Research to Support Evaluation of Truck Size and Weight Regulations (2019)

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1 Summary The U.S. Department of Transportation (USDOT) completed its Comprehen­ sive Truck Size and Weight Limits Study in 2016, responding to a directive of Congress for a study to compare the impacts of truck traffic operating under present federal size and weight limits with the impacts if trucks exceeding present limits were allowed to operate. The USDOT report recommended that a program of research be undertaken to overcome limitations in data and in models of impacts that had hindered the study. USDOT subsequently asked the Transportation Research Board (TRB) to convene a committee to develop a plan for a research program to reduce uncertainties in estimates of the impacts of changes in truck size and weight limits. The TRB Truck Size and Weight Limits Research Plan Committee prepared a first report that identified candidate topics in five categories of impacts of changes in size and weight limits (safety, bridges, pavements, enforcement of limits, and shares of freight transported by truck and other modes) for inclusion in a research plan, in addition to topics addressing the structure and methods of evaluations of size and weight regulations. This second report presents the committee’s research plan. The com­ mittee has defined a program of 27 research projects. For each project, the report provides a problem statement identifying the product, relationship to the overall objective of evaluating truck size and weight regulations, possi­ ble research approaches, and anticipated duration and cost. The program is selective: it includes projects on the categories of impact for which USDOT requested guidance and certain additional topics, but does not include re­ search on categories of impact for which estimates have not been critical sources of uncertainty in past evaluations of size and weight regulations.

2 EVALUATION OF TRUCK SIZE AND WEIGHT REGULATIONS The committee grouped the projects in three categories. Projects in­ cluded in one of the seven core research tracks identified below have a good probability of producing useful results within a practical time period and budget and would significantly contribute to reducing uncertainty in truck size and weight limit evaluations. The second category, projects that may be deferred, would have value, but would be less critical than the projects in the core research tracks in reducing uncertainties or would be of long duration. A third category of projects for consideration would have a rela­ tively high risk of producing inconclusive results or would be especially expensive and complex. The core research tracks defined by the committee (each corresponding to a research problem statement or to a series of problem statements) are the following: • Development of a truck traffic, weight, and configuration database from nationwide weigh­in­motion installations and other sources. • Development of a discrete continuous choice model, or suitable alternative, capable of estimating the effect of changes in truck size and weight regulations and other policies on shippers’ and carriers’ choices of freight mode, vehicle configuration, and shipment size. • Development of pavement analysis methods for heavier axle limits, multiaxle groupings, and alternative tire and suspension types. • Development of a comprehensive model of the relationship of bridge deterioration and service life to vehicle loads. • Comparative evaluations of crash risks of alternative configura­ tions by the case­control method. • Development of protocols for evaluating the performance of con­ figurations with simulation, track testing, and field trials. • Measurement of relationships between frequency of overloads and enforcement methods and level of effort. The core research tracks constitute a program of research that could meet the need for improved evaluation capabilities in the short term. The short­term goal would be to provide the capability to respond with the best available estimates to the kinds of proposals for changes in size and weight regulations that arise frequently at the federal and state levels. The full re­ search program (to which the core research would contribute) would have the long­term goal of identifying opportunities for major improvements in performance of the highway freight transportation system with respect to safety, infrastructure cost, and productivity. As noted in several of the problem statements, progress on the core research tracks would be aided by awareness of current international research and regulatory practice.

SUMMARY 3 In addition to the research problem statements, the report presents eight general conclusions of the committee on research needs and priorities: • The value of research on truck size and weight regulations. • Limitations of impact projections. • The value of broadly applicable research. • The importance of general purpose data programs. • The need to match research to policy objectives. • The value of tests and trials in evaluations of truck size and weight limits. • Emerging technologies and truck size and weight regulations. • Organizational needs and options. The conclusions emphasize that improvements in models for project­ ing infrastructure, safety, and freight cost consequences of changes in lim­ its will not guarantee the success of future truck size and weight policy studies. Future studies will be useful as guides for decisions only if policy objectives and practical policy options are clearly defined, the analysis is logically structured to reveal the most promising policies, and uncertainties are properly characterized.