While I realize Wikipedia is not infallible, this is what they say about the jurisdiction of CHP. It is indeed quite broad. This is consistent with the fact that counties are considered subdivisions of the State.
"The agency has specific jurisdiction over all California state routes (including all freeways and expressways), U.S. Highways, Interstate Highways, and all public roads in unincorporated parts of a county. Local police or the local sheriff's department having a contract with an incorporated city are primarily responsible for investigating and enforcing traffic laws in incorporated cities, but the CHP can still enforce traffic laws on any public road anywhere in the state."
http://en.wikipedia.org/wiki/California_Highway_Patrol
