A radar speed gun has a beam spread like a flashlight and shows the fastest object/car in it's beam. the cop must try to figure out which object that is. True story, I was with a friend running fast down the interstate. We passed a car that was running in the passing lane and would get over so we passed him on the right in the slow lane. Just as we shot past him, a cop came over the hill. My friend down shifted to slow down and didn't touch the brakes. the other car we had just passed hit his brakes. The cop turned around and pulled the other car over and I am sure he got a ticket for how fast we were running.
One day by the course there was one of the traffic radar trailers sitting there and I went over and threw a couple of discs past it and I got a speed of 43 MPH. I know I don't have much of an arm so I guessing that isn't very fast.