Speed guns, dont they have a target in a sight with a dot in the middle, the dot has to be on your numberplate (the flattest bit of the front of you car) and they press the trigger, it shoots a laser? and its bounced back off the numberplate into the gun about 6 times and the distance you have covered gives them a speed.

Im sure the further away you are, the safer you are, they have to get the dot on the numberplate for a second or two before they get a speed. even if they have it magnified, they would have a job keep it on your numberplate from far away.

Limited knowlage i know, CHIP should know more about it