Some new alignments are considered supplemental[4] and have a suffix of S. For example, an expressway replacement for State Route 86 between approximately three miles north of the Imperial/Riverside County line and Interstate 10 east of Indio was initially signed as State Route 86S until it was eventually transferred to SR 86.

Initially, signs were posted by the Automobile Club of Southern California (ACSC) and California State Automobile Association (CSAA), which had been active in signing national auto trails and local roads since the mid-1900s.

Since the 1990s, many non-freeway routes, especially in urban areas, have been deleted and turned over to local control.

Not all cities have been prepared to accept such routes from Caltrans simultaneously, so many have been decommissioned from the state system one fragment at a time.

In the case of the San Francisco Bay Area, the Caltrans district responsible for that region is granted permission to retain in the State highway system routes that run on conventional (non freeway or expressway) roadways unless a freeway is built to bypass the surface street route.

One cultural difference between Northern and Southern Californians is that the latter tend to put the definite article "the" before highway numbers (e.g. "taking the 5 to L.A."), while the former use the number alone (e.g. "taking 80 to San Francisco") or less frequently, with "I-" in the case of interstate freeways.

[5][6][7] The Southern California usage of the definite article is seen as stereotypical of Southern California "surfer" culture, and has been parodied in the recurring Saturday Night Live sketch "The Californians".

[9] It took several decades for Southern California locals to start to commonly refer to the freeways with the numerical designations, but usage of the definite article persisted.

The list of routes, as defined in the California Streets and Highways code, is split into the following pages: