Assessment of Collision Markings on Non-Used Vehicle Seat Belt Restraint Systems

Forensic investigators of automobile collisions are commonly tasked with determining whether physical evidence observed on restraint systems is consistent with the occupant’s use or non-use of the seat belt restraint. 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. As the state-of-the-art for seat belt assemblies has changed over time, so has the constellation of physical evidence typically created on seat belt restraint systems. Pretensioner deployment can leave physical evidence on restraint system hardware in the absence of occupant loading. 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. 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 physical evidence observed on the real-world restraint systems were consistent with the markings produced in the laboratory demonstrations. 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. These examples will be useful to investigators for the assessment of physical evidence and diagnosis of seat belt use.