The cameras, in yellow housings, use radar to detect the speed of passing vehicles. The camera is activated when a vehicle crosses a radar beam at excessive speed. It takes two photographs of the rear of the vehicle at an interval of 0.5 seconds. A series of short white lines are painted on the surface of the road, five feet apart, as an independent way of verifying the speed reading.
Because two photographs are taken of the speeding vehicle, it is possible to compare these and work out how far the vehicle has travelled in the interval using the checkmarks on the road. Using a simple formula, the viewer of the images can verify the speed given by the camera. The secondary check marks are there to safeguard the motorist against a faulty or false camera reading.