Spatial and Temporal Variations in Axle Load Spectra and Impacts on Pavement Design

The distribution of axle loads can vary geographically and seasonally; ignoring these variations may impact the accurate characterization of traffic and ultimately the design thickness of new and rehabilitated pavements. A study of axle load spectra at thirteen sites on rural arterials in Alabama examined the statistical and practical significance of these spatial and temporal differences. In most cases, daily and monthly axle load spectra exhibited statistically significant differences from the composite distributions. Differences between distributions observed at each direction at a site and the site average were statistically significant, as were differences between the distribution at each site and the statewide average. However, in most cases, the resultant impact on pavement thickness design was negligible. At 92% of the study sites, use of the statewide distribution in lieu of site-specific data resulted in no more than a difference of 0.3 for structural number for flexible pavement and a 0.7 inch (1.8 cm) difference in rigid pavement depth; these differences were considered insignificant from a practical perspective.