Injury Rates by Crash Severity, Belt Use and Head Restraint Type and Performance in Rear Impacts

This study assesses the exposure distribution and injury rate (MAIS 4+F) to front-outboard non-ejected occupants by crash severity, belt use and head restraint type and damage in rear impacts using 1997-2015 NASS-CDS data. Rear crashes with a delta V <24 km/h (15 mph) accounted for 71% of all exposed occupants. The rate of MAIS 4+F increased with delta V and was higher for unbelted than belted occupants with a rate of 11.7% ± 5.2% and 6.0% ± 1.5% respectively in 48+ km/h (30 mph) delta V. Approximately 12% of front-outboard occupants were in seats equipped with an integral head restraint and 86% were with an adjustable head restraint, irrespective of crash severity. The overall injury rate was 0.14% ± 0.05% and 0.22% ± 0.06%, respectively. It was higher in cases where the head restraint was listed as “damaged”. Thirteen cases involving a lap-shoulder belted occupant in a front-outboard seat in which “damage” to the adjustable head restraint was identified. Review of these cases showed that intrusion and crash severity were important factors and that the term “damage” was often used by NASS investigators to represent significant occupant loading.

A mathematical simulation was conducted using MADYMO to understand the relationship between seatback stiffness and head restraint loading using a typical modern seat design. This simulation was also used to investigate the relationship between the head restraint stiffness and occupant head acceleration and neck loads and moments. This analysis supports the findings that damage to an adjustable head restraint is most likely a result of a front seat that has been supported by intruding vehicle structures and is indicative of crash severity. Furthermore, the response of biomechanical metrics to stiffness variation demonstrates a complex coupling, in particular revealing that an increase to head restraint stiffness does not directly lead to improvement in biomechanical responses. As such, the assessment of the effect of head restraint stiffness on occupant response merits additional research.