1. A method of detecting a passing vehicle comprising the steps of: receiving a video sequence comprised of a plurality of image frames; measuring image intensity and estimating image motion in each image frame; formulating a hypothesis model describing background dynamics; using the measured image intensity and motion estimation to determine if the background dynamics has been violated in a given image frame; and if the background dynamics has been violated, using motion coherency to determine whether the violation of the background dynamics is caused by a passing vehicle.