Here is the CA vehicle code:
http://www.dmv.ca.gov/pubs/vctop/vc/tocd17c3a1.htm
Speed Trap Prohibition
40801. No peace officer or other person shall use a speed trap in arresting, or participating or assisting in the arrest of, any person for any alleged violation of this code nor shall any speed trap be used in securing evidence as to the speed of any vehicle for the purpose of an arrest or prosecution under this code.