SAFETY-BELT EFFECTIVENESS - THE INFLUENCE OF CRASH SEVERITY AND SELECTIVE RECRUITMENT

While theoretical considerations show that the effectiveness of occupa nt protection devices declines from 100% at very low crash severity to 0% at high severity, empirical details have been lacking. When overal l in-use effectiveness is estimated by applying traditional methods to data sets that lack a measure of severity, large biases are introduce d because non-wearing drivers are riskier drivers, an effect that has been called selective recruitment. These effects are investigated empi rically using National Accident Sampling System (NASS) data in which c rash severity is measured by delta-v, the estimated change in the spee d of the car as a result of the crash. Supplemental results are obtain ed using published police-reported data containing a more easily obtai ned but less objective severity measure. Both data sets provide inform ation on driver fatalities and injuries, thus allowing four comparison s of effectiveness estimates based only on total casualties with ones taking into account the different severities of crashes by belted and unbelted drivers. The data show consistently that the probability that a driver is belted declines as crash severity increases. Belt effecti veness estimates ignoring this effect are biased upwards by large amou nts (for example, 60% compared to 40% for injuries using NASS data). B elts appear more effective at preventing fatalities than at preventing injuries. The results are consistent with a prior estimate, derived u sing a method unaffected by the biases discussed here, which found tha t, averaged over all crashes, safety belts reduce driver fatality risk by (42+/-4).