Also in Cal they can't use radar unless they've recently done one of those surveys and set the radar speed threshold based on the criteria I mentioned. And they have to post a sign warning you of radar use. Of course there's no rule saying they have to remove those signs once the survey has expired (3 years I think...) so those radar in use warnings are all over the place and you can't tell which are actually true :-(

Also in California "speed traps" are illegal: this is when a highway runs through a town or district and the district puts up a sudden "25 MPH" sign so they can catch out of towners and charge them money.