Gatso

Gatso installations in the United Kingdom and in Queensland, Australia, are characterised by a measurement strip on the road surface, which is a series of white lines painted on the road, which are used with two photographs taken by the camera. These white lines are sometimes known as 'secondary check marks'. The camera is set to take two photographs, with a constant time interval between them, usually 0.5 seconds for faster roads and 0.7 seconds for slower roads. The difference in the vehicle's position in the two photographs can be used to calculate its speed.

In the UK, the evidence is admissible in court using the speed measured by the radar. The speed calculated from the distance travelled between the photographs using the secondary check mark white lines is used by police forces to verify the radar reading, and can be used as a defence in the event of an unreliable reading by the radar.

It can be a common misconception by drivers that the police do not use the distance travelled between the secondary check marks before issuing a notice in the post. The police can make mistakes, but the combination of radar reading and secondary check marks is a robust case for prosecution.

The speed indicated by the radar unit can be prone to error due to multiple reflections etc. With a minority of vehicle types such as lorries towing a curtain sided trailer. Further, it does not distinguish between multiple vehicles in shot, however secondary check marks will identify which vehicles are speeding, providing the police with corroborative evidence.