I need to know how police use the following things to find out the speed of moving vehicles:
A camera alone will not measure speed. It is hooked up to a speed measuring device, and if a car is over a certain preset speed, that triggers the camera to take a picture.
The car you are referring to is what is called a "drone". They place an unmanned car near the road with a cardboard cutout in it. People see it and slow down.
As far as a radar gun, that emits a radio wave of known frequency. If the wave reflects off a stationary target, the frequency does not change. If it reflects off a moving vehicle, the frequency changes in relation to the speed. The faster the car, the more the change. The radar unit calculates this frequency and displays the speed.
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