Federal truck size and weight study falls short of congressional requirements, says new report

Although a U.S. Department of Transportation report on federal truck size and weight limits acknowledges gaps in addressing its legislative charge, a more comprehensive and useful response would have been possible, says a new letter report from the National Academies of Sciences, Engineering, and Medicine. The DOT's Comprehensive Truck Size & Weight Limits Study lacks a consistent and complete quantitative summary of the alternative configuration scenarios, and major categories of costs—such as expected bridge structural costs, frequency of crashes, and infrastructure costs on certain roads—are not estimated.

DOT asked the Transportation Research Board of the Academies to convene a committee to review its congressionally mandated study of truck size and weight limits. The committee's 2014 letter report reviewed preliminary products of the study. In this final letter report, the committee considered how the DOT study addresses Congress' questions and assessed the appropriateness of the methods and data used to produce estimates of impacts of changes in federal truck size and weight limits on: bridges, pavements, shares of total freight traffic carried by trucks and other freight modes, safety, and enforcement of truck regulations.

The DOT report could have provided a framework for understanding all the costs and benefits, the committee found. Several components can be determined from results of the present and past studies, including a comprehensive list of the categories of costs and benefits; the features of a proposed regulatory change that influence each category; approximate sizes of impacts on shippers, truck operators, road users, and the public; and the categories that are likely to be critical to evaluating regulations. The committee also identified assumptions and simplifications in the DOT study that might result in misleading estimates of infrastructure, traffic, and safety impacts.

Although the Academies' letter report does not take a position on whether or how to change current federal truck size and weight limits, it offers recommendations for improving estimates in each of the impact categories, in order to increase the value of any future truck size and weight studies.

DOT should continue to support areas of research and data development begun in the present study in order to improve exposure data for estimating crash rates, understand the relationship of crash frequency on a road to the traffic volume and mix of vehicle types, and improve the cost-effectiveness of enforcement, the report says.