'Average Speed Cameras' in the UK use automatic number plate recognition technology. They 'read' your licence plate at set intervals and use that to work out how fast you're going on average, rather than at a single sample point like a traditional speed camera.

A few years back the police were agitating to get these cameras on every motorway and A road with a camera every half a mile.

The stated reason was specifically to allow tracking of vehicles after crimes had been committed, but everyone was suspicious that it was just to raise revenue by catching more people speeding and I think the scheme has been scaled back.