Leveraging Risk Tolerances and Simple Kinematics to Quantify Fault Tolerant Time Intervals for Commercial Trucks 2021-01-0066

The ISO 26262 series of standards for vehicle functional safety codify requirements to avoid unreasonable risk from the failure of electrical or electronic (E/E) systems. E/E failures may cause malfunctioning behavior that manifest as vehicle-level hazardous events. The ISO 26262 second edition includes commercial trucking, which employs significant variation from the passenger car development cycle. The highly distributed nature of E/E system development and integration in commercial trucks complicates forging unified safety concepts. For instance, the Fault Tolerant Time Interval (FTTI) quantifies the minimum time span from the occurrence of a fault to the possible occurrence of a hazardous event. Often, the subjectivity involved in defining unreasonable risk and hazardous event onset frustrates consensus among stakeholders. In order to provide some uniformity in the adoption of ISO 26262 across the commercial truck industry, this paper introduces the Risk Threshold (RT) Method to clarify the boundary between acceptable and unreasonable risk. RT is defined as the acceptable travel distance caused by a malfunctioning behavior. The RT Method includes:

• Selecting a malfunctioning behavior and a corresponding hazardous event from a Hazard Analysis and Risk Assessment (HARA)
• Designing a vehicle-level experiment that simulates the hazard
• Defining a RT that quantifies hazardous event onset
• Applying kinematic equations using the RT and experimental data to calculate FTTI

This paper applies the RT Method to four key hazards: unintended acceleration, unintended motion, unintended direction, and increased stopping distance. For ease of illustration, all motion described in this paper aligns with a truck’s longitudinal axis. The RT Method correlates FTTI to hazardous event onset using objective and repeatable measurements. For commercial trucks, consistently predictable velocity during the FTTI facilitates this correlation. The simplicity of this approach enables stakeholder comparison of differing risk tolerances in terms of RT. Driving consensus on RT then yields a corresponding FTTI.