Originally Posted by Xaccers

The radar in the Gatso determines your speed, not the white lines.
They can be used to give a rough confirmation of your speed should you contest the accuracy of the camera recorded speed.
A case I heard about involved a man going through roadworks.
The camera recorded him doing quite a bit more than the speed limit, he knew he wasn't speeding, so contested it.
The police "somehow" missed a few lines when they were counting so it showed him as speeding.
Thankfully his maths skills, along with his solicitor and the magistrate, were somewhat better than the police and showed that he hadn't been speeding.