Effects of Design and Site Factors on Roughness of Flexible Pavements in the LTPP SPS-1 Experiment

Although considerable mechanistic-empirical research exists on the factors affecting flexible pavement roughness, it is limited in scope because of inadequate field validation and the complex interactions of structural and site factors. This paper presents results of a study on the relative influence of design and site factors on roughness of in-service flexible pavements. The data from the SPS-1 experiment of the LTPP program were used in this study to investigate the effects of HMA surface layer thickness, base type, base thickness, and drainage on roughness of flexible pavements constructed in different site conditions (subgrade type and climate). Since this is the first comprehensive study of SPS-1 experimental data, a thorough methodology involving mean comparisons and multivariate ANOVA was adopted for data analyses. Among the design factors, base type was found to have the most significant effect on roughness. The best performance was shown by pavement sections with asphalt-treated bases. Drainage when combined with base type, also plays an important role in inhibiting roughness, while base thickness has only secondary effects. In addition, climatic conditions were found to have considerable influence. Pavement sections in the wet-freeze zone exhibited higher levels of roughness. Also, in general, pavements built on fine-grained soils have shown the worst roughness levels. While most of the findings from this study support the existing understanding of factors affecting roughness, important interactions between design and site factors were identified and provide new insights for achieving better long-term pavement performance.