Veh. Code, § 40804

(a) In any prosecution under this code upon a charge involving the speed of a vehicle, an officer or other person shall be incompetent as a witness if the testimony is based upon or obtained from or by the maintenance or use of a speed trap.

(b) An officer arresting, or participating or assisting in the arrest of, a person so charged while on duty for the exclusive or main purpose of enforcing the provisions of Divisions 10 (commencing with Section 20000) and 11 (commencing with Section 21000) is incompetent as a witness if at the time of that arrest he was not wearing a distinctive uniform, or was using a motor vehicle not painted the distinctive color specified by the commissioner.

(c) This section does not apply to an officer assigned exclusively to the duty of investigating and securing evidence in reference to the theft of a vehicle or failure of a person to stop in the event of an accident or violation of Section 23109 or 23109.1 or in reference to a felony charge or to an officer engaged in serving a warrant when the officer is not engaged in patrolling the highways for the purpose of enforcing the traffic laws.