Here's a concrete example: Embarcadero road in Palo Alto feeds from highway 101. It has a posted speed limit of 25, and there is a perennial complaint presented to the city council, letters to the editor etc: why won't the police come out and enforce the speed limit? The answer is that to do so they'd have to do a survey and then adjust the limit...up (I think traffic travels about 40-45 MPH on this road). If that is the reasonable and proper rate for that road, to enforce anything lower would be a speed trap. The city keeps the 25 signs up in the hope that they discourage even faster driving.
Likewise when they use radar they not only have to post a sign but do a survey, which has to be updated every few years -- you can't use radar to enforce something unfair. I'm not sure the radar signs on Embarcadero are even current (thus probably not even legal).
BTW as I learned in driving school years ago (got a ticket), apart from "reasonable and proper at all time" you can't go over 55 anywhere unless it's explicitly posted. Actually I got all this initially from driving school, and then when Jim Warren went to the effort of getting the entire CA legal code online in the 90s, I actually read a good chunk of the vehicle code in particular.
As a Californian, this interests me -- can you cite a source for me to follow? I found this page [0] from Caltrans, but it speaks to determining the posted speed limit, not overriding the posted speed limit.
[0] https://dot.ca.gov/programs/safety-programs/setting-speed-li...
> The most widely accepted method of determining the posted speed limit is to set the speed limit at what is called the “85th percentile speed”, which is the speed at or below which 85 percent of the traffic is moving.