Officer, how can you tell my speed?

Posted Tuesday, November 8, 2016 by Andrew Charles Huff

As a traffic attorney, I am asked regularly by friends about how an officer can determine your vehicle speed by using that handheld device they have pointed at traffic.

A very brief explanation….A radar gun is a Doppler unit that may be hand-held or vehicle-mounted. This device measures the speed of an object it is pointed at by detecting the change in frequency of the returned radar signal caused by the Doppler effect. This means the frequency of the returned signal is increased in proportion to the object’s speed of approach if the object is approaching, and lowered if the object is receding. From that difference, the radar speed gun can calculate the speed of the object from which the waves have been bounced.

Police normally use the more modern LIDAR speed guns that use pulsed laser light instead of radar because of limitations associated with small radar systems.