Technology

Vehicle-detection systems used in conjunction with road-rule enforcement cameras include the following:

  • Piezo-electric strips - pressure-sensitive strips embedded in the roadway (a set distance apart if speed is to be measured - typically 1-3 metres).
  • Doppler radar - a radio signal is directed at the vehicles and the change in frequency of the returned signal indicates the presence of a moving vehicle and the vehicle's speed.
  • Loops - inductive loops embedded in the roadway detect the presence of vehicles, and with two loops a set distance apart vehicle speed can be measured.
  • Laser - the time of flight of laser pulses is used to make a series of measurements of vehicle position, and from the series of measurements vehicle speed can be calculated.
  • Automatic number plate recognition (ANPR) systems that use a form of optical character recognition to read the vehicle's licence or registration plate.


Systems can be car- or van-mounted, hand held, or in a fixed site. In car-mounted systems, cameras and radars or lasers are fixed to a car. When deployed, the car is parked beside a road, and any speeding vehicles driving past are photographed. Most red-light cameras, and many speed cameras, are fixed-site systems, mounted in boxes on poles beside the road. They are also often attached to existing gantries that hold up signs over the road, or to overpasses or bridges.

Speed camera systems that measure the time taken by a vehicle to travel between two fairly distant sites (from several hundred metres to several hundred kilometres apart) are also being developed and introduced. From the elapsed time over the known distance, a speed infringement can be detected, or in the case of truck drivers driving long distances, avoidance of legally prescribed driver rest periods can be detected. Such systems take a picture of every vehicle passing the first site, and every vehicle passing the second, then find matches between the images from the two sites. Most commonly, this matching is done by using ANPR systems.



In the United Kingdom, automatic number plate recognition (ANPR) average-speed camera systems are known as SVDD (Speed Violation Detection Deterrent) by the Home Office. More commonly, they are known by the public by their brand name - SPECS (Speed Enforcement Camera System [2]), a product of Speed Check Services Limited, or just as speed cameras/traps. They are frequently deployed at temporary roadworks sites on motorways, but also increasingly on known accident blackspots such as the A77 between Ayr and Girvan in Scotland. A SPECS system also enforces a speed limit of 20 miles per hour (30 km/h) over Tower Bridge in central London to reduce vehicle damage to the bridge.



This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Red-light_camera".