Abstract:Industry-supplied data allowed a structured statistical comparison of the safety performance of tractor-semitrailers (singles) and doubles by comparing their accident experience on the same routes for 3 years. This paired structure essentially controls for roadway, environment and traffic conditions. Separate comparisons of vehicle safety performance were conducted for access- and non-access-controlled highways, local streets, and parking lots. In general, doubles experienced lower accident rates than singles in 1983 and 1985, but higher accident rates in 1984, which was a year of greatly expanding doubles operation. Doubles' accident rates are significantly lower than singles' accident rates for all types of operating environments over the entire period from 1983 to 1985. For the types of carriers represented in the data and for the conditions characterized by the routes in the sample, the consistent evidence is that doubles had better safety performance than singles except for the trnasition year 1984. The generalization derived from the study is that doubles are generally as safe or safer than singles, even when specifically controlling for roadway, traffic, and environmental conditions. This study was conducted on routes that are approved for doubles' operation. It is, therefore, not appropriate to extrapolate these findings to any specific route.