U.S. Route 101, or U.S. Highway 101, is a U.S. highway running north-south through the states of California, Oregon, and Washington, on the far West Coast of the United States. It is also known as El Camino Real (The Royal Road) where its route along the southern and central California coast approximates the old trail which linked the Spanish missions, pueblos and presidios. It merges at some points with California Route 1.
According to the AASHTO's numbering scheme for U.S. Highways, three-digit route numbers are generally subsidiaries of two-digit routes. However, the principal north-south routes were assigned numbers ending in 1. Rather than lose four available north-south numbers (93, 95, 97, and 99) or assign the primary west coast highway a "lesser" number, the AASHTO made an exception to its two-digit rule. Thus, U.S. 101 is treated as a primary, two-digit route with a "first digit" of 10, rather than a spur of U.S. 1. Thus U.S. Route 101, not U.S. 99, is the westernmost north-south route in the U.S. Highway system.
U.S. Route 101 was once the major north-south link along the Pacific coast. North of San Francisco, it has been replaced in importance by the highways of the Interstate Highway System, specifically Interstate 5, which is more modern in its physical design. Route 101 is still in use as a parallel freeway or highway between Los Angeles and San Francisco, and is an alternative to the Interstate for most of its length. In 1964, California truncated its southern terminus in Los Angeles, as Interstate 5 replaced it. The old road is known as county road S-21 or Historic Route 101 in northern San Diego County.
The highway's "northern" terminus is in Olympia, Washington, though the northernmost point on the highway is in Port Angeles. The southern terminus of U.S. 101 is in Los Angeles, California at the East Los Angeles Interchange, the world's busiest freeway interchange.
Bolded cities are officially-designated control cities for signs
In Northern California it is the primary coastal route providing motorists access in and out of the San Francisco Bay Area. It is also the primary commuter route carrying residents of Marin and Sonoma into San Francisco. For commuters of San Mateo County, San Jose, and other cities that make up Silicon Valley it shares this duty with Interstate 280. The route proceeds northward (via Van Ness Ave., Lombard St., and Richardson St.), leaving the City of San Francisco by crossing the Golden Gate Bridge. It then departs the immediate coast and continues through wine country and Redwood forests until it re-emerges coast-side at Eureka. The route provides access through the extraordinary terrain of the North Coast and into vast groves of protected Redwoods in area parks, including Redwood National and State Parks before reaching the Oregon State Line.
Unlike Washington, California does not sign the long east-west section of U.S. 101 between Point Conception and its junction with California State Route 134 and 170 in North Hollywood as "West" and "East." Instead, Caltrans observes the overall direction of the highway and marks these portions as "North" and "South". Local references to this potion of the freeway, including traffic reports, refer to the directions on this section as east for southbound lanes and west for northbound lanes. In the late 90s, Caltrans began placing guide signs on local streets in the San Fernando Valley adjacent to the 101 which identified eastbound and westbound entrances to the freeway. Other than replacing older guide signs which previously referenced the official northbound and southbound designations of the freeway, Caltrans made no other changes to mainline or street signing, and as such, pull-through signs, overhead signs, and freeway entrance signs all still reference north and south instead of east and west.
The highway runs north through Washington, running parallel to the Pacific Coast, all the way up the western side of the Olympic Peninsula. In the northwestern part of the peninsula, the highway turns east, and runs along the peninsula's northern edge, parallel to the Strait of Juan de Fuca. In the peninsula's northeast corner, the highway then turns south, running along the eastern edge of the peninsula and along the western shore of Hood Canal, until the highway terminates at a junction with Interstate 5 in Olympia. Between the intersection with State Route 112, on the western edge of the Olympic Mountains, U.S. 101 is signed east/west, and roughly south of the intersection with State Route 20, U.S. 101 is signed north/ south but having turned around 180 degrees. The direct route between the towns of Aberdeen (on the coast) and Olympia is US 12 and State Route 8.
The Washington section of U.S. 101 is defined at Washington Revised Code §47.17.165.
Parts of Historic Route 101 can still be found in San Diego County between Oceanside and the border with Mexico under a variety of different names. Through Oceanside it is called Coast Highway. In Carlsbad it becomes Carlsbad Blvd, but to the south in Encinitas it is Coast Highway 101. Solana Beach keeps it consistent with Highway 101, but Del Mar changes it to Camino Del Mar. All of those together make up San Diego County Route S21. From there it continues along Torrey Pines Road to Interstate 5 in La Jolla. The old 101 routing continues on I-5 in San Diego until Pacific Highway, the old U.S. 101 freeway a little west of current I-5. It then continued on Harbor Drive and Broadway through Downtown San Diego and Chula Vista, then onto National City Blvd in National City. The southern parts of I-5 were U.S. 101 for a period before I-5 was completed also. All have been decommissioned, but the roadways still exist and are occasionally signed as Historic 101.
Most of Historic 101 between Gilroy and San Francisco is still active, either signed as Business 101 or as State Route 82. The 101A bypass, however, is mostly discontinuous and is paralleled by the actual freeway, in some cases serving as an access road to the freeway.
Large parts of the old U.S. Business 101 and State Route 82 surface roads between San Francisco and San Diego are designated El Camino Real (The Royal Road), a designation originally given any thoroughfare under the direct authority of the King of Spain and his viceroys. These portions constitute the first major road in California.
Before the Golden Gate Bridge was completed, Highway 101 was divided in the San Francisco Bay Area. 101-W (west) followed the same general right-of-way of today's 101 from San Jose to San Francisco. 101-E (east) generally followed the right-of-way taken by today's I-880 from San Jose to Oakland. Since there was no Golden Gate Bridge yet, 101-W became a ferry ride across the Golden Gate Strait. As today, 101-W proceeded up Van Ness, but without any left turn at Lombard, ending at the Hyde Street Pier. From there, motorists would drive onto a ferry boat which would cross the Golden Gate to Sausalito, where they would drive off the ferry directly onto the main street of Sausalito, which was signed for U.S. 101 once again.
The 101-E designation was removed by the 1940s and became state Route 17 (later Interstates 880 and 580) between San Jose and Santa Rosa. As the Bayshore Freeway was built along the east side of the San Francisco peninsula in the early 1950s, old U.S. 101 along the El Camino Real was posted as U.S. 101 Alternate or 101-a, and the freeway was marked U.S. 101-Bypass or 101-B. In 1964, when California renumbering numerous state highways, El Camino was renumbered California 82 and the Bayshore Freeway lost the Bypass designation.