Those cameras with the round holes on them - that rely on sensors in the road to determine speed. There's some on a road near me, and they turn them around every few days, so they sometimes face traffic and sometimes face away from traffic. I reckon they cant work facing away from traffic, due to the sensor positioning in the road. I keep meaning to test this theory, but i daren't
(They are, by design, traffic facing cameras)
Just to confuse matters further, the road markings are such that facing traffic, the camera points at 3 thin white lines across the carriageway. Facing the direction of traffic flow, the camera points at "gatso-style" metre markings (but its not a gatso). I reckon its not a real camera