Safety cameras are calibrated so that vehicles traveling within the speed limit are "invisible". Only vehicles traveling above the limit are detected and photographed by the camera.

When processed at a secure location, each photograph shows to the second the precise time and date the image was taken.

It also shows the registration number and speed of the vehicle, a site identification code, an offence number and a film number.

The Gatso camera (rear-facing) is activated by a radar beam. A vehicle travelling at an illegal speed triggers two photographs, half a second apart. An on-board computer instantly calculates the distance travelled between two points. It's basic maths - distance traveled over time = speed. Secondary road markings, "ladder" marking. usually white strips across the road, provide another visual aid to motorists that they are travelling through a proven fatal crash area under camera enforcement.

The Truvelo camera (front facing) is activated by piezo-electric loop detectors in the road, which are marked by three thin white lines across the carriageway.

Cameras are rotated from one housing to another on each stretch of monitored road to ensure full coverage. However, all camera housings are fitted with speed detectors and will flash at any vehicle exceeding the speed limit, whether the housing is fitted with a camera or not. It is safer for drivers to assume that all cameras are 'live' and operational at all times, day and night.

Cameras only flash at vehicles whose speed is higher than the speed limit prevailing at that location. Vehicles travelling within the speed limit are 'invisible' to the cameras.