Take a look at 22351:
22351. (a) The speed of any vehicle upon a highway not in excess of
the limits specified in Section 22352 or established as authorized
in this code is lawful unless clearly proved to be in violation of
the basic speed law.
(b) The speed of any vehicle upon a highway in excess of the prima
facie speed limits in Section 22352 or established as authorized in
this 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 at the time, place and
under the conditions then existing.
You can be in violation of the basic speed law even if you are driving slower than the posted speed limit. Likewise, you can be driving in excess of the speed limit and NOT be in violation of the basic speed law. In short, 22351a says that the burden lies with the prosecution to prove that your speed was unsafe if you are below the speed limit. 22351b says that the burden lies with the defendant to prove that his speed was not unsafe if it was over the posted speed limit. So, effectively, the speed limit is only a line that defines who has the burden to prove their case. As per 22351b, if you are over the limit, you are assumed to be in violation of the basic speed law. However, that presupposes that the posted speed limit is valid. With a speed survey that hasn’t been updated since 2006, I’d argue that the speed limit is NOT valid. For a quick analogy, use the extreme as an argument. If the posted speed limit was 10mph on a major 4 lane highway. Would it be valid for the police to “pace” people and give them tickets for driving 50mph (especially if the speed survey showed the 85th percentile to be 55mph)?
Now take a look at 41100:
41100. In any action involving the question of unlawful speed of a
vehicle upon a highway which has been signposted with speed
restriction signs of a type complying with the requirements of this
code, it shall be presumed that existing facts authorize the erection
of the signs and that the prima facie speed limit on the highway is
the limit stated on the signs. This presumption may be rebutted.
This says that the posted speed limit is presumed to be to represent a justified speed limit… but it may be rebutted. By producing a copy of the most recent speed survey (from 2006), you would be rebutting the validity of the posted limit sign.

