Review of U.S. Department of Transportation Truck Size and Weight Study - Second Report: Review of USDOT Technical Reports (2015)

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33 4. PAVEMENT The MAP-21 requirements concerning pavement analysis parallel the bridge analysis requirements: the USDOT study is to evaluate the impacts on the infrastructure of vehicles that now operate with grandfather or other exemptions to federal size or weight limits and the infrastructure impacts of allowing operation of alternative configurations, compared with configurations now allowed nationally under federal law. The pavement analysis in the USDOT study uses a model of pavement wear as a function of traffic (the AASHTO Mechanistic–Empirical Pavement Design software package) to estimate the change in life-cycle pavement costs (i.e., the present value of the cost to the highway agency of all future pavement rehabilitations in a 50-year period necessary to maintain a specified average pavement condition) caused by replacing existing (baseline) traffic with the predicted traffic in each alternative configuration scenario. The estimates are based on analysis of a set of hypothetical representative pavement sections (Pavement, 28). Responsiveness to the Questions Identified by Congress Congress specifically required that the study “evaluate the impacts to the infrastructure in each State that allows a vehicle to operate with axle and weight limits that are in excess of the Federal law and regulations . . . in comparison to vehicles that do not operate in excess” and “the cost and benefits of the impacts in dollars. . . .” [Section 32801(a)]. The pavement analysis in the USDOT study does not adequately address this part of the charge. The study included an analysis comparing the time until the first required rehabilitation for selected pavement sections subjected to baseline traffic with the time to rehabilitation for the baseline traffic with all overweight axles removed (Pavement, 32). This analysis did not estimate impacts in the individual states that allow vehicles to operate with weights in excess of federal limits and did not take into account the pavement designs actually used in the states where heavier

34 axle loads are allowed. The report does not state whether the total quantity of truck cargo was held constant in the analysis. (If axle weights were reduced, a greater number of truck trips would be required to transport the same quantity of cargo.) Results of the pavement analysis are presented as a percent change in life-cycle cost for each scenario with respect to cost with the baseline traffic. Consequently, estimated changes in pavement costs cannot be compared with changes in bridge costs or in logistics costs, which are estimated in dollars and as annual costs (logistics) or as one-time costs for upgrading (bridges). Methods and Data The basic methodology for assessing the pavement impact of the various truck traffic configuration scenarios is appropriate. The USDOT study report acknowledges certain limitations of the analysis (Pavement, ES-5, 2): only the distresses and pavement types that the pavement model is capable of representing are considered, overlay pavement impacts could not be evaluated with the model, and local road impacts could not be estimated for lack of pavement and traffic data. The traffic analysis also appears not to have considered the possibility of changes in the existing overweight vehicle population due to increased legal limits in the alternative configuration scenarios. Vehicles currently operating over the legal load limits have a major effect on anticipated pavement life. 6 The report does not provide a complete description of the basis for selecting the pavement sections analyzed. Pavements typically are designed for a life of 20 to 30 years. Most of the pavement sections analyzed have a design life (with baseline traffic) outside the typical range. Four sections show a design life of less than 10 years, and three sections have a design life of more than 50 years (Pavement, 94–97). The method of computing the weighted averages of the results of the analyses of the selected pavement sections is not fully explained. 6 Appendix D, Pavement Effects of Illegally Overweight Trucks, of the 1990 TRB committee study of truck size and weight limits (TRB 1990, 254–255) provides an illustrative estimate of the likely range of the share of highway agency pavement costs attributable to illegal overloads.

35 The ranges shown for life-cycle cost estimates (Pavement, 28, Table 12) are simply the results using two values of the discount rate in the calculation of present value of future expenditures. This presentation appears likely to be misunderstood as an estimate of the overall uncertainty of the results, although it is not related to pavement analysis uncertainty. In general, the report does not adequately describe the uncertainties inherent in the estimates. The cost estimates are subject to uncertainty from two sources: uncertainties in the input data on pavement structure and truck traffic and uncertainties from the limitations of the modeling method used (in particular, the exclusion of some pavement types and distress mechanisms in the estimates). To interpret the importance of the pavement cost estimates, users of the report need information about the estimates’ reliability and about the magnitude of likely pavement costs (measured in dollars) compared with other categories of dollar-denominated costs and benefits. Recommendations In any future USDOT truck size and weight study, estimates of pavement impacts of changes in truck size and weight regulations should begin by defining a conceptual model of the effect of a change in truck traffic on actual pavements, identifying all mechanisms of impact for all important pavement structures. The absence of such a framework in the report of the USDOT study makes interpretation of the results of the partial analysis that the study conducted difficult. Future analyses should estimate the impacts of changes in truck traffic on local roads, on the basis of available data and necessary assumptions about pavements and traffic, and should systematically examine the sensitivity of costs on all roads to assumptions and data uncertainties. Finally, any future study should include an estimate of pavement costs derived by evaluation of a scientifically designed sample of actual existing pavements.

36 Reference Abbreviation TRB Transportation Research Board TRB. 1990. Special Report 225: Truck Weight Limits: Issues and Options. National Research Council, Washington, D.C.