What about that stretch of the M11 between the M25 and the North Circular. For years it was a 70mph zone, then literally overnight they casually introduced some new 50mph signs for a one mile stretch, with a speed camera, immediately followed by the familiar white circle with a black diagonal.
It couldn't have been any more blatant. The 50mph stretch had identical characteristics to the sections immediately before and after it. Nice and straight, 3 wide open lanes + Hard shoulder. Concrete barrier at the edge of the road, followed by a steep bank to discourage anyone encroaching from the side. No junctions, and furthermore, no history of accidents, UNTIL the speed camera installation.
As it turns out this was one of those cameras that actually caused accidents, due to the unanticipated emergency stops that ensued. Fortunately it was also one of the first to be removed through the "reduction of accidents only" policy. The fact that it ever existed, indicates what these cameras are all about.