A good radar detector can detect K (25% use in USA & Canada) and Ka (70% use in USA & Canada) radar (the two most used radar bands used today) at least from 2.5 miles away.

If you are driving down the road and nobody is driving in front of you and you get a K or Ka alert, your natural tendency is to hit the brakes. Even at two miles, a radar gun will detect your speed before you have a chance to hit the brakes. Thus, if you are the only person on the road, or the leader of the pack, a radar gun will detect your speed.

If you are driving in a group of cars, then the radar beam will generally strike all the cars at the same time, and the radar gun could not differentiate which vehicle was driving slower or faster. Then it is up to the officer using the gun to make a subjective decision as to which vehicle was driving too fast. This argument can often be contested in court, if the driver wishes to contest this particular driving situation.

This does not imply that you should speed, as this often presents an unsafe situation to the driver and other people on the road.