Seat Belt Entanglement in Rollover Accidents: Physical Evidence and Occupant Kinematics

In rollover accidents, physical evidence of seat belt usage is occasionally difficult to discern. Typically, if a seat belt is used by an occupant in an accident, various seat belt components will display characteristic marks in well-defined locations. These marks are known as “witness marks” or “occupant load marks.” Witness marks in a rollover accident may be faint in comparison to those caused by the occupant restraint forces in high-energy planar collisions. Additionally, in situations where a seat belt buckle is alleged to have unlatched early in a rollover accident, the lack of clear occupant load marks may in some cases be attributed to an alleged “buckle release” that occurred very early in the rollover sequence, so that the seat belt did not sustain loading while in a latched condition. However, a documented Case Study and preliminary testing indicate that release of a seat belt buckle during a rollover accident leads to entanglement of the occupant's torso and/or outboard arm in the webbing. This entanglement produces distinctive evidence on the seat belt assembly and vehicle, as well as injury to the occupant. To further evaluate occupant load marks unique to entanglement, dolly rollover tests were performed using a vehicle with a reinforced roof. Hybrid III Anthropomorphic Test Devices (ATDs) were seated in the front outboard seats with their respective seat belt tongues connected to their buckles using nylon cable ties to produce a simulated “buckle unlatching” very early in the rollover event. This testing showed that if the roll rate was high enough to produce occupant ejection, both driver and passenger-side occupants became entangled in the unlatched seat belts during ejection from the vehicle. This entanglement created occupant load marks substantially different from typical occupant restraint evidence, with a characteristic pattern observed on the vehicle and seat belt.