Determination of Minimum Gap in Congested Traffic

Accurate evaluation of site-specific loading can lead to cost and material savings in rehabilitation and replacement of bridges. Currently, bridge traffic load assessment is carried out using long run traffic simulations based on weigh-in-motion (WIM) data obtained at the site. Congestion is the governing load condition for long-span bridges. To correctly model congestion, a minimum gap between vehicles is usually assumed. Where the gap is overestimated, the calculated characteristic load is smaller than the actual characteristic load leading to an unsafe assessment. If the gap is underestimated, the safety assessment is too conservative, which is both costly and wasteful of finite resources. This paper outlines the development of an optical method to measure parameters required to model driver behaviour in congestion. Images are obtained using a camera with a wide angle, aspherical lens. Edge detection and Hough transforms are used to location wheels and bumpers. The resulting data can increase the accuracy of traffic microsimulation and hence, the assessment of long span bridge traffic loading.