user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: Model-Predictive Policy Learning with Uncertainty Regularization for Driving in Dense Traffic

Paper content:
Learning a policy using only observational data is challenging because the distribution of states it induces at execution time may differ from the distribution observed during training.
In this work, we propose to train a policy while explicitly penalizing the mismatch between these two distributions over a fixed time horizon.
We do this by using a learned model of the environment dynamics which is unrolled for multiple time steps, and training a policy network to minimize a differentiable cost over this rolled-out trajectory.
This cost contains two terms: a policy cost which represents the objective the policy seeks to optimize, and an uncertainty cost which represents its divergence from the states it is trained on.
We propose to measure this second cost by using the uncertainty of the dynamics model about its own predictions, using recent ideas from uncertainty estimation for deep networks.
We evaluate our approach using a large-scale observational dataset of driving behavior recorded from traffic cameras, and show that we are able to learn effective driving policies from purely observational data, with no environment interaction.
In recent years, model-free reinforcement learning methods using deep neural network controllers have proven effective on a wide range of tasks, from playing video or text-based games BID26 BID29 to learning algorithms (Zaremba et al., 2015) and complex locomotion tasks Zhang et al., 2015) .
However, these methods often require a large number of interactions with the environment in order to learn.
While this is not a problem if the environment is simulated, it can limit the application of these methods in realistic environments where interactions with the environment are slow, expensive or potentially dangerous.
Building a simulator where the agent can safely try out policies without facing real consequences can mitigate this problem, but requires human engineering effort which increases with the complexity of the environment being modeled.Model-based reinforcement learning approaches try to learn a model of the environment dynamics, and then use this model to plan actions or train a parameterized policy.
A common setting is where an agent alternates between collecting experience by executing actions using its current policy or dynamics model, and then using these experiences to improve its dynamics model.
This approach has been shown empirically to significantly reduce the required number of environment interactions needed to obtain an effective policy or planner BID1 BID7 BID28 BID6 .Despite
these improvements in sample complexity, there exist settings where even a single poor action executed by an agent in a real environment can have consequences which are not acceptable. At the
same time, with data collection becoming increasingly inexpensive, there are many settings where observational data of an environment is abundant. This suggests
a need for algorithms which can learn policies primarily from observational data, which can then perform well in a real environment. Autonomous driving
is an example of such a setting: on one hand, trajectories of human drivers can be easily collected using traffic cameras BID14 , resulting in an abundance of observational data; on the other hand, learning through interaction with the real environment is not a viable solution.However, learning policies from purely observational data is challenging because the data may only cover a small region of the space over which it is defined. If the observational
data consists of stateaction pairs produced by an expert, one option is to use imitation learning BID36 . However, this is well-known
to suffer from a mismatch between the states seen at training and execution time BID37 . Another option is to learn
a dynamics model from observational data, and then use it to train a policy BID31 . However, the dynamics model
may make arbitrary predictions outside the domain it was trained on, which may wrongly be associated with low cost (or high reward) as shown in FIG0 . The policy network may then
exploit these errors in the dynamics model and produce actions which lead to wrongly optimistic states. In the interactive setting,
this problem is naturally self-correcting, since states where the model predictions are wrongly optimistic will be more likely to be experienced, and thus will correct the dynamics model. However, the problem persists
if the dataset of environment interactions which the model is trained on is fixed.In this work, we propose to train a policy while explicitly penalizing the mismatch between the distribution of trajectories it induces and the one reflected in the training data. We use a learned dynamics model
which is unrolled for multiple time steps, and train a policy network to minimize a differentiable cost over this rolled-out trajectory. This cost contains two terms: a
policy cost which represents the objective the policy seeks to optimize, and an uncertainty cost which represents its divergence from the states it is trained on. We measure this second cost by
using the uncertainty of the dynamics model about its own predictions, calculated using dropout. We apply our approach in the context
of learning policies to drive an autonomous car in dense traffic, using a large-scale dataset of real-world driving trajectories which we also adapt into an environment for testing learned policies 1 . We show that model-based control using
this additional uncertainty regularizer substantially outperforms unregularized control, and enables learning good driving policies using only observational data with no environment interaction or additional labeling by an expert. We also show how to effectively leverage
an action-conditional stochastic forward model using a modified posterior distribution, which encourages the model to maintain sensitivity to input actions.
In this work, we proposed a general approach for learning policies from purely observational data.
The key elements are:
i) a learned stochastic dynamics model, which is used to optimize a policy cost over multiple time steps,
ii) an uncertainty term which penalizes the divergence of the trajectories induced by the policy from the manifold it was trained on, and
iii) a modified posterior distribution which keeps the stochastic model responsive to input actions.
We have applied this approach to a large observational dataset of real-world traffic recordings, and shown it can effectively learn policies for navigating in dense traffic, which outperform other approaches which learn from observational data.
However, there is still a sizeable gap between the performance of our learned policies and human performance.
We release both our dataset and environment, and encourage further research in this area to help narrow this gap.
We also believe this provides a useful setting for evaluating generative models in terms of their ability to produce good policies.
Finally, our approach is general and could potentially be applied to many other settings where interactions with the environment are expensive or unfeasible, but observational data is plentiful.

assistant: text: A model-based RL approach which uses a differentiable uncertainty penalty to learn driving policies from purely observational data.