Serious Injury in Very-Low and Very-High Speed Rear Impacts

This paper analyzed rear crashes for the risk of serious injury (AIS 3+) by delta V. Rear impacts were analyzed for occupants sitting in front seats of light vehicles. Data were obtained from the National Automotive Sampling System-Crashworthiness Data System (NASS-CDS) for calendar years 1991-2004. Tow-away crashes with \mL15 mph rear delta V account for 67% of rear impacts and 15% of serious injury. Even for crashes \ml30 mph delta V, the risk for serious injury is only 0.24% (less than 1 per 420 exposed occupants). Risks increase for higher delta Vs. Individual cases in the 1997-2004 NASS-CDS electronic database were reviewed for serious injury in crashes with \mL15 mph delta V and \mG35 mph for light vehicles with calendar year \mg1996 to better understand injury mechanisms. Nine cases were available where a front-seat occupant was seriously injured in \mL15 mph rear delta V impact. Most cases involved older occupants, some of whom had stenosis of the cervical spine. These occupants experienced spinal cord injury and paralysis with or without fracture of the cervical spine. Frailty of the cervical spine was a factor in the very low speed crash injuries. There were 28 occupants in 26 crashes with serious injury at delta V \mG35 mph. In 7 of the cases, the most serious injury was to the cervical spine. Intrusion of rear structures was common leading to the front-seat occupant being supported upright or being pushed forward causing serious injury. Intrusion forces were associated with the injury. In the most severe rear crashes, rearward rotation of the seatback was not a factor in the occupant's injury as the early occurrence of intrusion limited seatback rotation. This paper has identified a class of seriously injured occupants in very low speed rear crashes. Advanced age and degeneration of the cervical spinal canal can lead to spinal cord injury and paralysis in crashes otherwise not causing serious injury in normal adults. Yielding seats diminish the risk of cervical injury in frail occupants. Use of stiffer seats would increase the risk of injury in occupants with spinal stenosis. The very high speed crashes involved substantial intrusion, which occurred early in the crash and supported the seatback from rotating rearward. The collision forces and intrusion were associated with injury to front-seat occupants.

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