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Preface Through the Surface Transportation Assistance Act of 1982, the US. Congress required states to permit truck-tractors pulling twin 28-ft trailers on Interstate highways and other principal roads. Recognizing that the use of twin trailer trucks might have varied and unpredictable consequences, Congress directed the National Research Council to monitor the effects of twin trailer trucks, "determining the effects of the use of such vehicle combinations on highways and highway safety in urban and rural areas and in different regions of the country. . With support from the Federal Highway Administration, the National Research Council assembled a committee of 19 members with experience in the many areas involved, including vehicular safety, trucking opera- tions, highway administration, highway design, economics, statistics, law, insurance, traffic engineering, and vehicle handling and performance. The Committee for the Twin Trailer Truck Monitoring Study examined potential impacts of twin trailer trucks on trucking industry productivity, traffic operations, pavement wear, and bridge loadings, but devoted par- ticular attention to highway safety. The committee made judgments about the comparative safety of twin trailer trucks, but more definitive conclu- sions must await further data. Because the trucking industry has been adopting twins gradually, and will continue to do so over a number of years, it was difficult to observe directly the full impact of twins during the first few years of nationwide use. Potential differences in safety performance from one region to another were noted in a prelim- V
vi PREFACE mary way, but because route structures and shipment characteristics may create regional disparities in the use of twins that are not currently doc- umented, more detailed regional comparisons of accident experience were not possible. The safety performance of twin trailer trucks compared with that of other large trucks tends to be obscured by confounding factors, including differences in typical driver qualifications, highway routes, loadings, frac- tion of nighttime driving, and carrier safety practices. Making a well- founded judgment about the comparative safety of twins requires reliance on the totality of available information, not on any single statistic. Vehicle- handling studies, driver reactions, accident and use statistics, and trucking- company experience each offer useful insights, and all were considered by the committee in making its judgments. The findings presented here are based on experience with twins oper- ating chiefly on principal roads. The observations made about the com- parative safety of specific vehicle configurations apply mainly to these roads and may not apply to roads built to lesser design standards. Similarly, the safety findings are based on twins carrying general freight, their pre- dominant use, and do not assess the potential risks involved should twins be used to transport hazardous materials. In addition, the committee considered the effects of 48-ft semitrailers and 102-in, wide trucks, which were also legalized for nationwide use at the same time. Because these trucks were not used much before 1983, projections cannot be made with the benefit of a historical record. How- ever, the committee was able to draw general conclusions about the po- tential use of these vehicles and their impacts. The study benefited from data and assistance provided by state highway agencies, trucking companies, safety research organizations, trade orga- nizations, and the Federal Highway Administration. This report presents the committee's conclusions and summarizes underlying data developed over a two-year period under the chairmanship of Dr. Kenneth W. Heath- ington, Associate Vice President for Research, University of Tennessee. The study was performed under the overall supervision of Dr. Damian J. Kulash, Assistant Director for Special Projects. Robert E. Skinner, Jr., and Joseph R. Morris directed the project staff. Stephen R. Godwin, Dr. John A. Deacon, and Richard Margiotta made significant contributions. Finally, special appreciation is expressed to Nancy A. Ackerman, TRB Publications Manager, and Naomi Kassabian, Associate Editor, for editing the final report and to Marguerite Schneider and Margaret M. Sheriff for typing the many drafts and the final manuscript.
