There is the source for California -- I see it gives exemptions for safety and law (I assume for road works, etc) but generally speed limits are objective based on conditions. It doesn't mean you can drive slow and impede other traffic traveling around you though, which is why there are laws for it. I don't see it enforced as much in San Diego, but when I lived in the Lake Arrowhead area it was enforced quite often on the highway up and down the mountain.

Quick edit -- that law is for highways, I haven't checked whether this also applies for other types of roads.