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Standard route signage in California
|Notes||All classes of state-numbered highways are generally state-maintained.|
|Interstates||Interstate X (I-X)|
|US Highways||U.S. Route X (US X)|
|State||State Route X (SR X)|
Each highway is assigned a Route (officially State Highway Route  ) number in the Streets and Highways Code (Sections 300-635). Most of these are numbered in a statewide system, and are known as State Route X (abbreviated SR X). United States Numbered Highways are labeled US X, and Interstate Highways are Interstate X. Under the code, the state assigns a unique Route X to each highway, and does not differentiate between state, US, or Interstate highways.
California's highway system is governed pursuant to Division 1 of the California Streets and Highways Code. Since July 1 of 1964, the majority of legislative route numbers, those defined in the Streets and Highways Code, match the sign route numbers. For example, Interstate 5 is listed as "Route 5" in the code.
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.
Concurrences are not explicitly codified in the Streets and Highways Code; such highway segments are listed on only one of the corresponding legislative route numbers — for example, the I-80/ I-580 concurrency, known as the Eastshore Freeway, is only listed under Route 80 in the highway code while the definition of Route 580 is broken into non-contiguous segments, while other Routes such as US 101 and State Route 2 are concurrent and cosigned, but such concurrent portion of both Routes are internally referred to only as Route 101.  
The state may relinquish segments of highways and turn them over to local control. If the relinquished segment is in the middle of the highway's route, the local jurisdiction is usually required to install and maintain signs directing drivers to the continuation of that highway; they are not generally required to do so if the relinquishment effectively truncates the highway at one end, or is done as part of the process to re-route a highway. The state may also delete a highway completely and turn over an entire state route to local control.
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. Currently there are two such unrelinqushed routes, with State Route 14U, an old alignment of State Route 14, as the most recent example of such, where the process to relinquish 14U started on January 1 of 2018,  along with State Route 103U being the other unrelinquished route within the system.
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 included the "S" suffix on its signing until it was transferred to SR 86 in December 2012. 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 (ACSC) 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:
- 0 or 1 modulo 4: central and northern California
- 2 or 3 modulo 4: southern California
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. This transfers the cost of maintaining them from state to local budgets, but also gives local governments direct control over urban arterial roads that carry primarily local traffic. Once transferred, if a local government wants to add landscaping in the center median or install additional traffic lights or other traffic control devices, it can immediately do so itself rather than having to negotiate with Caltrans. 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" 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.    When the Southern California freeway system was built in the 1940s and early 1950s, local common usage was primarily the freeway name preceded by the definite article, such as "the Hollywood Freeway".  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. For example, it evolved to "the 605 Freeway" and then shortened to "the 605".  This did not occur in Northern California, where usage of the route numbers was more common.
- U.S. Route 40
- U.S. Route 40 Alternate
- U.S. Route 48
- U.S. Route 60
- U.S. Route 66
- U.S. Route 70
- U.S. Route 80
- U.S. Route 91
- U.S. Route 99
- U.S. Route 99E (central California)
- U.S. Route 99E (northern California)
- U.S. Route 99W (central California)
- U.S. Route 99W (northern California)
- U.S. Route 101A
- U.S. Route 101 Bypass (San Francisco Bay area)
- U.S. Route 101 Bypass (Los Angeles & Orange counties)
- U.S. Route 101E
- U.S. Route 101W
- U.S. Route 299
- U.S. Route 399
- U.S. Route 466
- California Roads portal
- Deleted California State Highways
- Unconstructed California State Highways
- Scenic California State Highways
- List of county routes in California
- California Streets and Highways Code, Section 231
- Caltrans, Other State Highway Routes
- Google Maps Access Date: December 15, 2018
- Google Maps Access Date: December 15, 2018
- Metadata for ST_HWY GIS data layer ( PDF)
- California Department of Transportation (July 2007). "Log of Bridges on State Highways". Sacramento: California Department of Transportation. (calls State Route 14U and State Route 180S, among others, spurs)
- Simon, Mark (2000-06-30). "'The' Madness Must Stop Right Now". San Francisco Chronicle. Retrieved 2009-09-22.
- Simon, Mark (2000-07-04). "Local Lingo Keeps 'The' Off Road". San Francisco Chronicle. Retrieved 2009-09-22.
- Simon, Mark (2000-07-29). "S.F. Wants Power, Not The Noise / Brown rejects docking floating plant off city". San Francisco Chronicle. Retrieved 2009-09-22.
- Geyer, Grant (Summer 2001). "'The' Freeway in Southern California". American Speech. 76 (2): 221–224. doi: 10.1215/00031283-76-2-221.