Criticality Metrics for Automated Driving: A Review and Suitability Analysis of the State of the Art

Abstract

The large-scale deployment of automated vehicles on public roads has the potential to vastly change the transportation modalities of today’s society. Although this pursuit has been initiated decades ago, there still exist open challenges in reliably ensuring that such vehicles operate safely in open contexts. While functional safety is a well-established concept, the question of measuring the behavioral safety of a vehicle remains subject to research. One way to both objectively and computationally analyze traffic conflicts is the development and utilization of so-called criticality metrics. Contemporary approaches have leveraged the potential of criticality metrics in various applications related to automated driving, e.g. for computationally assessing the dynamic risk or filtering large data sets to build scenario catalogs. As a prerequisite to systematically choose adequate criticality metrics for such applications, we extensively review the state of the art of criticality metrics, their properties, and their applications in the context of automated driving. Based on this review, we propose a suitability analysis as a methodical tool to be used by practitioners. Both the proposed method and the state of the art review can then be harnessed to select well-suited measurement tools that cover an application’s requirements, as demonstrated by an exemplary execution of the analysis. Ultimately, efficient, valid, and reliable measurements of an automated vehicle’s safety performance are a key requirement for demonstrating its trustworthiness.

Operational Design Domain-Driven Coverage for the Safety Argumentation of Automated Vehicles

“…Consequently, this means that using the TTC metric for evaluating CS' in complex urban intersections turns out to be worthless, as no new information can be gathered by applying this metric. A fact discussed as well in [47].…”

“…For a-posteriori traffic data analysis, the post encroachment time (PET) calculates the time gap between a traffic participant leaving and another entering a pre-defined area [46]. A comprehensive overview of criticality metrics is given in [47]. The authors of [47] also define a set of properties relevant to such metrics.…”

“…A comprehensive overview of criticality metrics is given in [47]. The authors of [47] also define a set of properties relevant to such metrics. These properties are (excerpt):…”

“…In this article, the focus is on the test evaluation of certain scenarios, based on pre-defined pass/fail criteria to determine the overall coverage of the target ODD for a specific ADS. If the focus is on an urban use case, including complex intersections, TTC is considered to not be an ideal choice, since it suffers from reduced validity and sensitivity [47].…”

Coupled Lateral and Longitudinal Controller for Over-Actuated Vehicle in Evasive Maneuvering With Sliding Mode Control Strategy

“…Safety metrics include, e.g., time-to-collision (TTC) and time-to-brake (TTB). TTC and TTB are among the most used safety metrics in critical situations [85]- [87]. The benefits of TTC include its formulation simplicity and effectiveness for rear-end scenarios.…”