The Effect of Unrestrained Rear-Seat Passengers on Driver Mortality

BACKGROUND: It is well documented that seat belt usage effectively reduces the severity of motor vehicle occupant injuries and fatalities in roadway crashes. This research examines how the presence of an unrestrained rear-seat passenger seated directly behind an airbag and/or belt-restrained driver affects the driver's risk of injury in two different idealized crash scenarios. Empirical data used in the study was obtained from four sled tests conducted with various size Hybrid III crash test dummies. METHODS: Three tests simulated a frontal (head-on) impact between two vehicles. The first established the baseline condition: a driver dummy restrained by a belt and an airbag system, with an identical belt-restrained dummy seated directly behind. The other two frontal-mode tests involved different size driver dummies restrained in the same manner, with different size unrestrained dummies behind them. A fourth test featured an angled driver-side impact crash with a restrained driver and unrestrained rear seat passenger. RESULTS: In both of the latter cases the driver incurred a high likelihood of severe head and chest injuries relative to that inferred in the baseline exposure. The last test featured two identical dummies in a simulated lateral (driver-side) inter-vehicular impact using a belt-restrained (only) driver and an unrestrained rear-seat passenger. Driver mortality was not significantly affected in this configuration. CONCLUSION: Unrestrained rear-seat passengers place themselves as well as their driver at great risk of serious injury when involved in a head-on crash.