Many electronic speed sensors use a magnetic principle called the Hall Effect. The Hall Effect device registers the presence of a nearby magnet. A computer counts how many times the magnet passes by and determines speed.
A magnetic speed sensor consists of a stationary Hall Effect device and a moving magnet. The device has no moving parts and can measure very high speeds. Whenever the magnet passes by it, it produces a small voltage pulse.
The speed sensor's magnet rides on a rotating wheel, such as the inside of a car's wheel. For every revolution of the wheel, the magnet moves past the Hall Effect sensor, which sends an electrical pulse to a computer.
A simple computer measures the time between pulses from the Hall Effect sensor. Using this data and the diameter of the wheel, it calculates the vehicle's speed.