2006 Issue

www.utahengineerscouncil.org 21 UECJOURNAL February 2006 Seat Belt Safety F E A T U R E continued on following page ▲ ➊ the nature of the occupant’s injuries, ➋ the existence and nature of occupant con- tacts inside the vehicle, ➌ detailed recon- struction of the sequence of events of the accident, ➍ occupant kinematics analysis, ➎ andmarkings on seat belt restraint system components. This article shares some in- sights regarding how to more decisively de- termine seat belt usage in a collision, focus- ing mainly on interpreting marks visible on the seat belt restraint system components. During visual inspections of seat belt hardware, it is not unusual for the police investigator or the reconstruction expert to find several individual and distinct ob- servable anomalies or “marks” on the re- straint components. The mere presence of marks does not necessarily indicate that the occupant used the restraint in a collision because a wide variety of marks from nor- mal everyday usage or “noncollision” con- ditions commonly exist in abundance on such components. Consequently, if the investigator is going to rely on his inter- pretation of marks as an indicator of usage in a collision, he must be able to identify which of the marks (if any) occurred only as a result of usage during the accident. This requires the investigator to have spe- cial knowledge of the characteristics of seat belt marks of all kinds. A comprehensive review of all types of seat belt marks is not practical in this article; however some se- lected examples should suffice. The seat belt strap or “webbing” usually exhibits the widest variety and density of noncollisionmarks, a natural consequence of its interaction with the occupant, the guide loop hardware and other proximate interior surfaces. Such marks include per- manent twists, folds and slight disfigurement; broken or disrupted strands (the strap is made up of hundreds of tiny, continuous man-made strands woven together to pro- vide both flexibility and strength); abra- sions, scrapes and impressions; and stains, deposits and discolorations. Figure 1 illus- trates visual similarities between collision and noncollision marks. Seat belt guide loop hardware has two purposes: it situates the strap at the proper locations about the occupant, and provides T his is not true because, although rollover crashes may not be predictable with great precision, vehicle dynamics and sequence of events can often be deter- mined well within a reasonable degree of engineering and scientific probability when based upon careful forensic inspection and correct interpretation of the available physical evidence. There are usually at least three distinct phases of a rollover crash: (1) the pre- rollover phase, (2) the “trip” phase, and (3) the rollover phase; each phase having unique elements that require appropriate methods of analysis. The pre-rollover phase includes the first events that lead to the vehicle over- turning; these events are caused by human, environment, or vehicle factors such as driver inattention and error, steep or uneven terrain, road conditions, mechanical problems, excessive and inappropriate cargo loading, or a combination of these. In this phase, typically the vehicle becomes sliding sideways beyond the driver’s ability to control the vehicle, such as when a series of heavy steering and braking inputs are applied in a typical off-road correction maneuver. Tire marks left by the sliding vehicle are usually the main focus, upon which calculation of vehicle speed is based. A first order analysis is to measure the curvature of the tire marks and apply a basic “critical speed” formula which assumes the vehicle is in a quasi-steady state cornering continued on following page ▲ A Primer on Reconstructing Automobile Rollover Crashes Rollover crashes can be one of the most challeng- ing types of accidents to reconstruct because of their random nature and multi-dimensional ele- ments. Some have described a rollover crash as a “chaotic” event that can’t be reconstructed.