Rangefinder type non-imaging traffic sensor

Abstract

A non-imaging traffic sensing system (,) employs three separate detectors (D,-D,) each positioned above a roadway (R) and spatially separated along the roadway. The detectors detect light reflected off the roadway surface. Each detector has its own field of view (FOV) of the roadway surface and a separate footprint (F,-F,) is defined on the surface by intersection of the respective fields of view with the surface. A disturbance passing over the roadway changes the amount of reflected light sensed by the detectors and the detectors generate respective signals indicative of the amount of reflected light they receive. A first pair of the detectors (D,, D,) measure the speed of a passing disturbance. A second pair of the detectors (D,D,) identify shadows so to eliminate their effects. The footprints defined by the fields of view of the second detector pair generally overlap. A processor (,) processes signals from the first detector pair to determine the speed of the disturbance. The processor further processes signals from the second detector pair to determine the disturbance's height. The disturbance is classified as vehicular if the height exceeds a predetermined threshold, but as a shadow if less than the threshold. This allows the effects of shadows on the roadway to be readily identified and distinguished from vehicle movement.