Pavement Smoothness at Weigh-in-Motion Sites: Developing Specifications for the Long-Term Pavement Performance Program

Accurate traffic data are vital to the success of the Long-Term Pavement Performance (LTPP) program. To minimize dynamic motions and therefore improve the accuracy of traffic loading data at weigh-in-motion (WIM) scales, pavement smoothness specifications were developed to screen sites for excessive truck dynamic loading that exacerbates scale error beyond ASTM recommended tolerances. WIM scale error was related to pavement profile characteristics by using a large simulation study of the response of virtual trucks over measured profiles. A distribution of error was compiled over the truck population for steer axle, tandem axle, and total vehicle weight at each site. The error distributions were summarized by their 95th percentile absolute error levels. The error levels assigned to each site were then used as a correlation standard for the proposed roughness indices and for the selection of corresponding threshold values. Instead of a single index, two versions of the Butterworth filter were selected for use in the specifications. One addresses short-range roughness and the other long-range roughness. The shortand long-range WIM error indices were then statistically related to WIM scale error to set the threshold values. An overview is presented of the development of the pavement smoothness specifications, including indices and threshold values.