Establishment and early development , California|alt=View from a sidewalk of several banners and signs with the U.S. 101 shield displayed in decorative fashion alongside an American flag pattern. US 101 was established as part of the initial United States Numbered Highway System that was developed by the
American Association of State Highway Officials (AASHO) in the 1920s. The preliminary plan recommended in 1925 had the highway terminate at
San Diego in the south and in
Port Angeles, Washington, to the north. The final plan extended US 101 within Washington around the east side of the Olympic Peninsula to Olympia and was adopted by the AASHO on November 11, 1926. The first section of US 101 to be signed in California was between San Diego and Los Angeles in January 1928, which was followed by the rest of the route later in the year. Portions of the coastal highway had already been constructed by the respective state governments and also followed foot and wagon routes developed in earlier centuries. Among these was
El Camino Real in California, which was formed by the
Portolá expedition in 1769 and 1770 and connected the historic
Spanish missions, pueblos, and
presidios. The California state government chose a section of El Camino Real in San Bruno in 1912 to become the first paved highway in the state. The San Diego–San Francisco section of El Camino Real was incorporated into the multi-state
Pacific Highway in the 1910s; other sections of US 101 in California used the existing Redwood Highway, which was constructed from 1917 to 1923, and Coast Highway. , photographed in 1938|alt=Black-and-white photograph of a narrow road winding around the cliffs overlooking the ocean. Construction of the Oregon Coast Highway began in 1921, two years after a state
referendum that voted in favor of funding the development of highways with a one-cent
gas tax. At the time, several short wagon roads and plank roads connected settlements on the coast, and overland travel primarily used beaches. The highway was gradually constructed and paved in the 1920s, but a set of six
ferry crossings remained. These ferries were operated by private companies until the state government acquired them in 1927 with plans to replace them with bridges. The five major Oregon bridges on US 101 were designed by state engineer
Conde B. McCullough and opened by 1936 using funding from the federal government's
New Deal programs. As automobile traffic on the Oregon Coast increased, other sections were realigned to avoid rugged terrain or bypassed with tunnels. The Oregon Coast Highway was declared complete on October 3, 1936, and cost $25 million to construct (equivalent to $ in dollars). Washington completed the final sections of the Olympic Loop Highway between 1927 and 1931 for $11 million (equivalent to $ in dollars). The project paved several existing sections of the state roads and also constructed of new highway from the
Queets River to Ruby Beach near
Kalaloch. Portions of US 101 passed through lands that were later incorporated into Olympic National Park when it was established in 1938; under the
National Park Service, sections of the Crescent Lake Highway were widened and improved in 1949. The
Washington State Highway Commission submitted an application to AASHO in 1955 to extend US 101 northeast from Discovery Bay to
Whidbey Island and
Mount Vernon, where it would terminate at
US 99. The proposal was rejected by AASHO for being too long of a detour and including a tolled ferry crossing.
New alignments and freeways in San Francisco, part of the pre-
Golden Gate Bridge alignment of US 101|alt=An overhead structure labeled "Hyde St. Pier" frames a historic ferryboat alongside a US 101 shield and older sailing ship. US 101 was split into two routes in the San Francisco Bay Area between San Jose and San Francisco in 1929: US 101W followed El Camino Real on the San Francisco Peninsula for ; US 101E traversed the
East Bay for to
Oakland, where it turned west on a
cross-bay ferry to San Francisco. The branches converged in Downtown San Francisco and traveled along city streets to the
Hyde Street Pier, where the highway continued on automobile ferries to
Sausalito at the south end of the Redwood Highway. By 1936, US 101E had been eliminated in favor of the route on the west side, which was re-designated as US 101. The Hyde Street–Sausalito ferry was removed from US 101 following the May 1937 completion of the
Golden Gate Bridge, which would carry the highway from San Francisco to Marin County. It was the
longest suspension bridge in the world at the time of its construction and was funded by a $35 million regional
bond (equivalent to $ in dollars) that was paid off in 1971. The San Jose–San Francisco section of US 101 was initially moved from El Camino Real to the
Bayshore Highway, a four-lane undivided highway that was constructed between 1924 and 1937 to bypass several towns on the peninsula. El Camino Real was re-designated as US 101 Alternate in 1936, which sparked outcry from businesses and groups who lobbied for a reversal of the change that was submitted by state officials to AASHO. In 1938, US 101 was moved back to El Camino Real and the Bayshore Highway was designated as US 101 Bypass. In the late 1940s, the California state government announced plans to convert most of US 101 between Los Angeles and San Francisco to freeways using funds from the
Collier–Burns Highway Act of 1947. Prior to the act, the Cahuenga Pass Freeway had opened in June 1940 between Hollywood and the San Fernando Valley in Los Angeles to replace a narrow, winding mountain road. It was the city's second freeway and was later extended southeast towards Downtown and renamed the Hollywood Freeway when it was completed on April 16, 1954. In
San Luis Obispo, a freeway around the northwest side of downtown was completed by the early 1960s, alongside bypasses of nearby rural towns. The San Diego–San Francisco section of US 101 was designated as El Camino Real by the California state government in 1959 as part of a program to add historic markers on the highway. The
Santa Ana Freeway was planned as a Los Angeles–
Irvine connector in the late 1930s and constructed in phases, beginning with a section near Downtown Los Angeles that opened in December 1947. US 101 was later moved onto sections of the freeway, which was completed in 1958 and served as a continuation of the Hollywood Freeway. By the time it was completed, sections of the freeway between Anaheim and Los Angeles were carrying over 113,000 vehicles per day and were planned to be widened to six lanes within a few years. The south end of the Santa Ana Freeway merged into the
San Diego Freeway, which began construction in 1954 and was completed in 1968. Both freeways were incorporated into plans for the new
Interstate Highway System in 1955 and assigned to I-5 three years later. US 101 was truncated to Los Angeles during a 1963 AASHO meeting at the request of the California state government, as I-5 had replaced the stretch to San Diego; the changes were made ahead of a
major restructuring of the state's highway system that took effect on July 1, 1964. The old sections of the highway from San Diego to Los Angeles were given local names and later signed as Historic US 101 in the late 2010s by local governments. The freeway had been proposed to address congestion and frequent collisions on the highway, nicknamed "Bloody Bayshore", and opened in stages between 1947 and 1962. From the north end of the Bayshore Freeway at
I-80 in San Francisco, US 101 was routed west along a section of the
Central Freeway, which opened in 1955 and was extended four years later to Van Ness Avenue. Plans to extend the Central Freeway and other thoroughfares through San Francisco to the Golden Gate Bridge were later cancelled by the mid-1960s following
widespread opposition and protests from city residents. , opened in 1966, carries US 101 over the
Columbia River north of
Astoria, Oregon.|alt=A green-colored truss bridge with two towers crossing a wide river as seen from a distance. Several sections of the Oregon Coast Highway were rebuilt in the 1950s and 1960s to eliminate curves and move the highway further from the coastline in cities such as Cannon Beach. A realignment from Brookings to Gold Beach and a more direct route from Bandon to Coos Bay to bypass
Coquille were completed as part of this program in the early 1960s. In 1955, Oregon congressman
A. Walter Norblad unsuccessfully proposed that the US 101 corridor be included in the
Interstate Highway System to allow for federal funds to construct a bridge across the Columbia River; the proposal was also endorsed by a
U.S. Army official, who also sought a similar designation for the Washington section. The Seaside–Astoria section was straightened and realigned onto a
new bridge over Youngs Bay in 1964. The
Astoria–Megler Bridge over the Columbia River opened to traffic on July 29, 1966, replacing a ferry and comprising the final "link" in US 101. The northern terminus of US 101 was originally at Capitol Way (US 99) in downtown Olympia until it was moved to a freeway bypass in December 1958. The freeway section was extended northwest from Olympia to
Shelton in 1965.
Modern projects The final traffic signal on the section of US 101 between Los Angeles and San Francisco, located at Anacapa Street in Santa Barbara, was removed in November 1991. The removal was spurred by the construction of a freeway through Santa Barbara, which was completed the following year and bypassed four signalized intersections. The Central Freeway's northernmost leg in San Francisco was demolished in the early 2000s after it had sustained damage in the
1989 Loma Prieta earthquake, which required the upper deck to be removed in 1997. A portion of the corridor was replaced by
Octavia Boulevard, which opened in 2005, while US 101 was rerouted onto Van Ness Avenue further east. From 2016 to 2022, Van Ness Avenue was rebuilt by the
San Francisco Municipal Railway to add center bus lanes and landscaped medians as part of the
Van Ness Bus Rapid Transit project. Several existing freeway sections in California were expanded to add
high-occupancy vehicle lanes (HOV lanes) beginning in the 1980s to address increased congestion, especially in the San Francisco Bay Area. By 1984, a section in Marin County had been opened to traffic; it was followed by sections in San Mateo and Santa Clara counties that were funded by a sales tax approved in a 1984 ballot measure. The HOV lanes were extended south through San Jose to Bernal Road in 1990. A section of the existing HOV lanes from
Redwood City to
San Bruno was converted to
high-occupancy toll lanes in 2023 with the use of
electronic toll collection. , in 2013|alt=The remains of an asphalt road that has been destroyed with several pieces lifted up from the ground and others with large cracks. Sections of US 101 in Oregon have been rebuilt or relocated due to erosion or landslides that caused considerable damage to the highway. The use of
riprap to reinforce new sections of the roadway was banned by the
Oregon Department of Land Conservation and Development in 1977 due to its effect on beach degradation. An effort to repeal the ban and rebuild sections of US 101 began in 2002 and resulted in an amendment in 2022. Several sections of US 101 between Newport and Lincoln City are identified as persistent sunken grade and rockfall areas in need of frequent repairs. Severe erosion of the highway along the
Hoh River in Washington was addressed through the construction of eight engineered
logjams by the
Washington State Department of Transportation in 2004. The project redirected the river's flow through the use of dense piles of
spruce logs reinforced by steel piles; the highway previously underwent four emergency repairs in the area that included installation of riprap and other conventional materials. US 101 was relocated onto an expressway bypass of
Sequim, Washington, in August 1999 that replaced a congested route on the city's main street, Washington Avenue. The project cost $40.7 million (equivalent to $ in dollars) to construct and included the installation of
warning signals triggered by the presence of nearby
Roosevelt elk wearing
radio collars for tracking. The section of the highway between Port Angeles and Sequim was widened to four lanes in November 2014 following a two-year project to fill the final, gap. The section west of Port Angeles along Lake Crescent was rehabilitated from 2017 to 2019 to repave the roadway and repair structures—among them retaining walls and guardrails. A freeway bypass of
Willits, California, for the Redwood Highway opened in November 2016 at a cost of $460 million (equivalent to $ in dollars). The bypass was expected to divert away tourists and cause a drop in local
sales tax revenue due to lost traffic. A portion of US 101 in the North Bay region near San Francisco, nicknamed the "Novato Narrows", was widened to three lanes with the addition of an HOV lane; construction on the section began in 2011 and is scheduled to be completed in 2026. A four-phase widening through the Santa Barbara area to add a third lane in each direction began construction in 2008. Its final phase is estimated to cost $700 million and be completed in 2027. The world's largest urban
wildlife crossing, named the
Wallis Annenberg Wildlife Crossing, is under construction over US 101 in
Agoura Hills, California, and is scheduled to open in 2026. ==Major intersections==