How unsafe was the scenario? A criticality measure for scenario-based testing of automated vehicles

Scenario-Based Testing (SBT) is the most widely used method for safety assurance of Automated Driving System equipped Vehicles (ADS-Vs). To comply with ISO 21448, a key standard for ADS-V safety assurance, SBT can be formulated as an optimization problem to identify unknown hazardous scenarios within the ADS-V's Operational Design Domain. To identify hazardous scenarios through optimization, an appropriate cost function for scenario hazardous-ness is needed that satisfies specific requirements. Existing criticality measures in the literature do not satisfy these requirements. We interpret scenario hazardous-ness as the proximity to a collision for the ADS-V. We propose a Criticality Measure (CM) using the set of alternative futures of a Reference Vehicle (RV) following the trajectory of the ADS-V. The CM indicates the proximity to a collision, both in time and space. The proposed CM provides a posteriori knowledge about the criticality of a scenario already generated by the optimization where the future evolution of every scene is available. To showcase the developed CM, multiple scenarios in two road networks, an intersection, and a highway segment are evaluated for criticality. We briefly discuss how to incorporate the severity of collisions/potential future collisions in the CM. The possible extensions and limitations of the proposed CM are discussed.