Portable WIM systems : comparison of sensor installation methods for site-specific traffic data measurements

Abstract

As an alternative to costly permanent weigh-in-motion (WIM) stations that are mostly limited to major interstate highways, portable WIM systems are often used as a substitute or supplement to routinely collect site-specific traffic data (both volume and weight) for pavement design and analysis applications. By comparison, portable WIM systems are cost effective and much easier to install at any desired highway site/location. However, accuracy, reliability, and data quality have been some of the key challenges of portable WIM systems. As a means of addressing these challenges, this field pilot study was undertaken to comparatively evaluate two different sensor installation methods for routine traffic data measurements: the pocket tape and metal plate methods. The two methods were comparatively evaluated in terms of their practicality, simplicity of installation, cost effectiveness, resource/manpower needs, environmental sensitivity and endurance, consistency, data accuracy, and statistical reliability of the traffic data measurements. Along with a side-by-side field validation using permanent WIM data, the findings from the study indicated that the metal plate sensor installation method is superior to the pocket tape method, particularly in terms of data accuracy, data quality, statistical reliability, and endurance. Its traffic data accuracy rate was found to be 87∼91 % compared with 79 % for the pocket tape method, which exhibited a significant loss of sensitivity and data accuracy after 7 d of traffic measurements. Overall, the conclusions of this study provide technical merit and preference to the metal plate method over the pocket tape sensor installation method, particularly for traffic data measurements exceeding 7 d.