MEPDG Traffic Loading Defaults Derived from Traffic PooledFund Study

As part of traffic loading inputs, the Mechanistic-Empirical Pavement Design Guide (MEPDG), Interim Edition: A Manual of Practice requires detailed axle loading information in the form of normalized axle load spectra (NALS), number of axle per truck class and axle group types, and axle spacing inputs. These data are obtained from weigh-in-motion (WIM) sites. The objective of this project was to evaluate the applicability of the existing MEPDG global traffic loading defaults and to use research-quality WIM data from the Long-Term Pavement Performance (LTPP) Specific Pavement Studies (SPS) Traffic Data Collection pooled fund study to revise and improve the global default axle loading values. This report provides an assessment of the original MEPDG axle loading defaults, describes WIM data selection criteria, including data reliability assessment, presents findings from the LTPP SPS traffic pooled fund study traffic data review, describes a methodology to generate new MEPDG traffic loading defaults, and provides a description of the new traffic loading defaults and recommendations for their use. The report also discusses a sensitivity analysis of MEPDG pavement performance models to NALS. Significant differences found in the MEPDG outcomes support the need for axle loading characterization beyond a simple default value for heavy trucks that dominate vehicle class distributions, especially for class 9 trucks. The effect of WIM accuracy on axle weight measurements, NALS estimates, and the associated MEPDG outcomes was also investigated. It was found that drift in WIM system calibration leading to over 5 percent bias in mean error between true and WIM-measured axle weight could lead to significant differences in MEPDG design outcomes. In addition, two new statistical parameters were developed in this study: (1) a summary statistic used to describe traffic loads for comparison and grouping of similar NALS called the relative pavement performance impact factor and (2) a parameter used to quantify errors associated with NALS and to assess NALS reliability called the pooled weighted load error.