The simplified point-mass, flail-space model (FSM) was developed in 1981 by Michie and is currently used in the AASHTO Manual for Assessing Safety Hardware (MASH) crash test procedures to assess vehicle occupant injury risk in roadside hardware crash tests. Similar Canadian, Australian, and New Zealand crash test standards also use the FSM. European CEN procedures use a variation of the FSM in conjunction with the Acceleration Severity Index (ASI) to gauge occupant injury risk. Both metrics are used in roadside crash tests as a substitute for an instrumented anthropometric crash test dummy (ATD). Using an ATD in MASH certification compliance testing would significantly increase the cost of crash testing.
Because of advancements in vehicle design, these methods to evaluate occupant injury risk in roadside hardware crashes should be reevaluated. In frontal crashes, the flail-space crash injury metric might be too stringent because it does not consider that occupants are now required to wear seat belts, airbags are used as supplementary restraint systems, and vehicles have crumple zones, all specifically designed to provide controlled ridedown decelerations. In contrast, in side crashes, the flail-space model, which does not account for intrusion, may not be sufficiently stringent.
Despite their long use in the evaluation of occupant risk in full-scale crash tests of roadside safety hardware, there is little information correlating either FSM or ASI to occupant injury. FSM predictions might be unrepresentative of the injury risk experienced by occupants in modern vehicles. The newer ASI was designed for belted occupants, but has not been validated for occupant injury risk in the current vehicle fleet. In addition, both FSM and ASI are acceleration-based measures which are most suited to head and chest impacts. They are less than ideal for predicting the risk of leg injuries, such as those observed in some end terminal collisions. Also, neither metric is suited for predicting injury in crashes where the occupant compartment is compromised, including broken side windows in rigid and semi-rigid barrier impacts and A-pillar cutting that can occur in crashes with cable barrier.
Research is needed to compare predictions from the current MASH occupant risk procedure with data from crash tests and real world crash events where longitudinal and lateral decelerations have been measured with instrumentation in vehicles impacting roadside safety hardware. Evaluation of alternative vehicle-based methods of determining occupant injury risk is also needed. The key anticipated products are revised MASH occupant risk tolerance values and an improved method of determining occupant injury risk in roadside hardware crash tests.
The objective of this research is to compare predictions from the current MASH occupant risk model and alternative models with data from crash tests and real-world crash events where longitudinal and lateral decelerations have been measured with instrumentation in vehicles impacting roadside safety hardware. Required deliverables for consideration by the AASHTO Technical Committee on Roadside Safety (TCRS) are:
An assessment of the MASH occupant risk tolerance values relative to occupant injury risk type as defined by current driver demographics;
Proposed adjustments to current risk tolerance values, if needed;
Development of an improved method of determining occupant injury risk in roadside hardware crash tests for incorporation in MASH, if needed; and
A proposed roadmap for future research to aid in possible updates of the evaluation method in MASH or to incorporate an alternative method in MASH.
The Phase I interim report is available on request to NCHRP. The draft final report is in review.