The police might also use a "laser gun". Then a radar detector will not work.
A "laser gun" measure the time for the infra red light to travel to the moving object and back. After a number of successful measurements a speed is calculated. As one example Kustoms Pro Laser II are sending 250 laserbeams per second and to get a speed value it needs 80 approved measurements.
Two manufacturers of speed meusuring equipment:
http://www.kustomsignals.com/
http://www.lasertech.com/
Devices that detect laser/radar or disturbs measurement are also illegal in some places.