Measuring traffic load on Forth Road Suspension Bridge using Weigh-InMotion and image data

This paper uses a camera and image analysis to complement Weigh-In-Motion (WIM) data on a long-span bridge and determine characteristic maximum load effects in congested traffic situations. The camera system measures vehicle positions and hence car/truck mix and inter-vehicle gaps, all key data for long-span bridge loading. It also measures vehicle lengths and approximates weights from the lengths, using data from a WIM system adjacent to the bridge. The approach is shown to give good accuracy and to be in sensitivity to inaccuracies in the weights of individual vehicles. The high-resolution camera captured image data on the Forth Road Bridge in Scotland (Figure 1) at1 second intervals over a five-month period. Standard image processing approaches are applied to extract the lengths of the vehicles from the images (Figure 2).Calibration factors are used to convert the lengths in the images from pixels to metres and to correct for perspective. Free-flowing traffic captured by the camera is compared with the same traffic recorded by the on-site WIM system to calibrate and confirm the accuracy of the calibration process. A year of data from the WIM system is used to establish the correlations between vehicle lengths and weights and hence to provide estimates of weights for vehicles identified in the images. The approach is used to gather bridge traffic load information including car/truck mix and intervehicle gaps. Bridge load effect data are fitted to Extreme Value statistical distributions to calculate characteristic maximum values (Figure 3). For fully jammed conditions, results are shown to be sensitive to the frequency of traffic jams on the bridge.