The article surveys the available options for aboveground technologies concerning road vehicle detection, which are becoming increasingly important due to the problems of underground magnetic loop detectors. The outstanding advantage of aboveground detectors is their minimal ongoing cost of maintenance. Their installation is relatively inexpensive on multiple-lane high-speed roads with existing overhead structures. The options for aboveground detection include: (1) active infrared, where a stream of coded infrared, with typical wavelength 0.8-1.1 micron, is emitted onto a road surface, and a receiver examines the reflected signal; (2) passive infrared, which is sensitive to wavelengths, typically 8-14 micron, which are naturally broadcast by the targets; (3) ultrasonic, emitting ultrasonic waves which are reflected by targets; (4) CCD cameras, which are imaging systems operating in visible light or near infrared; (5) Doppler radar, which seeks small frequency shifts of radar reflected from moving targets; and (6) FMCW radar, which emits frequency modulated radar signals. In the UK and Belgium, Doppler radars have been used widely instead of loop detectors. Typical applications include: (1) pedestrian detection; (2) incident detection; and (3) traffic counts, speed estimation, and classification. Loop sensors are still best for detecting stationary vehicles.
Samenvatting