The Future of Speeding Tickets

Radar guns work by way of the Doppler Effect principle.  A signal is sent from the radar gun to the moving vehicle.  The signal reflects off of the vehicle and strikes the radar gun where it originated from.  The radar gun compares the received signal to the one it sent, and then, based on the frequency shift, computes the speed of the moving vehicle.  This very same principle is invoked to determine the relative velocity of distant stars based on the frequency shift of the light they emit.

When I first learned about the Doppler Effect and its application to handing out speeding tickets, I wondered why policeman were always stationary while using it.  After all, the devices measure relative velocity, so if the motion for the radar gun itself were accounted for, it could still compute the absolute speed of moving cars with sufficient accuracy.  For example, a radar gun could be mounted onto a police car, and have the vehicle's speedometer as an input - the device could then fire radio signals to cars behind and in front, receive the echo, compute the relative velocity, and then the absolute speed.  When I mention this to my students, they sometimes respond with, "Don't give the cops any ideas."