here, the previously-derived models are used to predict approximately the long-term development of traffic scenes.
Another idea: adapt the combination of model-based and manoeuvre-estimation-based predictions, depending on how far the rollout looks into the future.

"As we go deeper into the history tree (that is, into the future), the observed dynamics of the targets at the root node become less relevant and so we rely increasingly in the model to predict the behaviour of the obstacles."

Other related works:
- The Hierarchical architecture of (Sonu, E., Sunberg, Z., & Kochenderfer, M. J. (2018). "Exploiting Hierarchy for Scalable Decision Making in Autonomous Driving").
- The Double Progressive Widening (DPW) or progressive unpruning of (Sunberg & Kochenderfer, 2017) to deal with continuous observations.
- The general approach of (Bouton, M., Cosgun, A., & Kochenderfer, M. J. (2017). "Belief state planning for autonomously navigating urban intersections"). The main difference is the substitution of the Interacting Multiple Model (IMM) with DBN-based model to consider the interactions between vehicles.
One quote about the (relative) benefits of POMDP formulations:

"There have not been significative differences between the decisions taken by the proposed POMDP planner and the reactive SUMO model. This is due to the fact that neither of those scenes truly required to analyse the long-term consequences of a maneuver".

"Value Sensitive Design for Autonomous Vehicle Motion Planning"

[ 2018 ] [📝] [ 🎓 Stanford University ] [ 🚗 Ford ]
[ POMDP, QMDP ]

Click to expand


The `POMDP` policy better deals with uncertainties in the detection of the pedestrian. It accounts for the possible transition between `detected` and `not detected` cases, leading to smoother `action`s across the `state` space. Source.

Authors: Thornton, S. M., Lewis, F. E., Zhang, V., Kochenderfer, M. J., & Christian Gerdes, J.

Motivation:
- Apply the VSD methodology / formalism to the problem of speed control for the scenario of an occluded pedestrian crosswalk.
About Value Sensitive Design (VSD):
- "[Wikipedia]: A theoretically grounded approach to the design of technology that accounts for human values in a principled and comprehensive manner."
- In (PO)MDPs, engineers account for some "human values" in the design of the reward function.
- Values are converted to specifications:
  - safety: harm reduction or collision avoidance.
  - legality: care when approaching a crosswalk.
  - mobility: efficiency.
  - smoothness: comfort.
- Stakeholders are also identified:
  - the AV and its occupants.
  - the obstructing vehicle parked.
  - the pedestrian potentially crossing the street.
  - the authority of traffic laws.
About the POMDP formulation:
- The belief of a pedestrian crossing is tracked with some Bayesian filter.
  - The pedestrian detection is a Boolean value because the pedestrian is either crossing or not.
  - "There is observation uncertainty for the pedestrian crossing with a false positive of 5% for detecting and a false positive of 5% for not detecting the pedestrian, which captures sensor uncertainty.
  - "When the pedestrian is detected, there is a 90% probability the pedestrian will continue to be detected at the next time step. When the pedestrian is not detected, then there is a 50% chance he or she will continue to not be detected, which captures the uncertainty due to the occlusion.
- The QMDP solver from JuliaPOMDP is used.
  - "Although QMDP assumes that at the next time step the state will be fully observable, it is well suited for this problem because the actions are not information gathering, meaning the actions do not directly reduce the uncertainty of the scenario."
  - I do not agree with that statement: information gathering is key in this scenario to resolve the ambiguity in the detection.
- state and action spaces are discrete.
  - But "continuousness" is maintained using "multilinear grid interpolations" for the state transitions, as in (Davies 1997).
Benefits of POMDP:
- "A stochastic optimization problem can account for modeled uncertainty present in the driving scenario while balancing the identified values through the objective function."
- The baseline on the other hand is reactive:
  - if detection, then decelerate to stop at crosswalk.
  - else target the desired velocity with a proportional controller.
- For this scenario where the detection is key but uncertain, one need to anticipate transitioning from one set of logic to the other.
  - When the pedestrian is detected: the baseline is safe, but it lacks efficiency.
  - When the pedestrian is not detected: the baseline is efficient, but not safe.
- This rule-based dichotomy makes the baseline control have full speed when the pedestrian appears and prevents it from to legally yielding to the pedestrian.

"Decision Making Under Uncertainty for Urban Driving"

[ 2018 ] [📝] [] [ 🎓 Stanford ]
[ POMDP, MCTS, julia, probabilistic risk assessment, value iteration ]

Click to expand

One figure:


Comparing the vanilla POMCP and proposed safe variant of it. Source.

Authors: Weingertner, P., Autef, A., & Le Cleac’h, S.

One algorithm: POMCP.
- Presented in 2010, POMCP is an extension of the traditional MCTS algorithm to POMDP.
- Together with [DESPOT], POMCP is an often-used POMDP online solver.
One term: "observation class".
- Different extensions of POMCP and DESPOT have been proposed. In the presented approach, the goal is to work with continuous observations, while ensuring safety.
- The idea is to limit the number of observation nodes in the tree by grouping observations based on some utility function.
- This utility function should not to be confused with the offline-learn value function representing the probability of collision.
- The safe clusterization of observations can be based on smallest TTC or smallest distance to other participants.
One idea: guide the online graph search using an offline methods to improve safety.
- This is based on the work of (Bouton, Karlsson, et al., 2019), where offline VI (value iteration) is used to compute P_collision(s, a).
- This safety criterion is then used to limit the set of safe available actions.
- In the presented work, the author reason over the belief instead of the state.
Another idea: Use a Kalman Filter (instead of some particle filters) for belief updater.
One quote:

"Using an online method based on sparse sampling may lead to safety issues. Rare events with critical consequences may not be sampled leading to sub-optimal and potentially dangerous decisions."

One promising tool: POMDPs.jl
- POMDPs.jl is an interface for defining, solving, and simulating MDPs and POMDPs on discrete and continuous spaces. It has been developed and is supported by a team from Stanford.
Two ideas for future works:
- In their repo, the authors suggest combining learning (e.g. model-free RL used as a heuristic and/or for rollout) with planning (MCTS), mentioning the success of AlphaGo Zero.
- Another improvement concerns the transition model for the observed vehicles. Instead of CV (constant velocity) models, one could assume surrounding vehicles are following a driver model (e.g. IDM) and the task would be to infer its hidden parameters.

"On Monte Carlo Tree Search and Reinforcement Learning"

[ 2017 ] [📝] [ 🎓 Universities of Ljubljana and Essex ]
[ RL, MCTS, learning, planning ]

Click to expand


Four parameters introduced in a TD-Tree Search (`TDTS`) algorithm related to `forgetting`, `first-visit updating`, `discounting` and `initial bias`. Source.

Author: Vodopivec, T., Samothrakis, S., & Ster, B.

Goal: The authors aim at promoting a "unified view of learning, planning, and search".
- First, the difference planning / learning is discussed.
  - It depends on the source of experience: simulated / real interaction.
- Then, sample-based (RL and MCTS) search algorithms can all be described as a combination of:
  - 1- A learning algorithm, = how the estimates get updated from gathered experience.
  - 2- A control policy, = how actions get selected.
  - 3- And a representation policy = how is the underlying representation model adapted.
Some RL/MCTS similarities:
- They are somehow connected to MDP formulations.
- Both cope with the exploration-exploitation dilemma.
- Both are value-based and share the concepts of:
  - Policy evaluation: backpropagation phase
  - Policy improvement: selection phase.
- Both exhibits anytime behaviours.
Two major RL/MCTS differences:
- RL methods do not recognize the playout phase, i.e. a separate policy for non-memorized (i.e., non-represented) parts of space.
  - In MCTS it is often called the "default policy".
  - It would be beneficial to have a more complex default policy (where expert knowledge could be included).
- They present two different memorization approaches and two different approximation accuracies:
  - RL methods based on value function approximation (e.g. NN) can weakly describe the whole state space.
    - The RL theory should acknowledge a non-represented (i.e., non-memorized) part of the state space, i.e. the part that is not described (estimated) by the representation model at a given moment.
  - MCTS algorithms initially approximate only a part of the state space (with high accuracy).
    - Therefore MCTS maintains the distinction between a memorized and non-memorized part of the state space.
    - The state-space representation is changed online: it is an "adaptive (incremental) representation method".
    - Indeed “incomplete” representations can be beneficial: it might be better to accurately approximate the relevant part of the space and less accurately (or not at all) the remaining part.
Contribution: Based on this comparison, the authors introduce a framework called "temporal-difference tree search" (TDTS) which aims at combining the advantages of both RL and MCTS methods.
- How it extends classic MCTS methods:
  - Replace MC backups with bootstrapping backups.
    - This is the idea of TD search: do not wait for the end of the trajectory to backup but instead update state value estimates based on previous estimates.
      - TD errors are decayed (γ) and accumulated (λ). It boils down to standard UCT if λ = γ = 1.
    - The motivation is to reduce the variance of the estimates, I guess similarly to the introduction of a TD-based Critic in Actor-Critic methods.
    - This is done using "Eligibility Traces" ("traces" because it tracks which states were previously visited and gives them credit, i.e. "eligibility"), as in n-step SARSA.
      - The authors note that the eligibility trace decay rate λ can be hard to tune.
- How it extends classic RL methods:
  - Contrary to TD search methods, TDTS uses:
    - Some playout and expansion phases.
      - It has some representation policy for incremental or adaptive representation of the non-memorized part of the state space.
    - A playout policy and playout values (as opposed to the already existing control policy).
      - The idea is to replace the missing value estimates in the non-memorized part of the state space with "playout values".
      - These values can be regarded as a placeholder (entry point) for expert knowledge.
- All in all: recreate the four MCTS iteration phases:
  - (1) Selection – control in the memorized part of the search space.
  - (2) Expansion – changing the representation (adaptive incremental model).
  - (3) Playout – control in the non-memorized part of the search space.
  - (4) Backpropagation – updating the value estimates with bootstrap.
- TDTS is applied to UCT (UCB selection policy in Tree search), leading to Sarsa-UCT(λ).
One takeaway: as identifies in previous summaries, one idea (e.g. in AlphaGo) is to combine:
- Prior knowledge (value estimates from the RL-pre-trained network),
- And online feedback (MC evaluations based on playouts).
Two terms I learnt:
- "Transposition tables":
  - As I understand, it plays the role of generalisation in function approximation: two similar states should have similar values.
  - It originates from search of the game tree, where it is possible to reach a given position in more than one way. These are called transpositions.
  - The transposition table is a kind of cache: on encountering a new position, the program checks the table to see whether the state has already been analysed. If yes, the value (stored) can be used instead of calculating it (which would require expending a subtree).
- "afterstates":
  - I understand it as a third metric to quantify the "value":
    - V(s): state value of being in state s and following policy π.
    - V(s'): afterstate value of arriving at state s' and thereafter following policy π.
    - Q(s, a): action value of being in state s, taking action a and thereafter following policy π.
  - This is used in the presented Sarsa-UCT(λ) algorithm.

"Online decision-making for scalable autonomous systems"

[ 2017 ] [📝] [🎞️ (slides)] [ 🎓 University of Massachusetts ] [ 🚗 Nissan ]
[ POMDP, scaling, scene decomposition ]

Click to expand

Two solvers have been developed offline: one to deal with one vehicle (Decision-Problem 1 (DP1)), and another that can deal with one pedestrian (DP2). DPs are instantiated for each detected entity: here two cars and one pedestrian. At each timestep, three recommendations are issued. The most conservative one is kept and implemented (here stop). Source.

Two solvers have been developed offline: one to deal with one vehicle (Decision-Problem P1), and another that can deal with one pedestrian (P2). DPs are instantiated for each detected entity: here two cars and one pedestrian. Therefore, at each timestep, three recommendations are issued. The most conservative one is kept and implemented (here stop). Source.


The proposed solution offers number advantages over the direct use of a massive monolithic `POMDP` for planning and learning. First, it remains tractable by growing linearly in the number of decision-making problems encountered. Secondly, its component-based form simplifies the design and analysis and offers easier interpretability. Source.


The authors consider that urban* deployment of AVs requires mid-level decision-making. Hence both `state` and `action` are rather abstract. Source.*

Author: Wray, K. H., Witwicki, S. J., & Zilberstein, S.

Motivation:
- Scale!
- More precisely, provide a general solution to intersection decision-making that can cope with complex real-world scenarios, with varying number of lanes and number of entities (e.g. car and pedestrian).
One option: a single (big) policy.
- "A single all-encompassing POMDP with these state factors quickly becomes utterly infeasible, and will vary greatly among intersections."
- Instead, the authors suggest decomposing the problem.
Main idea:
- 1- Decompose the scene into canonical simple scenarios.
- 2- Solve each instance separately with solvers that have been learnt offline. It leads to one recommendation per instance.
- 3- Aggregate all the recommendations using some (conservative) "aggregation operator".
  - It could have been min() if the action were about the acceleration.
  - Here, the action is more abstract (go, stop) and the operator uses lexicographic preference for safest action (concept called LEAF): stop > go.
About the POMDP offline solver:
- The authors parallelize the Point-Based Value Iteration (PBVI) algorithm using GPU.
- They called it nova: paper slides.

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My Bibliography for Research on Autonomous Driving

Motivation

`Architecture` and `Map`

`Behavioural Cloning` `End-To-End` and `Imitation Learning`

`Inverse Reinforcement Learning` `Inverse Optimal Control` and `Game Theory`

`Prediction` and `Manoeuvre Recognition`

`Rule-based` `Decision Making`

`Model-Free` `Reinforcement Learning`

`Model Based` `Reinforcement Learning`

`Planning` and `Monte Carlo Tree Search`

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My Bibliography for Research on Autonomous Driving

Motivation

Architecture and Map

Behavioural Cloning End-To-End and Imitation Learning

Inverse Reinforcement Learning Inverse Optimal Control and Game Theory

Prediction and Manoeuvre Recognition

Rule-based Decision Making

Model-Free Reinforcement Learning

Model Based Reinforcement Learning

Planning and Monte Carlo Tree Search

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`Architecture` and `Map`

`Behavioural Cloning` `End-To-End` and `Imitation Learning`

`Inverse Reinforcement Learning` `Inverse Optimal Control` and `Game Theory`

`Prediction` and `Manoeuvre Recognition`

`Rule-based` `Decision Making`

`Model-Free` `Reinforcement Learning`

`Model Based` `Reinforcement Learning`

`Planning` and `Monte Carlo Tree Search`

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