Evaluation of the Acceleration Severity Index Threshold Values Utilizing Event Data Recorder Technology

The Acceleration Severity Index (ASI) is used to evaluate the potential for occupant risk in full-scale crash tests involving roadside safety hardware. Despite its widespread use across Europe, there is a lack of research relating this metric to occupant injury in real-world collisions. Recent installation of Event Data Recorders (EDRs) in a number of late model vehicles presents a different perspective on the assessment of the validity of occupant risk based on the Acceleration Severity Index. EDRs are capable of electronically recording data such as vehicle speed, brake status and throttle position just prior to and during an accident. Of particular interest is the EDRs ability to document the deceleration of a vehicle during a collision event. This paper utilizes EDR technology to investigate the correlation between the ASI threshold limits and the potential for occupant injury in crash events. The longitudinal ASI is found to be a good predictor of overall injury and the intent of the current recommended threshold value of 1.0 appears valid. Limitations include investigation of the longitudinal direction only, lack of injuries in excess of AIS 3, and no control for occupant compartment intrusion. Gabauer and Gabler 3 INTRODUCTION Full-scale crash testing has been the traditional method used to evaluate roadside safety hardware. The goal of crash testing procedures is to evaluate hardware performance in representative worst-case impact scenarios. Although there are minor differences between US and European (CEN) procedures, both test protocols evaluate devices based on vehicle behavior, the response of the device, and the potential for injury to vehicle occupants. The US procedures are prescribed in NCHRP Report 350 (1), while the CEN procedures are presented in EN-1317 (2). Because the goal of roadside hardware is to perform its intended task while minimizing injury to vehicle occupants, the assessment of occupant risk is crucial to the full-scale crash test evaluation of these devices. Unlike vehicle crashworthiness testing, crash tests of roadside safety devices do not utilize a crash test dummy. Roadside hardware collisions typically involve an oblique impact and, to date, no crash test dummies have been validated for use in oblique crash loadings. Instead, occupant risk is based on metrics derived from vehicle kinematics measured during the crash test. Since 1981, the US procedures have calculated occupant risk with the flail space model. The CEN procedures use a variant of the flail space model in conjunction with the Acceleration Severity Index (ASI) to assess occupant injury risk. Although there is evidence that the ASI originated in the US in the late 1960’s, current US procedures do not utilize this acceleration-based model for the assessment of occupant risk. The currently underway NCHRP Project 22-14[2], however, will provide an update to the NCHRP 350 procedures. The preceding project, NCHRP 22-14[1], recommended the need for an update to these procedures. Findings from this study suggest the potential inclusion of an acceleration-based occupant risk metric, such as ASI, in the update assuming sufficient evidence supporting its effectiveness (3). To date, there is little research relating the ASI to actual occupant injury in collision events and the biomechanical basis for the threshold values is not well documented. OBJECTIVE The purpose of this study is to investigate the correlation between the ASI threshold limits and the potential for occupant injury in crash events. BACKGROUND The Acceleration Severity Index Using measured vehicle acceleration information, CEN test procedures (2) indicate the ASI is computed using the following relationship: