Longer Trucks
In the Surface Transportation Assistance Act of 1982, Congress permitted nationwide use of trucks pulling dual 28-foot trailers on Interstate highways. At the same time, Congress established more uniform limits on maximum truck dimensions, which effectively raised the maximums in a few states that had opposed such increases. Twin trailer trucks (twins) operating with 28-foot trailers raised a number of operational and safety concerns, particularly on the more densely trafficked Interstates on the East Coast, where these vehicles had not previously been permitted. Because such trailers offer more cubic space, they might be expected to see widespread use by carriers of lightweight cargoes. In the 1982 legislation allowing twin trailers, therefore, Congress called for a TRB study to evaluate the effects of their use.
The committee formed to conduct this study concluded that twins would see fairly limited application in the trucking industry (Special Report 211: Twin Trailer Trucks: Effects on Highways and Highway Safety; TRB 1996; TRB 1996; TRB 1996). Although twins provide more storage capacity, the general freight carriers most likely to use them accounted for only about 15 percent of intercity combination-truck travel in the early 1980s. The committee found further that the greater carrying capacity of twins would probably cause about 2 percent more pavement damage than that due to the trucks they replaced but would result in no additional effects on bridges. Moreover, twins would probably not replace other trucks on a one-to-one basis; instead, slightly fewer twins would be required to move the same amount of cargo, resulting in a modest reduction in total truck miles traveled.
Truck safety is a serious issue. Large trucks are involved in roughly 5,000 fatal crashes each year. Prior experience with twins and computer modeling of their handling implied that they might have a somewhat higher crash rate. Because they would allow more cargo to be moved per shipment, however, the committee concluded that the reduction in truck travel would offset this somewhat higher risk.
The TRB twin trailer truck study highlighted one of the key policy debates surrounding the use of larger trucks. Because the incremental (marginal) cost of adding longer trailers is relatively small, the productivity potential of larger trailers is quite substantial. The intensely competitive private market drives shippers to seek such gains. The safety consequences, however, are not so obvious. Thus policy makers are faced with making choices that involve nearly certain benefits but uncertain costs. Better data on truck safety and truck travel would help reduce this uncertainty. (Safety issues associated with trucking are discussed in the section on managing risk.)