ACCIDENT RATES OF MULTIUNIT COMBINATION VEHICLES DERIVED FROM LARGE-SCALE DATA BASES (WITH DISCUSSION AND CLOSURE)

The operating characteristics of multitrailer vehicles could be expected to make them more dangerous than other vehicles, but previous accident involvement studies have produced mixed results, with no consistently strong indications of greater hazard. A review of these studies, however, indicates sufficiently severe limitations in their sample sizes and data reliability to readily explain the great degree of scatter in their findings. The size and reliability issues of previous studies are overcome by using large national data sources to calculate overall involvement rates of various vehicle configurations. No suitable sources of nonfatal accidents or disaggregate travel information were located. Use of national data rather than state and highway-type-specific data obscures the safety effects of differences in vehicle operations but at least allows an overall comparison of fatal accident involvement rates. Because current multitrailers are concentrated more than single trailers on the safest highways, rural Interstates, multitrailers appear in this study to be safer than they would if differences in operations were considered. The most reliable sources of fatal accident and travel data indicate that multitrailers, single trailers, and single-unit trucks have fatal accident involvement rates of 9.96, 6.01, and 3.00 per 100 million mi traveled, respectively. The ratio of fatal accident involvement rates for multitrailers to single trailers is 1.66. The multitrailer to single-unit truck ratio is 3.32. Most previous studies have indicated doubles or multitrailer fatal accident rates to be higher than singles, but with less difference. The higher ratios here can be attributed in part to larger and more reliable data sources than have been used in the past.