In order to get a large Doppler shift, the source or observe must be traveling at a reasonable fraction of the velocity of the wave. However, police officers can use the Doppler shift of microwaves to accurately measure the velocity of a car, which is MUCH less than the speed of light. The precision comes from measuring beats.
Police radar uses a frequency of 109 Hz.
If the police officer is standing by the side of the road, first calculate the frequency of the radar that you would detect in your car after you pass by at 86 miles/hr.
The microwaves reflected by the car now act as a moving source as they travel back to the police officer. Now find the frequency of the reflected microwaves that the police officer would detect.
Finally, what is the beat frequency between the original radiation and the reflected radiation? Enter this beat frequency below. This beat frequency can be very accurately measured.