Since July 1 of 1964, the majority of legislative route numbers, those defined in the Streets and Highways Code, match the sign route numbers. On the other hand, some short routes are instead signed as parts of other routes - for instance, Route 112 and Route 260 are signed as part of the longer State Route 61, and Route 51 is part of Interstate 80 Business.
Business routes are not maintained by the state unless they are also assigned legislative route numbers. A few routes or sections of routes are considered unrelinquished - a new alignment has been built, or the legislative definition has changed to omit the section, but the state still maintains the roadway - and are officially Route XU. State Route 14U, an old alignment of State Route 14, is the only one signed as such. Some new alignments are considered supplemental and have a suffix of S; State Route 86S, a replacement for State Route 86 between approximately three miles north of the Imperial/Riverside County line west of the Salton Sea and Interstate 10 east of Indio is the only one that includes the "S" suffix on its signing. Both types of suffixed routes are also considered spurs. Current or former unsigned suffixed routes include State Route 156U, signed as State Route 156 Business through Hollister, and State Route 180S, the freeway replacement for State Route 180 in Fresno (now signed as SR 180).
The first legislative routes were defined by the State Highway Bond Act in 1909, passed by the California State Legislature and signed by Governor James Gillett. These, and later extensions to the system, were numbered sequentially. No signs were erected for these routes.
The United States Numbered Highways were assigned by the American Association of State Highway Officials (AASHO) in November 1926, but posting did not begin in California until January 1928. These were assigned to some of the main legislative routes in California. Initially, signs were posted by the Automobile Club of Southern California (ASCS) and California State Automobile Association (CSAA), which had been active in signing national auto trails and local roads since the mid-1900s.
In 1934, after the major expansion of the state highway system in 1933 by the California Legislature, California sign route numbers were assigned by the California Division of Highways (predecessor to Caltrans). The California sign route numbers were assigned in a geographical system, completely independent of the legislative routes. Odd-numbered routes ran north-south and even-numbered routes ran east-west. The routes were split among southern California (ACSC) and central and northern California (CSAA) as follows:
For instance, State Route 1 and State Route 4 were in central and northern California, and State Route 2 and State Route 3 (since moved) were in southern California. A rough grid was used inside the two regions, with the largest numbers - all less than 200 (except for State Route 740, which was related to State Route 74) - in eastern California (north-south) and near the border between the two regions (east-west).
The Interstate Highway System numbers were assigned by AASHO in late 1959. In 1963 and 1964, a total renumbering of the legislative routes was made, aligning them with the sign routes. Some changes were also made to the sign routes, mostly related to decommissionings of U.S. Routes in favor of Interstates.
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 division in that region is granted permission to keep state routes that run on city streets in commission unless a freeway is built to bypass the surface street route.