Field Verification of Smoothness Requirements for weigh-in-Motion approaches

The smoothness of the pavement surface approaching a Weigh-In-Motion (WIM) scale directly affects the device’s ability to accurately estimate static loads from measured dynamic forces. Lack of smoothness creates difficulties in obtaining an acceptable weighing error. Efforts were made in this study to determine appropriate threshold values of profile length and roughness approaching the WIM. Roughness data for 19 WIM sites across Texas were collected, and the corresponding calibration records from those WIM sites were employed to verify the smoothness threshold values proposed by AASHTO MP14 and ASTM E1318. Since there are multiple lanes at each WIM site, a total number of 70 profiles was surveyed. Based on the calibration records from the WIM sites and field roughness measurements, it was found that the upper threshold value of the Long Range Index (LRI) of AASHTO method MP14 needs to be adjusted in order to screen out the locations that yield unacceptable weighing error. The lower threshold value was found to be unnecessary because more than 90% of WIM sites yielded acceptable weighing error even when exceeding the lower threshold value. The LRI is preferable to the Short Range Index (SRI) as it matched well with the field WIM calibration results. Based on the 70 profiles used to determine appropriate lengths, 25.8 m (85 ft) preceding the WIM, is found to be sufficient, which is the length required according to the LRI of AASHTO MP14. The ASTM E1318 requirement of 60 m (198 ft) is too conservative. The study concluded that an LRI of 1.74 m/km (110 in/mi) for the 25.8 m (85 ft) leading up to the WIM is the only critical factor in AASHTO MP14 and is the threshold value recommended.