There are 9 routes assigned to the "N" zone of the California Route Marker Program, which designates county routes in California. The "N" zone includes county highways lying in Los Angeles and Orange counties.

State Route 138 is an east–west state highway in the U.S. state of California that generally follows the northern foothills of the San Gabriel Mountains and the western Mojave Desert. The scenic highway begins in the west at its junction with Interstate 5 located south of Gorman in the Sierra Pelona Mountains, continues eastward through the Antelope Valley and Cajon Pass, to its junction with State Route 18 in the east, located in the San Bernardino Mountains south of Crestline.