Which device measures speed using the Doppler effect?

The device that measures speed using the Doppler effect is radar. Radar systems work by emitting a radio wave signal that travels away from the device and reflects off of moving objects, such as vehicles. When the signal reflects back to the radar unit, the frequency of the returned signal changes based on the speed of the object. This phenomenon is known as the Doppler effect, which describes how the wavelength and frequency of waves change when the source of the waves and the observer are in motion relative to each other. By calculating this frequency shift, the radar unit can determine the speed of the vehicle.

In contrast, lidar uses laser beams to measure distance and speed but does not rely on the Doppler effect. A pace car measures speed based on a predetermined speed limit without utilizing any specific technology for speed measurement. Speed cameras typically rely on different technologies, such as timing how long a vehicle takes to travel between two points, rather than using the Doppler effect to calculate speed.