Originally Posted by the mail wrote

But, under Home Office rules governing the camera equipment, prosecutions are only valid if a driver is filmed in the same lane at the start and finish of each section by a linked pair of cameras.

That means a three-lane motorway would require three separate sets of cameras - one for each lane. If drivers leave the speed-camera zone via a different lane to the one they entered in, they cannot normally be prosecuted.

But under Home Office 'type approval' rules, each individual set cannot be linked to any of the others. So cars are timed only between sets of number plate readers 'paired' for the same lane.

A spokeswoman for the Home Office said: 'The manufacturers applied for the camera to be type-approved to measure one lane only. It has been type-approved for this use - this can be either the lane under the camera or a lane to either side of it.'