TRB Special Report 211: Twin Trailer Trucks examines the potential safety effects of the new federal truck size rules (the Surface Transportation Act of 1982 requires states to permit twin 28-ft trailers on Interstate highways and on principal roads). The study also addressed other effects of the new rules on highway transportation including the trucking industry use of twins, safety consequences of twins, pavement wear and other highway features affected by twins, and safety and pavement wear affected by 48-ft long semitrailers and 102- in. wide trucks. The use of twin trailer trucks will be concentrated in the general freight common carriers sector and will result in a 9% reduction in combination-truck miles in the portion of their hauling that is switched from tractor-semitrailers to twins. It is not expected that either the scope of the current highway network or restrictions on access to that network will have an impact on travel by twins. The increased use of twins will have little overall effect on highway safety. However, twins will accelerate pavement wear and increase rehabilitation costs. The use of 102-in. wide trucks and 48- ft semitrailers (also permitted by the 1982 law) also increases pavement wear. Recommendations are presented that would help coordinate the efforts of various data producers and users of programs that collect nationwide information on truck travel, safety, and highway impact, and thus enhance their ability to provide data addressing policy questions. Recommendations are also made for improving the safety of the vehicles. The various chapters of the book cover the study method and report organization; the historical and regulatory background; use characteristics and cost impacts; safety impacts; highway condition, operation and design impacts; long-term monitoring; and conclusions drawn from the study.