Speed limit enforcement methods
Speed limits were originally enforced by manually timing or "clocking" vehicles travelling through "speed traps" defined between two fixed landmarks along a roadway that were a known distance apart; the vehicle's average speed was then determined by dividing the distance travelled by the time taken to travel it. Setting up a speed trap that could provide legally satisfactory evidence was usually time consuming and error prone, as it relied on its human operators.
Average speed cameras
Automatic number plate recognition (ANPR) systems that use a form of optical character recognition to read the vehicle's licence or registration plate. A computer system reads vehicle registration plates at two or more fixed points along a road, usually hundreds of meters or even kilometers apart, then uses the known distance between them to calculate a vehicle's average speed. If the average speed exceeds the speed limit, then a penalty is automatically issued.
Instantaneous speed cameras
Instantaneous speed cameras measure the speed at a single point. These may either be a semi-permanent fixture or be established on a temporary basis. A variety of technologies can be used:
- VASCAR is a device that semi-automates the timing and speed calculation of the original manually-operated "speed trap". An observer on the ground, in a vehicle or in the air simply presses a button as a vehicle passes two landmarks that are a known distance apart.
- Radar guns use a microwave signal that is directed at a vehicle; the Doppler effect is used to derive its speed.
- LIDAR - the time of flight of laser pulses is used to make a series of timestamped measurements of a vehicle's distance from the laser; the data is then used to calculate the vehicle's speed.
- Sensors embedded in the roadway in pairs, for example electromagnetic induction or Piezo-electric strips a set distance apart
- Infra-red light sensors located perpendicular to the road, e.g. TIRTL
Types of camera include Gatso, Truvelo Combi and D-cam.
Pacing
Officers in some jurisdictions may also use pacing, particularly where a more convenient radar speed measuring device is not available-a police vehicle's speed is matched to that of a target vehicle, and the calibrated speedometer of the patrol car used to infer the other vehicle's speed.
(Source: Wikipedia)