Hazard and Safety Analysis of Automated Transit Bus Applications

Becker, Christopher; Nasser, Ahmad; Brewer, John C. · 2020 · ROSA P / United States. Department of Transportation. Federal Transit Administration

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Summary

This report, sponsored by the Federal Transit Administration and prepared by the Volpe National Transportation Systems Center, addresses the safe deployment of Level 0 to Level 2 driving automation systems in transit buses. The research was motivated by the release of ISO 26262 Edition 2 in 2018, which expanded functional safety standards to include buses, and the need to identify hazards specific to transit bus operations compared to light-duty vehicles. The study aimed to accelerate the development of these systems by providing a baseline for hazard analysis and risk assessment tailored to the unique operational context of a generic 40-foot transit bus. The researchers applied the concept phase methodology of ISO 26262, utilizing Hazard and Operability Analysis (HAZOP) and Systems-Theoretic Process Analysis (STPA) to identify vehicle-level hazards. The analysis considered nine categories of driving automation systems, including adaptive cruise control, lane keeping, and docking systems. To tailor the assessment to transit buses, the study incorporated insights from subject matter expert interviews with manufacturers, suppliers, and transit agencies, as well as a literature review. Key operational factors examined included the presence of standing, unrestrained passengers; interactions with vulnerable road users (VRUs) in high-density environments; and unique system interfaces such as door controls. The risk assessment evaluated hazards based on severity, exposure, and controllability to determine Automotive Safety Integrity Levels (ASILs). The study identified 18 potential vehicle-level hazards, two of which were unique to transit buses: "vehicle motion when passenger door is open" and "vehicle too far from the curb at station/stop." While most hazards overlapped with those found in light vehicles, the risk assessment revealed that steering-related hazards required a more stringent ASIL rating (ASIL C) for transit buses compared to light vehicles (ASIL B). Although transit buses faced higher severity and exposure ratings due to frequent VRU interactions, these risks were offset by improved controllability ratings attributed to the presence of skilled, trained operators. However, the authors noted this tradeoff may not apply to higher automation levels where human operators are absent. The analysis also highlighted that current ISO 26262 guidance lacks specificity for exposure assessments in fixed-route scenarios, such as repeated encounters with railroad tracks. The findings indicate that while many functional safety measures from light vehicles are transferable, they require modification to account for transit bus-specific constraints, such as longer stopping distances and the need to protect standing passengers from excessive acceleration or deceleration. The report concludes that engineered safety measures are more effective than operator training for mitigating system failures. It provides specific safety goals and functional safety measures for transit bus applications, serving as a reference for manufacturers to ensure safe system design and deployment. The study underscores the importance of addressing unique transit bus dynamics, such as wide turning radii and door-system interfaces, to facilitate the safe integration of automation technologies in public transit.

Key finding

Transit bus operations introduce unique vehicle-level hazards, specifically vehicle motion with open passenger doors and insufficient docking proximity to the curb, which require additional functional safety measures beyond those used for light vehicles.

Methodology

review

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• bus coach
• driverless ads
• automation surprise