Yep, you've had it! There are 2 makes of cameras. Gatso's take your picture when you are travelling away from the camera (ie they photograph the rear of the vehicle). Truvelo's take the front of the vehicle (incl driver) so they can record who was actually driving.They even use infra red to get you when its dark. If you pass a mobile camera(usually in a Transit or similar), they can get you both ways! If you approach the rear of the van, it is usual to accelerate once past is. The operators know this, so they spin the camera round and get you through their front windscreen!! There are of course digital cameras now, though these are few and far between as they are very expensive. They measure your speed over a distance (usually about a mile). If you do the distance too quick, you must have been speeding at some point, QED you are nicked!!