There have been many studies that have attempted to ascertain any difference in the safety of various large truck configurations. Special attention has been paid to double trailer combinations (doubles) compared to tractor-semitrailer combinations (singles). Most researchers have found little if any difference in their respective accident rates. The major exception is a large, widely quoted study conducted by BioTechnology, Inc., for the Federal Highway Administration. It concluded, among other things, that doubles have significantly higher accident rates than do singles. The research reported in this paper consisted of a thorough review of all aspects of that study, with particular emphasis on the structure and contents of the three major data bases used. These included data on 2,112 large truck accident involvements, vehicle classification count data taken at 78 sites, and driver and truck data (including size and weight) on more than 32,000 trucks. It was found that the conclusions drawn by BioTechnology, Inc., regarding doubles versus singles, as well as most of the other major conclusions, are not supported by the data bases. Although the accident data base may be useful to other researchers, if used with care, the two data bases needed to estimate exposure are totally inaccurate, and results derived therefrom are probably meaningless.

The passage of the 1982 Surface Transportation Assistance Act<.!.>(STAA) by the us Congress accomplished many things. Among these was a mandate that the states not prohibit truck combinations involving two trailers from using the Interstate highways or other highways designated by the Secretary of the US Department of Transportation. Although such truck combinations, often called doubles or double bottoms, were already legal and regularly operating in many states--notably in the midwest and west--they were not legal in many of the southeastern and eastern states. Their legalization by the STAA raised many questions in the minds of concerned officials and citizens, especially questions about the safety of doubles compared with that of the more common tractor-semitrailers (singles)• The literature contains a number of original studies of this issue (~-10) as well as several reviews and summaries (11-13). Most of the studies found little differenCe in accident involvement rates of doubles and singles. The major exception to this trend is a study by Vallette et al.(10), often referred to as the BioTech study. It concluded that" twin trailer combinations have a significantly higher accident rate than single tractor-trailer combinations."(Vallette et al. use the word" twin" instead of" double"; however," twin" is often taken to be the more limited configuration of a tractor plus twin van-type trailers. This distinction will be addressed further at a later point in this paper.) Their findings were met with much skepticism and criticism (14). Nevertheless, they were ultimately" accepted" and widely quoted (11-13). Among those reviewing and criticizing the study were members of the trucking industry. Subsequently, Midwest Research Institute (MRI) was asked to conduct an in-depth, critical review of the work. This paper covers some of the major findings of that review.