Which device is used to measure a vehicle's speed?

A radar gun is an essential tool used in traffic law enforcement to measure the speed of vehicles. It operates by emitting a radar signal toward a moving vehicle and then calculating the vehicle's speed based on the Doppler effect, which involves measuring the change in frequency of the returned signal. Radar guns are widely employed by law enforcement officers because they provide accurate, real-time speed readings and can be used effectively from a distance, allowing officers to monitor traffic speed without being in close proximity to the vehicles they are measuring.

Other devices mentioned have different functionalities: a speedometer is commonly found in vehicles to inform drivers of their current speed, but it doesn't serve law enforcement purposes. A speed limiter electronically restricts a vehicle's maximum speed but is not used to measure speed. Lastly, a GPS device can provide speed data by calculating movement based on location changes, but it is not specifically designed or widely used for enforcement purposes as a radar gun is.