IMPROVING TRAFFIC CHARACTERIZATION TO ENHANCE PAVEMENT DESIGN AND PERFORMANCE: LOAD SPECTRA DEVELOPMENT

This research addresses the understanding of, and need for, load spectra in future pavement design procedures and as a stepping stone toward more complete pavement design. The primary objective of this project was to develop truck axle load spectra for Washington State. To do this, axle load data collected at weigh-in-motion (WIM) stations throughout Washington State were used. The developed load spectra encompass the principal truck axles on the roadway network: single, tandem, and tridem. Achieving this objective allows the Washington State Department of Transportation, or any state highway agency with analogous traffic patterns, to accommodate the requirements of the 2002 Design Guide, developed through National Cooperative Highway Research Program (NCHRP) Project 1-37A. A secondary objective of this project was to determine whether Equivalent Single Axle Loads (ESALs) obtained from the developed load spectra are significantly different from historical values. Because the developed load spectra are transformable to ESALs, state highway agencies that decide not to use the new guide can still choose to employ the ESALs produced with the load spectra. The project concluded that the developed load spectra are reasonable. For single axles they are comparable to the 2002 Design Guide and MnROAD defaults. For tandem and tridem axles they are slightly more conservative than defaults of the 2002 Design Guide and MnROAD, but they are still within reason. In addition, the ESALs per vehicle class associated with the developed load spectra are comparable to Washington State historical ESALs for vehicle classes 9, 10, and 13. The use of the newly developed ESALs per vehicle will generally increase design ESALs, but that increase will be due to inclusion of the less predominant vehicle classes (4, 6, 7, 8, and 11).