Various strategies have been developed to improve motor vehicle occupant protection, including an automatic shoulder belt, combined with a manual lap belt or knee bolster. Since 1987, more than 27 million cars have been manufactured with automatic 2-point belt systems and more than 10 million are still in use. Studies have found that drivers do not use the manual lap belt 50-71% of the time. Prior studies examining the effectiveness of automatic systems have produced conflicting estimates. Numerous studies also report injuries from use of the shoulder harness alone. This study was conducted to examine the effectiveness of automatic shoulder belts when used with and without a manual lap belt. Data was collected and analysed from the 1993-96 National Highway Traffic Safety Administration Crashworthiness Data System on front-seat occupants involved in 25,811 tow-away crashes of passenger cars, light trucks, vans, and sport utility vehicles. Results show that the use of automatic shoulder belts without lap belts was associated with a decrease in the risk of death vs. no restraint use, but was not statistically significant for all crashes or for frontal crashes after adjustment for age, sex, vehicle year, airbag deployment, estimated change in vehicle speed during the crash, and principal direction of force. This association was significantly weaker than the 86% lower risk observed for use of automatic shoulder belts with lap belts. Use of automatic shoulder belts without lap belts was associated with an increased risk of serious chest injuries for all crashes. Data indicates that improperly used automatic restraint systems may be less effective than properly used systems and are associated with an increased risk of serious chest and abdominal injuries. (A)
Abstract