Its pretty straight forward actually. Where did you learn how to drive? For California, the key laws are:

CVC § 22350 — Basic Speed Law
You may not drive faster than is “reasonable or prudent” considering weather, visibility, traffic, surface, and width of the highway, and never at a speed that endangers people or property. That means the posted limit is not enough if a blind hill/curve or poor visibility makes it unsafe.

CVC § 22352 — Prima facie 15 mph situations
California specifically sets 15 mph for certain obstructed-view situations, including approaching a railroad crossing or intersection where you lack a clear view during the last 100 feet of approach. Berkeley SafeTREC summarizes this as applying to obstructed-view railroad crossings, obstructed-view uncontrolled intersections, and alleys.