Note that exceeding a properly posted speed limit is prima facie evidence of a violation of 22350 CVC.
From the CA Attorney General in the CA Peace Officers Legal Sourcebook):
Speed Law Violations - Procedural (Veh. Code, § 22351)
The speed of any vehicle upon a highway not in excess of the limits specified in section 22352 (prima facie speed limits) or established as authorized in the Vehicle Code is lawful unless clearly proved to be in violation of the basic speed law (reasonable or prudent).
The speed of any vehicle upon a highway in excess of the prima facie speed limits in section 22352 or established in the Vehicle Code is prima facie unlawful unless the defendant establishes by competent evidence that the speed in excess of said limits did not constitute a violation of the basic speed law (Veh. Code, § 22350) at the time, place and under the conditions then existing.
In cases where the driver was not exceeding a posted speed limit, the arresting officer must prove both the speed and that it was a speed greater than was reasonable or prudent under the conditions.
In cases where the driver was exceeding the posted speed limit, the arresting officer must only prove the speed. The burden of proof then shifts to the defendant who must prove that the speed was safe at the time, place, and under the conditions existing. You should be prepared to overcome this defense by your notes containing the number of cars passed, intersections traversed, lane changes, pedestrians, other hazards and conditions.
The defendant has no such defense in cases involving maximum (absolute) speed limits.