Speed camera margin: from measured speed to recorded speed

A device measuring on the move combines its own uncertainty with the movement of the vehicle carrying it. The 2009 order therefore gives it double the permitted error of a fixed device: 10 km/h below 100 km/h, 10% above. The measured speed that triggers a penalty is higher by as much.

Yes: an average-speed camera is a fixed device that works out an average between two points. The 5 km/h or 5% margin is taken off that average. The calculation from distance and travel time is on the average-speed camera page.

Speed cameras must be checked periodically, and a notice can be contested if the device was not compliant. That is hard to prove alone: you have to request the photo and the device’s records, with no guarantee. The margin is already deducted: it cannot be claimed twice.

No. A car speedometer usually reads slightly above true speed, which tends to help the driver, but only the approved device’s measurement counts. A GPS app has no evidential weight against a checked speed camera, which is inspected every year by an approved body.