The modern day electronic speed trap can fire a radar signal at a moving vehicle in order to measure its speed. If the vehicle is approaching towards the radar, then the frequency of the returning signal will increase. Similarly, if the vehicle is travelling away from the radar, the frequency will decrease. The speed is determined by comparing the two frequencies, the difference is then converted into a speed reading and displayed in miles or kilometres per hour.
One of the most popular forms of speed trap is the hand held unit, which is aimed at a moving vehicle and gives an instant digital readout of speed, however, doubt over their safety to the user is now under investigation.
