Normally you would compare the timestamps on photos of the license plate in two places; you don't know whether or not the driver was speeding until you have the

Normally you would compare the timestamps on photos of the license plate in two places; you don't know whether or not the driver was speeding until you have the second timestamp. Or does it only take a picture in response to an instantaneous speed measurement? This seems much less useful, as you could just speed everywhere except the tiny range of the speed trap.