Abstract
Accurate trajectory prediction and motion planning are crucial for autonomous
driving systems to navigate safely in complex, interactive environments
characterized by multimodal uncertainties. However, current
generation-then-evaluation frameworks typically construct multiple plausible
trajectory hypotheses but ultimately adopt a single most likely outcome,
leading to overconfident decisions and a lack of fallback strategies that are
vital for safety in rare but critical scenarios. Moreover, the usual decoupling
of prediction and planning modules could result in socially inconsistent or
unrealistic joint trajectories, especially in highly interactive traffic. To
address these challenges, we propose a contingency-aware diffusion planner
(CoPlanner), a unified framework that jointly models multi-agent interactive
trajectory generation and contingency-aware motion planning. Specifically, the
pivot-conditioned diffusion mechanism anchors trajectory sampling on a
validated, shared short-term segment to preserve temporal consistency, while
stochastically generating diverse long-horizon branches that capture multimodal
motion evolutions. In parallel, we design a contingency-aware multi-scenario
scoring strategy that evaluates candidate ego trajectories across multiple
plausible long-horizon evolution scenarios, balancing safety, progress, and
comfort. This integrated design preserves feasible fallback options and
enhances robustness under uncertainty, leading to more realistic
interaction-aware planning. Extensive closed-loop experiments on the nuPlan
benchmark demonstrate that CoPlanner consistently surpasses state-of-the-art
methods on both Val14 and Test14 datasets, achieving significant improvements
in safety and comfort under both reactive and non-reactive settings. Code and
model will be made publicly available upon acceptance.