State highways in California

The state highway system of the U.S. state of California is a network of highways that are owned and maintained by the California Department of Transportation (Caltrans).

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.

The California Highway Patrol (CHP) is tasked with patrolling all state highways to enforce traffic laws.

California's highway system is governed pursuant to Division 1 of the California Streets and Highways Code, which is one of the 29 California Codes enacted by the state legislature. 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.

Some short routes are instead signed as parts of other routes regardless of legislative definition. For instance, Route 112 and Route 260 are signed as part of the longer State Route 61. Route 51 is signed as Interstate 80 Business, but is done so under legislative mandate in section 351.1.

The Streets and Highways Code allows for non-contiguous segments of state highways, which are logged by Caltrans using the route's postmile data, picking up where it leaves off at any legislative breaks in the state highway as originally measured. To avoid duplication, concurrencies are only listed in one of their respective highway's definition, and are treated as non-contiguous segments in their other highways' definition. For example, the I-80/I-580 concurrency, known as the Eastshore Freeway, only falls under the Route 80 description in the highway code while the definition of Route 580 is broken into non-contiguous segments. However, Caltrans may not sign all concurrences. For example, while Route 1 is generally co-signed along Route 101 between its disconnected segment from San Francisco to Sausalito, Route 1 is not co-signed along Route 101 between near the Ventura-Santa Barbara county line and Las Cruces.

The state highway system may be expanded by either construction of new state highways or adoption of local highways as state highways. Section 75 allows the California Transportation Commission to select, adopt and determine locations for state highway routes, while Section 90 empowers Caltrans to construct and maintain said state highways between the termini designated by the routes listed in Chapter 2, Article 3. Section 81 allows the commission to adopt local highways as state highways, and without compensation as stated in Section 83, as long as the adopted route meets eligibility requirements. Said requirements include that the road be traversable from end-to-end, must exist along a route defined in Chapter 2, Article 3 to be superseded or yet to be constructed, be contiguous to a portion of the state highway system currently maintained by Caltrans, and be constructed to adequate standards. For example, Seal Beach Boulevard in the City of Seal Beach between Route 1 and Route 405 would be eligible for adoption under Section 81, as its traversable alignment matches an unconstructed portion of Route 605 as defined in Section 619, subdivision (a). By law, an adoption under this section would sign the route as State Route 605.

Section 73 of the code permits the commission to relinquish any portion of a state highway to a city or county that has been deleted via legislative enactment or superseded by relocation, but only after the highway has been placed in a state of good repair as defined in Section 23. Legislative deletion can involve the supersession of a state highway route by a different state highway route that serves a similar corridor, such as the completion of Route 105 forcing the deletion of Route 42, or an agreement between the state and city or county to transfer a portion of state highway to local control based on the desires of either entity. Depending on the relinquishment, the local jurisdiction may be required to install and maintain signs directing drivers to the continuation of that highway. These portions of state highway relinquished are deemed by the legislature to be ineligible for future adoption under Section 81. In most cases, these highways are ineligible by default as the route's definition is amended by the legislature.