Summary: Twenty states, most of them in the West, allow longer combination vehicles to operate but have very different restrictions on the types of vehicles allowed, the routes on which they can operate, and their length and weight. Although such vehicles have been on the roads for more than 30 years, little definitive information on their safety is available. Due to data problems and different study approaches, existing studies on the accident rates of multiple-trailer trucks have reached widely different conclusions about the safety of longer combination vehicles. Longer combination vehicles may be less stable and maneuverable than single-trailer trucks, which can affect their safety. However, the operational characteristics of longer combination vehicles can be affected by many factors, including the type of vehicle, the driver, the distribution of the load, the equipment used, and road conditions. Until data shortcomings are overcome, the actual impact of all types of longer combination vehicles on highway safety will remain unknown. Congress has passed legislation prohibiting the expansion of longer combination vehicle use. Although this law also requires several actions to improve transportation data, it does not specify the kind of truck data required.
