The instrument that measures speed using the Doppler effect of radio waves is a radar gun, also called a radar speed gun.
A radar gun transmits a focused radio signal toward a moving object, such as a vehicle or a baseball. The signal reflects from the object and returns to the instrument with a slightly different frequency. The size of that frequency difference reveals the object’s speed along the radar beam.
If the target approaches, the returned frequency is higher; if it recedes, the frequency is lower. For ordinary traffic and sports measurements, the target’s speed is tiny compared with the speed of light, so the relationship between frequency shift and speed can be treated with a simple approximation. The factor of two in common formulas comes from the signal’s trip to the target and back.
Radar guns are used by police, in baseball and tennis, and in cricket to measure deliveries. They should not be confused with lidar speed guns, which use laser light rather than radio waves. A stationary radar gun measures speed relative to itself; moving-radar systems compensate for the gun’s own motion by comparing signals from the target and stationary surroundings.