Comprehensive Truck Size and Weight Limits Study: Comparison of Results Report
Chapter 5: Pavement Comparative Analysis
5.1 Purpose
The purpose of this section is to compare principal results of the Pavement Comparative Analysis with other similar studies available in the literature. This involves two main objectives. First, those documents summarized in the revised desk scan that contain quantitative results pertaining directly to pavement analysis (i.e., the 2014 CTSW Study) are identified. Second, the results from each of the selected documents are reviewed and objectively compared with the results of the 2014 CTSW Study.
5.2 Comparison of Pavement Study Findings
Unlike most other recent truck size and weight studies, the 2014 CTSW Study considered some scenarios that result in anticipated increases in average axle loads and some that resulted in decreases. In the 2000 CTSW Study, all scenarios resulted in significant reductions in average axle loads, as did the 2004 Western Uniformity Scenario Study and state studies in Minnesota and Wisconsin. Only the Vermont pilot study resulted in increases in average axle loads.
As discussed more thoroughly in the pavement desk scan report, the Vermont pilot study and all recent previous federal studies have all used a different approach than was applied in the current CTSW. In each of these studies, pavement performance or design models were used to derive load equivalence factors for various types of pavement distresses and incorporated into specialized national pavement cost models designed to be used for cost allocation and truck size and weight analysis studies.
Most state truck size and weight studies have used a much simpler approach of estimating traditional ESALs for a base case and for each scenario, then applying a cost-per-ESAL-mile estimate to the change in ESALs.
The current CTSW used an approach of applying the most current pavement design model to a small number of pavement sections to directly estimate changes in initial pavement life for each pavement section under each scenario. Initial lives were translated to life cycle costs and expanded to represent the entire highway system.
Differences in the study approaches as well as in the types of scenarios considered render direct comparison of the results of the various studies somewhat difficult, but Table 5-1 presents summary results from each of these recent state, regional, and national studies. Note that scenarios with lower average axle loads tended to result in reduced pavement costs, while cases with higher average axle loads tended to result in increased costs. Note, however, that some scenarios resulted in somewhat more subtle interactions between reduced VMT and increased average loads per axle. Average axle loads, after all, are not as important as the distribution of axle loads at the higher ends of the axle load range, given the non-linearity of pavement damage as a function of axle load.
Note in Table 5-1 that the last major national study, the 2000 CTSW Study, used scenarios that resulted in truck VMT reductions of 11 to 23%, while the current scenarios resulted in much more modest overall changes in truck VMT. Note, however, that pavement costs decreased by small amounts for each of the 2000 scenarios, but increased for some of the current scenarios.
Note also that the only previous study that used a pavement design model (MEPDG) similar to the design model used in this study (AASHTOWare Pavement ME Design®
) was also the only previous study that showed an increase in pavement costs on the Interstate system (and a slight decrease off the Interstate System). It should be noted that the similar design models were applied in very different ways, but still resulted in similar results.
| Study |
Vehicles and Weights Analyzed k = thousands of pounds |
Change in truck VMT | Change in Pavement Costs |
|---|---|---|---|
| Nationwide Studies | |||
| USDOT, Comprehensive Truck Size and Weight Limits Study (2014) |
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| USDOT, Comprehensive Truck Size and Weight Study (2000) |
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| Regional Studies | |||
| USDOT, Western Uniformity Scenario Analysis (2004) |
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| WsDOT, Wisconsin Truck Size and Weight Study (2009) |
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| FHWA, Vermont Pilot Program Report (2011) |
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| MnDOT, Minnesota Truck Size and Weight Project (2006) |
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