Beyond Safe Execution: Safe Online Reinforcement Learning for Autonomous Driving with Value Calibration and Adaptive Guidance

Online reinforcement learning (RL) enables driving policies to adapt to changing deployment conditions, but requires safe exploration throughout training. Safety filtering alone does not ensure stable policy improvement or reduced dependence on corrections. We propose Safe Online Adaptive Refinement (SOAR-Flow), a framework for safe and stable online fine-tuning of offline-initialized Flow-based driving policies. A control barrier function (CBF)--model predictive control (MPC) filter corrects potentially unsafe actions before execution. To support stable policy updates, we introduce Gaussian-return-aware value calibration based on calibrated Q-learning (Cal-QL), using recorded behavior returns and predicted return variability to smooth conservative critic updates and mitigate policy degradation. Paired corrective learning teaches feasible action adjustments, while an intervention-to-go predictor encourages decisions requiring fewer future corrections. Its predictions also regulate guidance strength, allowing recovery supervision to weaken as predicted dependence decreases. Together with gradually reduced calibration regularization, this supports continued task improvement while preserving hard execution constraints. We analyze conditions for safe execution at control steps and diminishing auxiliary learning influences. We evaluate SOAR-Flow in simulation and on an uncrewed ground vehicle (UGV). The results demonstrate improved driving performance and reduced reliance on safety corrections during protected online fine-tuning. The demonstration video and project overview are available at https://lzqw.github.io/SOAR-Flow/.