Eh, I'm bored so I went and found it for you all. Enjoy yet another ticket killer courtesy of Mr. Wizard.
Technowizard.
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.
40802. (a) A "speed trap" is either of the following:
(1) A particular section of a highway measured as to distance and
with boundaries marked, designated, or otherwise determined in order
that the speed of a vehicle may be calculated by securing the time it
takes the vehicle to travel the known distance.
(2) A particular section of a highway with a prima facie speed
limit that is provided by this code or by local ordinance under
subparagraph (A) of paragraph (2) of subdivision (a) of Section
22352, or established under Section 22354, 22357, 22358, or 22358.3,
if that prima facie speed limit is not justified by an engineering
and traffic survey conducted within five years prior to the date of
the alleged violation, and enforcement of the speed limit involves
the use of radar or any other electronic device that measures the
speed of moving objects. This paragraph does not apply to a local
street, road, or school zone.