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Publication Number: FHWA-HRT-13-089
Date: October 2013
Many State highway agencies are evaluating and implementing the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical Pavement Design Guide (MEPDG) and the DARWin-ME software.(1) The main MEPDG traffic inputs are normalized axle load spectra (NALS), a percentile truck classification volume distribution, truck volumes, and truck growth rates. In most earlier pavement design and analysis methodologies, the effect of traffic loads was accounted for in the design by using the AASHTO equivalency factors to compute the number of equivalent single-axle loads (ESAL).(2) Instead of using the computed ESAL values, the MEPDG and DARWin-ME software use the load spectra that can be used directly to estimate truck statistics.
NALS are percentile distributions of axle type counts by load range. Individual NALS are computed for each axle type and truck class. The axle types included in the MEPDG design procedure are single, tandem, tridem, and quad, and the truck classes are vehicle classes 4 through 13 from the Federal Highway Administration (FHWA) classification scheme.(3) The MEPDG uses NALS representative of a "typical day of the month." For roads that do not show significant seasonal variations in percentages of heavy and light loads, the same NALS are used for all calendar months.
Of all MEPDG traffic loading inputs, NALS are the most challenging owing to the high cost and considerable time to collect an adequate quantity of accurate axle load data. Therefore, the majority of pavement designs and analyses using the MEPDG and DARWin-ME software do not use site-specific NALS. Instead, most analyses rely on regional, agency-wide, or national NALS traffic loading defaults, combined with site-specific truck volume counts. It is important that these default NALS tables be as representative as possible of the actual truck traffic loading characteristics at the project site.
The original default traffic datasets for the MEPDG were developed during National Cooperative Highway Research Program (NCHRP) Project 1-37A using Long-Term Pavement Performance (LTPP) traffic data available in 1998.(4) At that time, these data represented the best available and most comprehensive national set of weigh-in-motion (WIM) and automated vehicle classification (AVC) data. However, some concerns existed regarding the lack of documented quality of the data that were used to determine the truck traffic default values - specifically, the NALS values for each axle type within each class of trucks. Just as important, many agency personnel also questioned the quality and adequate quantity of their own State's truck traffic data. Consequently, a new set of default NALS has been produced for use with the MEPDG and DARWin-ME software. In addition, detailed instructions have been developed to help States create NALS tables that reflect the unique truck types and trucking patterns in their State.
To develop the improved default tables, LTPP undertook a Specific Pavement Studies (SPS) Transportation Pooled-Fund (TPF) study that focused on installation of highly reliable, well-calibrated, permanent WIM systems, and collection of axle loading data using a uniform vehicle classification scheme and rigorous quality control procedures. One outcome of that ongoing effort is the production of research-quality traffic data (classification and weight), primarily to support LTPP analysis projects. The SPS TPF study has generated high-quality traffic loading information for 26 LTPP SPS sites located in 23 different States that represent moderate- and high-volume rural principal arterial interstate and non-interstate highways. These data also allow the update of the original MEPDG axle loading defaults. The SPS TPF data and their use as default NALS tables are described in Part I of this guide.
This guide addresses two issues critical for MEPDG use:
This guide primarily focuses on NALS, but it also addresses other defaults computed from WIM data, including axle spacing, wheelbase, and axles-per-truck defaults.
This guide is designed for analysts working with LTPP pavement performance data. It is designed specifically to aid in the selection of the default axle loading inputs computed using the SPS TPF data and application of those defaults to LTPP sites that do not have reliable site-specific axle loading data, when such data are required for use in pavement design analysis.
This guide also can be used outside of LTPP research and analysis. For example, the new SPS TPF based axle loading defaults can be used for any MEPDG-based pavement analysis when the pavement sites do not have reliable site-specific axle loading data. Agencies can also use this guide to help develop their own NALS defaults for use with the MEPDG and the DARWin-ME software.
As noted above, Part I of this guide provides guidance on selecting LTPP axle loading defaults for MEPDG use. Part II provides guidance on developing additional axle loading default tables for MEPDG use. Appendix A contains an operator's manual for an accompanying software application and database application CD, and appendix B contains the data dictionary for the LTPP Pavement Loading User Guide (PLUG) database application. The LTPP PLUG database application can also be requested through the LTPP Customer Support Service Center at:
Telephone: (202) 493-3035
Fax: (202) 493-3161
Topics: research, infrastructure, pavements and materials
Keywords: research, infrastructure, pavements and materials, Long-term performance, Transportation Pooled Fund Study, Mechanistic-Empirical Pavement Design Guide, weigh-in-motion, load spectra
TRT Terms: research, facilities, transportation, highway facilities, roads, parts of roads, pavements