The police use the Doppler Effect when checking for speeding vehicles. A radar gun sends out radar waves at a particular frequency. As the radar wave hits a vehicle, the wave reflects back toward the radar gun at a different frequency. The frequency of the reflected wave depends upon the direction and speed of the vehicle. The faster the speed, the greater the frequency change. The radar gun determines the speed of the vehicle by measuring the difference between the emitted frequency and the reflected frequency and computing the speed from that measurement.
Radar guns used by police to monitor traffic speeds work by taking advantage of the Doppler Effect.