The Federal Highway Administration [FHWA) identifies functional classification as a key item in transportation data. Streets and highways are grouped into classes according to the service they provide. The California Road System (CRS maps) on this web site display functional classification which is used in determining Federal funding to maintain the roads. The FHWA stamp of approval is in the lower left corner of each CRS map (PDF), which makes it the official functional classification.
(Click on the above legend to view a larger image and/or save it.)