A COP is NOT SUPPOSED TO JUST SIT BEHIND A BUSH WITH RADAR GUN AIMED AT PASSING CARS: NEED PROBABLE CAUSE
A cop cannot legally leave the RADAR on and then just wait until a big number appears as a reason to throw on the lights and sirens – they have to suspect it was your car and estimate it is travelling at a certain speed (typically within 5 mph of actual speed), and then and only then can they measure your speed with a device. It’s harder to challenge, but asking the police about this process can occasionally result in the officer admitting they haven’t established probable cause prior to using the radar, or else they may claim to have done it in a way that makes it circumstantially questionable (like they claim to have estimated your speed in their rear view mirror while in freeway traffic when your car was just a quarter inch blob among dozens of others).
http://skattertech.com/2011/02/how-my-smart-phone-got-me-out-of-a-speeding-ticket-in-traffic-court/
