California uses a postmile highway location marker system on all of its state highways, including U.S. Routes and Interstate Highways. The postmile markers indicate the distance a route travels through individual counties, as opposed to milestones that indicate the distance traveled through a state. The postmile system is the only route reference system used by the California Department of Transportation (Caltrans). The system was operative by 1966.

State Route 14 (SR 14) is a north–south state highway in the U.S. state of California that connects Los Angeles to the northern Mojave Desert. The southern portion of the highway is signed as the Antelope Valley Freeway. Its southern terminus is at Interstate 5 in the Los Angeles neighborhoods of Granada Hills and Sylmar just immediately to the south of the border of the city of Santa Clarita. SR 14's northern terminus is at U.S. Route 395 (US 395) near Inyokern. Legislatively, the route extends south of I-5 to SR 1 in the Pacific Palisades area of Los Angeles; however, the portion south of the junction with I-5 has not been constructed. The southern part of the constructed route is a busy commuter freeway serving and connecting the cities of Santa Clarita, Palmdale, and Lancaster to the rest of the Greater Los Angeles area. The northern portion, from Vincent to US 395, is legislatively named the Aerospace Highway, as the highway serves Edwards Air Force Base, once one of the primary landing strips for NASA's Space Shuttle, as well as the Naval Air Weapons Station China Lake that supports military aerospace research, development and testing. This section is rural, following the line between the hot Mojave desert and the forming Sierra Nevada mountain range. Most of SR 14 is loosely paralleled by a rail line originally built by the Southern Pacific Railroad, and was once the primary rail link between Los Angeles and Northern California. While no longer a primary rail line, the southern half of this line is now used for the Antelope Valley Line of the Metrolink commuter rail system.