What is the prima facie speed limit?

Prima Facie literally means "on it's first appearance"...it's the requirement that you are to follow in the first instance with no other overriding circumstances...

In California the Prima Facie limits are defined in each state's vehicle codes. For instance, in California 15mph limits are assumed when traversing a railway grade crossing and your visibility is limited, when visibility if obstructed near and intersection, in Alleys, etc...

Hope this helps,
Scott