Quote Quoting CDornsife
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If you need to win this because of priors you need a very competent attorney that can challenge the safety value of the 50 mph posting. The speed trap law only applies when radar/lidar is used, but due process applies in all cases.

In California we have found reverse engineering a prevalent practice to cause limits to be under posted and California's standards for setting limits can also be challenged. The number posted is never the maximum safe speed because the 85th percentile speed is the safest speed, and limits in California are as a matter of practice posted at or below this threshold so the number on the sign does not in itself represent probable cause for an unsafe act.

This is a prima facie zone that provides a rebuttable presumptive challenge. Prior to the stop the officer must know what the range of safe speeds are for that particular segment of roadway as determined by the study, this is an extremely rare occurrence. Moreover, because California studies are only spot studies, the range of safe speeds over the course of a day is an unknown nor do their studies quantify this range, thus the officer becomes in-law incompetent to testify per se as to prior knowledge of what a safe speed was.

Defenses 1. Value posted on the sign not supportable by law, 2. Safe for conditions range of speeds not quantified or unknown because of inadequate study or quantification, 3. Absent prior knowledge of 1 and 2 traffic stop lacked probable cause because range of safe speeds is to be determined by an engineering finding of fact, absent prior knowledge the officer lacked foundation for stop, 4. and of particular importance in this instance, by citing the posted as the safety threshold they have demonstrated their ignorance of engineering tenants that govern this field and the shape of the parabolic risk curve for being involved in an accident because the posted is never that threshold per California posting practices. Keep in mind that the mean plus 16 mph is still within the safe for conditions speed according to the FHWA, and on higher speed roads the safest speed shifts to the 90th percentile speed.

The Basic Speed Rule trumps: The Highway Safety Act of 1966 incorporated into federal law the progeny of the 1926, Uniform Vehicle Code (UVC) which in part required motorists to drive at speeds “reasonable and prudent” (Basic Speed Law UVC § 11-801) where speed in itself is not a violation of law; and, the 1927 the American Association of State Highway Officials published the “Manual and Specifications for the Manufacture, Display, and Erection of U.S. Standard Road Markers and Signs (for rural roads)” which evolved into the Manual on Uniform Traffic Control Devices (MUTCD), 23 U.S.C. 109(d) and 402(a) et al.
You might want to re-read VC22351(b) and bone up on CA speed trap law a little bit.