Impact Axle Load Spectra Loadings on Flexible Pavement Performance and Design

Accurate characterization of axle load spectra (ALS) is essential for mechanistic–empirical pavement design. This study evaluates the impact of overweight trucks on pavement service life using one year (2024) of weigh-in-motion (WIM) data from 41 stations across Michigan. A bimodal mixture model was employed to characterize tandem-axle load distributions, distinguishing between empty/partially loaded and fully loaded truck populations. To capture the influence of heavy-tailed distributions, a severity metric—the Fourth Root of the Fourth Moment (FRFM) factor—was computed for each station. Mechanistic–empirical analyses were then performed using the PerRoad software, in which ALS served as direct inputs to fatigue-based design simulations. Results demonstrate a strong correlation between FRFM and predicted pavement service life, with thickness adjustments of up to 0.93 inches required to maintain a 21-year design life under higher traffic volumes. These adjustments translate into significant increases in life-cycle costs, confirming that traffic volume and overloading interact to accelerate pavement deterioration. The findings establish FRFM as a robust, distribution-sensitive severity metric for linking WIM data to pavement design outcomes, underscoring the policy relevance of monitoring and managing overweight-vehicle traffic. Future work will extend the analysis by simulating upward shifts in the loaded-axle peak to quantify additional cost impact.