Researchers found seeing a speed camera can distract drivers and cause them to slam on the brakes.

Accident rates were studied on two of the main roads in and out of Leeds.

In both cases, accidents have increased since cameras were installed.

Road conditions

The research was headed by Dave Keenan from the University of Leeds.

He studied the effect of cameras on the A61 Scott Hall Road and the A58 Easterly Road.

In his report he says: "The installation of a speed camera systems does not automatically imply that road conditions will improve at that location.

"The effect of the camera on driver behaviour can create new problems such as erratic braking and acceleration and distract drivers from the traffic flow."

Secret cameras might be more effective

Mr Keenan's conclusion is that speed cameras would be more effective if their exact location was not obvious.

If motorists knew speed cameras were around but not where they were, he argues, they would be forced to obey the speed limit along the whole stretch of the road.

But he concedes that covert cameras are not popular with the public.

Government policy is that cameras should be painted yellow, making them more visible.