Safety-Polarized and Prioritized Reinforcement Learning

Motivated by the first priority of safety in many real-world applications, we propose MAXSAFE, a chance-constrained bi-level optimization framework for safe reinforcement learning. MAXSAFE first minimizes the unsafe probability and then maximizes the return among the safest policies. We provide a tailored Q-learning algorithm for the MAXSAFE objective, featuring a novel learning process for optimal action masks with theoretical convergence guarantees. To enable the application of our algorithm to large-scale experiments, we introduce two key techniques: safety polarization and safety prioritized experience replay. Safety polarization generalizes the optimal action masking by polarizing the Q-function, which assigns low values to unsafe state-action pairs, effectively discouraging their selection. In parallel, safety prioritized experience replay enhances the learning of optimal action masks by prioritizing samples based on temporal-difference (TD) errors derived from our proposed state-action reachability estimation functions. This approach efficiently addresses the challenges posed by sparse cost signals. Experiments on diverse autonomous driving and safe control tasks show that our methods achieve near-maximal safety and an optimal reward-safety trade-off.