Seat belts dramatically reduce the risk of injury in motor vehicle collisions. Forensic investigation of automobile collisions commonly involves analysis of restraint system physical evidence to determine if a seat belt restraint system was used during a collision event.
This study presents examples of physical evidence collected from seat belt systems involved in real-world collisions, which were initially alleged to affirm proper belt use, but were ultimately proven to be evidence of non-use.
Several laboratory demonstrations were conducted to investigate physical evidence created on restraint system hardware as a result of pretensioner deployments of non-used seat belts in a variety of incompletely stowed conditions. Seat belt pretensioners are engineered to tighten the seat belt around the occupant during the early stages of moderate to severe crashes. The demonstrations show the initial positions of the seat belt assemblies necessary to produce the distinct physical evidence documented in the real-world samples.
The photographs above show examples of collision-induced markings on non-used restraint systems from this study. These markings could easily be misinterpreted as evidence of seat belt use.
The characteristics of collision-induced markings generated on seat belt systems are not solely dependent on the belted status of the occupant, but also the technological features incorporated in the seat belt assembly. Pretensioner deployment can leave physical evidence on restraint system hardware and webbing in the absence of occupant loading and may be misinterpreted as evidence of seat belt use.
The presence of physical evidence on restraint system hardware and webbing is inadequate to conclude that a seat belt restraint was in use. Careful consideration of restraint system physical evidence combined with restraint system geometry and incorporated technological features can distinguish physical evidence related to seat belt use or non-use during a collision event.
Gregg R. and Petroskey K. Assessment of Collision Markings on Non-Used Vehicle Restraint Systems. Society of Automotive Engineers. SAE 2020-01-0975.