All fixed speed control safety cameras are designed to measure the speed of approaching vehicles, or departing vehicles, or both, depending upon the type of camera. They are calibrated so that vehicles travelling within the speed limit are invisible. Only vehicles travelling above the limit are seen and photographed by the camera. Gatso cameras are activated as the vehicle crosses a radar beam and takes two photographs half a second apart.
The white line markings on the road surface provide a secondary back-up check, verified by an operative who examines the film taken by the camera.
Every camera is checked to ensure it is properly calibrated and working every single time it is placed inside its yellow housing.
In addition, the camera conducts a self-test every time it takes a photograph. Furthermore, every single photograph is subsequently checked 'by eye' by an operative. The calibration and checking regime is rigorous in the extreme to ensure that no innocent motorists are accidentally caught by the cameras.