Most statistics regarding the growth of California's highway system focus on state-owned highways. But the so-called "state highway system" itself proves to be a poor measure for gauging the growth in California's road capacity. "State highways" are really just those roads that are owned and operated by the State of California (Caltrans, the California Department of Transportation), a mere 13 percent of all lane-miles in the state's overall road system. A majority of the total mileage owned by Caltrans, and thus comprising the state highway system, are actually two lane roads like Highway One on the Pacific coast and function as both local streets and main streets for many small towns throughout California.

The state highway system does include Interstate highways (I-5, I-80, I-10), US routes (US 50, US 101, US 395) and other numbered state routes with the familiar rounded green shield. Yet it doesn't include other freeways and expressways under the jurisdiction of city and county governments. The state is also often "shedding" segments of its system (especially two lane roads that function as local streets) and turning them over to local government control, a move that will appear as a loss of overall state highways when the roads themselves still exist but merely shift to a different jurisdiction.

A much more useful classification for different types of roadways is provided by the Federal Highway Administration based on usage, design and traffic levels. From largest to smallest the classifications include: Interstate highways, other freeways and expressways, principal arterial streets, major collector streets, minor collector streets, and finally local streets. The state highway system includes all the Interstate highways and some pieces of every other type of classification of roadway.