Abstract

Diffusion-based trajectory planners achieve strong nominal performance in autonomous driving, but sparse safety intervention remains difficult to evaluate and realize effectively. This study addresses this problem by proposing a safety-aware event-triggered intervention framework on top of a fixed DiffusionDriveV2 planner. The method uses candidate-level risk signals and an auxiliary semantic risk trigger to decide when intervention should be activated, and realizes the intervention through conservative re-selection and mild action-space augmentation. To evaluate sparse interventions beyond global validation metrics, we further construct normal, conservative, and actual trajectories and introduce triggered-subset counterfactual evaluation. On NAVSIM navtest, global planner metrics remain nearly unchanged across sparse trigger policies, but the semantic trigger achieves better triggered-subset final score, TTC, and progress than matched-random and TTC-based triggers. Qualitative cases show that behaviorally distinct safety responses mainly arise from action-space augmentation rather than candidate reranking alone. These results show that the proposed framework can diagnose and partially alleviate the gap between risk recognition and action realization, while revealing that stronger semantic-conditioned action generation is needed to fully overcome the trigger-to-action bottleneck.

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