U.S. Route 101 in California

Southern California

US 101 begins in Boyle Heights at the East Los Angeles Interchange, a major freeway junction that includes I-5, I-10, and SR 60. The six-lane portion of the route is the northerly continuation of the Santa Ana Freeway, inheriting that title from I-5. After 2 miles (3.2 km), US 101 turns west at the San Bernardino Split, a three-way junction with the San Bernardino Freeway that transitions into I-10 to the east. US 101 travels on a 1944 bridge across the Los Angeles River before passing Los Angeles Union Station.^[14]

Proceeding in a generally northwestern direction, US 101 travels through downtown Los Angeles via the Downtown Slot^[15] between the Los Angeles Civic Center and El Pueblo de Los Ángeles, then intersects SR 110 at the Four Level Interchange. Starting here, US 101 is named Hollywood Freeway. It passes through Echo Park, Silver Lake, Los Feliz, and Hollywood, sharing a short segment with SR 2 through the area. North of Hollywood, US 101 traverses the Santa Monica Mountains via the Cahuenga Pass before entering the San Fernando Valley. It then passes Universal Studios Hollywood, after which is the Hollywood Split where US 101 shifts westward, superseding SR 134 on the Ventura Freeway, while the Hollywood Freeway continues north as SR 170.^[14]

From the Hollywood Split, US 101 runs east–west despite its north–south designation. Because of this, the same freeway entrance can be signed as 101 North and 101 West or 101 South and 101 East; this is most common in the San Fernando Valley where local signage does not match the Caltrans' statewide designation.^[16] In this segment, US 101 intersects I-405 in Sherman Oaks and SR 27 in Woodland Hills. In Calabasas, US 101 changes from heavily urbanized to a somewhat rural character as it enters Conejo Valley, where it leaves Los Angeles County and enters Ventura County.^[17] In Thousand Oaks, US 101 and SR 23 run concurrently for about 2.44 miles (3.93 km), from Westlake Boulevard to a three-way interchange with the Moorpark Freeway.^[14]

Continuing westward, US 101 climbs the Conejo Grade on a 7% incline^[18] before descending into the Oxnard Plain, where it travels through Camarillo and Oxnard. SR 1 and US 101 run concurrently in Oxnard; there is no signage confirming the overlap^{[citation needed]} despite maps depicting this. Upon crossing the Santa Clarita River, the two routes reach Ventura, where they intersect SR 126 and then SR 33.^[14]

Central Coast

US 101 crosses the Ventura River at Emma Wood State Beach, at which point the Pacific Ocean comes into view. The route then passes La Conchita before entering Santa Barbara County, where it intersects SR 150; this is also where the Ventura Freeway name ends. In Santa Barbara, US 101 intersects SR 154, then continues into Goleta, where it intersects SR 217.^[14] US 101 then narrows to four lanes and switches from freeway to expressway,^[a] albeit with several interchanges serving beaches and parks along the Gaviota Coast.^[b] This stretch of SR 101 often comes within one to two miles (1.6 to 3.2 km) of the coast.^[20][17] At Gaviota, US 101 turns sharply north, thus resuming its original north–south alignment, and heads inland across the Santa Ynez Mountains via the Gaviota Pass, where the northbound lanes pass through the Gaviota Tunnel, while no tunnels exist in the southbound direction.^[14]

Throughout the rest of the Central Coast, US 101 switches intermittently between freeway and expressway status,^[17] but despite occasional cross-traffic it has unimpeded traffic, as there are no stop signs or traffic signals.^[21]

About 1+1⁄2 miles (2.4 km) north of the Gaviota Tunnel, SR 1 splits from US 101 to resume its role as California's primary coastal highway. US 101 then enters Buellton, where it intersects SR 246. North of Buellton, US 101 intersects SR 154 again, and in Los Alamos, it intersects SR 135. In Orcutt, US 101 and SR 166 run concurrently for three miles (4.8 km), until north of Santa Maria, where US 101 again intersects SR 135. Vineyards frequently surround US 101 between its northern SR 154 intersection and Orcutt.^[14]

In San Luis Obispo County, US 101 intersects SR 227 in Arroyo Grande. The highway once again runs concurrently with SR 1 as it passes Pismo Beach. SR 1 and US 101 then turn inland into San Luis Obispo, where US 101 reconnects with SR 227, after which SR 1 splits off again.^[14] North of San Luis Obispo, US 101 ascends the Santa Lucia Range on a 7% incline via the Cuesta Grade, cresting at 1,522 feet (464 m) before descending north of the range.^[22] Here, US 101 intersects SR 58 near Santa Margarita, then enters Atascadero, where it intersects SR 41. Upon reaching Templeton, US 101 and SR 46 run concurrently for about three miles (4.8 km) to Paso Robles. North of San Miguel, US 101 enters Monterey County, then passes Camp Roberts.^[17][14]

North of Camp Roberts, US 101 bypasses the San Ardo Oil Field about five miles (8.0 km) south of San Ardo. Between San Lucas and King City, US 101 crosses the Salinas River three times and also intersects SR 198.^[14] US 101 then travels through the Salinas Valley, known as America's Salad Bowl,^[23] before traveling through Greenfield, Soledad, Gonzales, Chualar, and Spence. US 101 then enters Salinas, where it intersects SR 68 and SR 183. North of Salinas, US 101 and SR 156 runs concurrently for approximately eight miles (13 km). SR 156 splits off near San Juan Bautista, after which US 101 intersects SR 129.^[14] US 101 crosses the San Andreas Fault in this same area.^[24]

San Francisco Bay Area

US 101 enters the San Francisco Bay Area by crossing the Pajaro River watershed, where it enters Santa Clara County.^[25] US 101 then intersects SR 25, then continues north to Gilroy, where it changes from expressway to freeway. US 101 run concurrently with SR 152 for one mile (1.6 km) in this area; after which US 101 traverses San Martin and Morgan Hill in the Santa Clara Valley.^[14]

US 101's surroundings become increasingly urbanized as it approaches San Jose, where it intersects SR 85.^[17] US 101 bypasses downtown San Jose to the east, passing through largely residential areas and also intersecting I-280 and I-680 at the Joe Colla Interchange. US 101 then turns west-northwest as it continues through San Jose and intersects I-880, then partially intersects SR 87. US 101 then enters Sunnyvale, where it intersects SR 237 and SR 85, the latter for the second time. US 101 continues northwest through the South Bay, where it passes the campuses of several major technology corporations, including IBM, Intel, Nvidia, and Google.^[14]

US 101 enters the San Francisco Peninsula in Palo Alto, where it travels along the western edge of the San Francisco Bay. It enters San Mateo County in East Palo Alto, where it intersects SR 84, then passes through Redwood City, Belmont, and San Carlos. US 101 then intersects SR 92 in San Mateo, then passes the San Francisco International Airport in San Bruno, with direct ramp and flyover connections between the freeway and airport terminals. US 101 then intersects I-380, then traverses the east side of South San Francisco.^[14] Here, the scenery briefly becomes more natural as the freeway passes Brisbane east of San Bruno Mountain, at one point traveling on a causeway that separates Brisbane Lagoon from the San Francisco Bay.^[26] US 101 then passes Bayview Park, the former location of Candlestick Park.^[27][14]

US 101 enters the city and county of San Francisco at Bayview Park, after which it again intersects I-280, this time via the Alemany Maze.^[14] US 101 then traverses a three-way interchange with the western terminus of the San Francisco Skyway, the latter of which is signed as I-80 but not defined as such.^[28] US 101 then turns west and follows the old Central Freeway viaduct near San Francisco's Civic Center before transitioning to a surface street on Van Ness Drive. Here, MUNI bus lanes traverse the middle of the alignment, which also passes the rear of San Francisco City Hall. US 101 then turns west on Lombard Street, then northwest on Richardson Avenue where it passes the Palace of Fine Arts, after which it continues west and onto the grade-separated Presidio Parkway. Here, US 101 travels through the Presidio of San Francisco, where it passes through the Main Post and Battery tunnels,^[29]then turns northwest as it and SR 1 join near Crissy Field. This shared highway continues onto and crosses the Golden Gate Bridge.^[14]

North of San Francisco, US 101 travels through the North Bay,^[30] first entering Marin County as Redwood Highway,^[31] then climbing the Waldo Grade and passing through the Robin Williams Tunnel. US 101 then passes Sausalito and descends to Richardson Bay, where it splits from SR 1 and travels through Mill Valley. Here, US 101 intersects the west end of SR 131 (Tiburon Boulevard), then continues through the twin towns of Corte Madera and Larkspur. US 101 then enters San Rafael, where it partially intersects I-580, then travels through the city east of downtown. Farther north, US 101 intersects SR 37 in Novato, then travels through sparsely populated areas along the northern outskirts of Olompali State Historic Park.^[14][32]

North of Olompali State Historic Park, US 101 enters Sonoma County,^[14] where vineyards again frequently surround the freeway.^[33] US 101 and SR 116 run concurrently from Petaluma to Cotati, where SR 116 splits off and US 101 continues north through Rohnert Park. US 101 then travels through Santa Rosa, where it intersects SR 12 and passes over Railroad Square via the Robert L. Bishop Memorial Bridge.^[31] At Windsor, US 101 begins to traverse a more rural setting.^{[citation needed]} It crosses the Russian River in Healdsburg, then parallels the river through the Alexander Valley, where it traverses the west hills as it overlooks the winegrowing valley. US 101, then enters Geyserville, where it and SR 128 run concurrently to Cloverdale.^[14]

Northern California

From the Bay Area to the south, US 101 first enters Mendocino County, then crosses the Russian River about one mile (1.6 km) north of the county line. US 101 then changes to an expressway before following the river along the Russian River Canyon.^[14] While in the canyon, US 101 passes Frog Woman Rock, a massive rock feature central to local Pomo lore,^[34] and after exiting the canyon, it enters Sanel Valley, where it narrows to a two-lane road and crosses the Russian River again. US 101 then enters Hopland, where it intersects SR 175.^[17][14]

North of Hopland, US 101 widens to a four-lane freeway as it approaches Ukiah, where it intersects SR 253 and Talmage Road (unsigned^{[citation needed]} SR 222). In Calpella close to Lake Mendocino, US 101 and SR 20 merge then run concurrently for 15.5 miles (24.9 km),^[14] ascending the 1,956-foot (596 m) Ridgewood Summit, US 101's highest elevation in California, in the process.^[35] US 101 and SR 20 separate south of Willits, where SR 20 travels through the town while US 101 bypasses it to the east.^[14]

US 101 intersects SR 162 north of Willits, then travels through Laytonville inside Long Valley. US 101 then traverses 1,796-foot (547 m) Rattlesnake Summit,^[35] after which it follows the South Fork Eel River. In Leggett, US 101 and SR 1 meet once again, this time at an at-grade intersection. The highway then makes a series of sharp curves, following the South Fork Eel River into Piercy, where it crosses the river on the Confusion Hill Bridges,^[14] a section prone to landslides.^[36] US 101 then reaches Cooks Valley. SR 271 closely parallels US 101 for two sections of this route: between Cummings and Legget, and Piercy and Cooks Valley; ^[14] these sections were US 101’s old alignment.^{[citation needed]}

US 101 enters Humboldt County in Cooks Valley,^[14] then bisects Richardson Grove State Park, where standard-sized trucks in compliance with the Surface Transportation Assistance Act of 1982 are prohibited.^[6][7] North of Richardson Grove State Park, US 101 traverses Benbow, then a preserved portion of the original alignment named Avenue of the Giants parallels US 101 for more than 30 miles (48 km), during which both routes travel through Myers Flat, Weott, and Redcrest, while US 101 bypasses Phillipsville, Miranda, and Pepperwood. Both routes also traverse the eastern boundary of Humboldt Redwoods State Park in this segment, and north of this stretch, US 101 crosses the Eel River three times as it travels through Stafford, Scotia,^[37] and Rio Dell.^[14]

North of Rio Dell, US 101 crosses the Van Duzen River, then intersects the western terminus of SR 36. It then enters Fortuna, then traverses the Humboldt Bay National Wildlife Refuge, Humboldt Hill, and Fields Landing. US 101 then enters Eureka, where it parallels the North Bay Channel, then transitions to a surface street. As a surface street, US 101 continues north past Fort Humboldt State Historic Park, then turns east on a one-way couplet through downtown, where it intersects SR 255. The couplet ends east of downtown, after which US 101 becomes an unobstructed highway as it crosses Freshwater Slough via the Eureka Slough Bridge. US 101 then curves around Arcata Bay, leaving Eureka in the process, and travels north through Arcata, where it intersects SR 255 again, then intersects SR 299.^[17]

US 101 continues north, past McKinleyville on a bluff north of the Mad River. The Pacific Ocean comes into view at Clam Beach,^{[citation needed]} after which US 101 crosses Big Lagoon north of Trinidad. US 101 then narrows to two lanes and follows a bar between Freshwater Lagoon and the Pacific Ocean. It then turns inland to Orick, after which it becomes a 14-mile (23 km) undivided freeway that traverses Redwood National and State Parks east of Prairie Creek Redwoods State Park.^[17]

US 101 continues north into Del Norte County, where it crosses the Klamath River before entering Klamath.^[14] Further north, US 101 enters Crescent City, where it separates into a one-way couplet for nine blocks. North of Crescent City, US 101 turns northeast and becomes a freeway for a three-mile (4.8 km). US 101 then intersects the southern terminus of US 199, then continues northward as a two-lane road through the eastern outlying area of Fort Dick. US 101 then crosses the Smith River, then enters the town named after the river. US 101 returns to the coast, where it continues north as it passes Pelican State Beach and enters Oregon.^[14]

El Camino Real

Long before California entered statehood, the 16th-century Alta California had a loose-knit network of transportation paths deemed by the monarchy of Spain as caminos reales, or "royal roads".^[51] In the late 18th century, Gaspar de Portolá, under the stewardship of Junípero Serra, led Spanish missionaries on two expeditionary runs in the coastal regions; missions, pueblos and presidios were also established between San Diego and Monterey during this time.^[52] Six years after Portolá's last expedition, Juan Bautista de Anza followed Portolá's trail from present-day Los Angeles to the Presidio of San Francisco.^[53] George Wharton James's 1908 travelogue Through Ramona's Country describes the road these expeditions traced out;^[54] El Camino Real then became synonymous with it.^{[55][clarification needed]}

Despite modest improvements in the mid-19th century, El Camino Real was difficult for stagecoaches and freight wagons to navigate.^[56] The movement to preserve and memorialize El Camino Real began in 1902, when Anna Pitcher of Pasadena presented a plan to restore the route.^[57] Later that year, the California Federation of Women's Clubs adopted the project; government agencies and other organizations signed on as well.^[58] In 1904, the Los Angeles Chamber of Commerce formed a convention to support commemorating El Camino Real;^[59] a second convention was held in Santa Barbara that same year.^[60][61] The first distinctive bells marking El Camino Real were erected in 1906^[62][63] and in 1915, the Automobile Club of Southern California produced a map detailing El Camino Real and the missions.^[64] In 1924, a roadway that would eventually become part of US 101 replaced an outdated segment of El Camino Real in Ventura;^[65] the replaced segment was later re-signed as part of SR 1.^[66]

El Camino Real was part of the Pacific Highway, a chain of highways that existed until 1926.^[67] The route was declared California Historical Landmark No. 784 in 1963,^[68][69] the same year the state assumed maintenance of the bell markers.^[70] The route was also codified in the California Streets and Highways Code.^[when?][71]

Redwood Highway

Quick facts Nearest city, Built ...

Redwood Highway
U.S. National Register of Historic Places
Redwood Highway c. 1930s
Nearest city	Klamath, California
Built	1900–1949
NRHP reference No.	79000253[72]
Added to NRHP	December 17, 1979

Close

Prior to the 20th century, almost all commerce on the Redwood Coast was done by sea, whereupon ships would dock at portside towns to transport passengers and deliver and receive goods.^[73][74] Amid the backdrop of a sprawling automobile industry, locals and logging companies called for a reliable highway north of the Golden Gate, as at the time, travel by motorcar across this regions could take days.^[75][76] In 1910, the California government passed the State Highway Bond Act, authorizing funds for a statewide road system.^[74][77]

Construction of Redwood Highway began in Del Norte County in 1917,^[77] with another section constructed on tidal wetlands bordering Humboldt Bay between 1918 and 1919.^[78] In 1921, Crescent City hotelier A.D. Lee proposed the name Redwood Highway to honor Save the Redwoods League and their work preserving redwood forests in the area.^[79] Local communities and the city of San Francisco endorsed the proposal,^[80] which was adopted in 1957.^[81] The California Highway Commission also set stringent limits on the number of trees that could be cleared during the highway's construction,^[77] while the trees that were cut down were often used as guardrails,^[82] drainage control devices,^[82] or to build a 4,000-foot (1,200 m) trestle over Big Lagoon, at the time the largest native wood structure in the state highway system.^[83] Ultimately, increased awareness of the destruction of the redwoods led to the establishment of Redwood National Park.^[84]

Landslides and washouts repeatedly frustrated progress on Redwood Highway,^[85] with overall construction lasting about nine years, during which the highway opened segment by segment. The first paved section opened in Ukiah^[when?], after which Jack London became the first person to ride the stretch on his way to Crescent City.^[74] The final segment was completed with the dedication of the Douglas Memorial Bridge on May 17, 1926^[86] and the full highway opened on October 29.^[87][88] To boost tourism in the area, the Redwood Highway Association marketed the highway's beauty,^[89][90] most successfully with the internationally publicized 480-mile (770 km) Redwood Highway Marathon, held in 1927 and 1928.^[91][92] The National Automobile Club also surveyed a split route for Redwood Highway in 1928,^[93] but it was not adopted.^[94]

Hollywood Freeway

In 1924, Los Angeles voters backed the Major Traffic Street Plan to address the city's worsening traffic congestion. A "stop-free express highway" between downtown Los Angeles and the San Fernando Valley via the Cahuenga Pass was included in the project and the city acquired many residential and commercial properties to make room for it.^[95][96][97]

Mexican border to Los Angeles

At its inception, US 101 traversed San Diego suburbs on Beyer Boulevard in San Ysidro, Broadway in Chula Vista, and National City Boulevard in National City.^[105] From there, it took an erratic path through the city, coming within a few blocks of Balboa Park before turning west and traveling through downtown, then turning north onto India Street. North of Mission Bay, US 101 turned west through Pacific Beach, before turning back north on La Jolla Boulevard and serving the business areas of La Jolla. By the 1940s, however, the downtown section was truncated onto Harbor Drive close to the San Diego Bay^[106] and the La Jolla segment rerouted inland on Rose Canyon Road.^[107][108]

In Del Mar, US 101 aligned with the Torrey Pines bypass, dubbed the Million Dollar Highway ^[109] and built in 1933 to bypass a dangerous highway that predated federal designations.^[110] Also in this area, the route traversed the Surf Line and Los Peñasquitos Creek estuary via bridges,^[111][112] after which, it continued along the Pacific Coast through Solana Beach, Encinitas, Carlsbad, and Oceanside.^[113][114]

US 101 continued past Marine Corps Base Camp Pendleton, where it entered Orange County and passed through San Clemente. At Doheny Park north of San Clemente, US 101 originally split into two routes, with US 101 Alternate serving the beachside cities before converging with the main route in Oxnard,^[115] while the main route veered inland through San Juan Capistrano, Laguna Niguel, and Mission Viejo, after which it zigzagged through Irvine and Tustin,^[116] with a concentrated section in Tustin once containing the Tustin Garage and up to ten fuel stations for motorists traveling from the south.^[117] North of Tustin, US 101 followed Katella Avenue and Spadra Road,^[118] then traversed southeastern Los Angeles County on Montebello Boulevard and entered Los Angeles on Whittier Boulevard.^[119]

Due to increased traffic, US 101 between Carlsbad and Oceanside was rolled over to Interstate 5 in the 1950s.^[120][107] In 1963, US 101 Alternative was re-designated as SR 1^[121] and in 1964, California both streamlined its numbered routes across the state^[122] and stipulated that U.S. highways be removed in favor of Interstate highways.^[123] As a result, I-5 supplanted US 101 entirely from Los Angeles to the Mexican border,^[107][124] after which, many of US 101's original surface roads were relinquished for municipal and county control.^[125] The section through Camp Pendleton was closed off to the public,^[107] while a majority of the route in northern San Diego County was designed County Route S21.^[114] Historic route signs have also been added to former sections in San Diego, Del Mar, and Solana Beach.^[125]

Los Angeles and the Central Coast

The first section of downtown Los Angeles–San Fernando Valley expressway to open was the Cahuenga Pass Freeway in 1940.^[97] This section was extended by ten miles (16 km) at a cost of $55 million in 1954 ($659 million in 2025),^[97] to Vineland Avenue in the San Fernando Valley and Spring Street in downtown's Civic Center, at which point it was renamed Hollywood Freeway.^[126] The Downtown Slot also opened in 1954,^[127][103] while the Four Level Interchange, the world’s first stack interchange, was completed in 1949 but not put into full use until 1953.^[128] Proposed interchanges with the Beverly Hills Freeway near Glendale, with the never built Malibu Freeway near Malibu Canyon, and with the never built Laurel Canyon Freeway at the Hollywood Split were cancelled in response to the freeway revolts in the 1960s and 1970s.^{[129][130][131]}

In 1952, the State Highway Commission announced plans for a freeway along El Camino Real through Los Angeles's San Fernando Valley.^[132] The route, which previously ran along Ventura Boulevard^[133] between Sepulveda Boulevard and the western city limit, was constructed and opened in segments between from 1956 to 1960,^[132] and by 1961, US 101 had been upgraded to freeway standards from Los Angeles through Santa Barbara.^[134][135] The Conejo Grade was built as part of this upgrade; it replaced a windy two-lane road built in 1914.^[136] Grade separation in Camarillo was also part of the upgrade; completed in 1954, it eliminated the last railroad crossing on US 101 between Los Angeles and San Francisco.^[137]

West of Santa Barbara, US 101 traversed the Gaviota Pass, which was originally part of El Camino Real and was declared California Historic Landmark No. 248 in 1937.^[138][139] The Gaviota Tunnel opened to northbound traffic in the pass in 1953,^[140] with the previous road repurposed for southbound traffic.^[141] By 1954, the entire freeway was upgraded to expressway standards through the pass.^[142]

North of Gaviota, bypasses funded by the Collier–Burns Highway Act of 1947^[143][144] allowed US 101 to bypass most town centers by the 1950s,^[145] although a US 101/SR 1 interchange was constructed in Las Cruces in 1967, destroying the town.^[146][147] Conversely, a 1951 study determined that frontage-road businesses along a new Santa Barbara–Buellton alignment saw minimal impact on the businesses' customer volume.^[148] This combined with improvements into Santa Ynez Valley lengthened US 101 by twelve miles (19 km), but shortened travel time because it allowed for higher speeds.^[149]

In the northern central coast, the Prunedale Cutoff replaced a more hazardous route over the Gabilan Range via the San Juan Grade in 1932^[150] and the Cuesta Pass opened as an expressway in 1938.^[151][152] In Salinas, US 101 was built as a freeway for ten miles (16 km) by 1954;^[153] the Sala Road interchange, adopted in 1964, connected to this segment.^[154]

San Francisco Bay area

Bayshore Highway, located on the San Francisco peninsula, was completed in 1929,^[155] the first section opened being a tolled arterial between San Bruno and Visitacion Valley.^[156] Between 1929 and 1936,^[157] US 101 was signed as US 101W along the San Francisco Peninsula and US 101E along the East Bay to Oakland, the latter then overlapping with US 40 westward across the San Francisco Bay through a ferry connection. The split routes then rejoined at Hyde Street Pier in San Francisco.^[158] Additional ferries traversed the Golden Gate prior to completion of the Golden Gate Bridge in 1937.^[159]

In 1936, US 101 on the San Francisco Peninsula was truncated at Bayshore Highway, while El Camino Real was re-signed as US 101 Alternate.^[157] However, due to local outrage, this section of US 101 was reverted to El Camino Real two years later, while Bayshore Highway was renamed US 101 Bypass.^[160] US 101E was also re-signed as SR 17 during this time.^[161]

Bayshore Highway was upgraded to a freeway starting in 1940.^[162] The first segment, a two-mile (3.2 km) stretch between San Mateo and Burlingame, was completed in 1947,^[163] making it the Bay Area's first freeway.^[164] The freeway was extended to San Jose and San Francisco in 1962,^[165] after which it was designated part of US 101 while El Camino Real was re-signed as SR 82.^[166] Plans to build a direct connection to the Golden Gate Bridge on the Central Freeway, a northerly continuation of the Bayshore Freeway,^[167] were canceled in 1959 due to backlash by residents;^[168][169] as a result, only one section of this segment of US 101 was completed.^[170]

In Marin County, US 101's Sausalito–San Rafael segment was completed in the early 1930s, concluding with a one-quarter-mile (0.40 km) redwood bridge over Richardson Bay.^[171][172] Twin bores of the Waldo Tunnel, which connect Sausalito and Marin City at the highest point of the Waldo Grade, opened individually in 1937 and 1954,^[173][174] and the bridge across Richardson Bay was replaced in 1956.^[175] Additionally, numerous North Bay improvements in the 1950s centered on upgrading US 101 from expressway to freeway, as well as bypassing cities.^[176][177] An exception was Santa Rosa, where US 101 was routed through the city;^[178] even so, it was brought to full freeway standards with the completion of the Steele Lane interchange in 1965.^[179] Additional bypasses were constructed in Novato and Geyserville in the 1970s.^[180][181]

In Santa Clara County, a four-lane alignment replaced a winding roadway across the Pajaro River in Sargent in the early 1940s^[182] and a 24.6-mile (39.6 km) bypass through the southern Santa Clara Valley was adopted in 1961 and constructed in the early 1970s.^[183][184] Additionally, the Joe Colla Interchange was completed in 1981, after lengthly delays.^[185][186]

Northern California

While Redwood Highway was completed in 1926,^[86] some remote sections in Mendocino, Humboldt, and Del Norte counties were redeveloped soon after to improve grades and provide curve corrections. For example, a segment built between Outlet Creek and Reeves Creek in the late 1930s eliminated forty-seven turns from the previous alignment^[187] and a new 1940s alignment in Piercy lowered the segment's total curvature from 2,978 degrees to 807.^[188]

Despite these and other upgrades, freeway and expressway segments are not as prevalent on US 101 in this area compared to elsewhere.^[189] A variety of factors, most of which came into play in the middle of the 20th century, contributed to this, most notably local business owners opposed bypasses^[190][191] and conservationists opposed the destruction construction of a multi-lane highway would require.^[192] Conversely, the Eureka-based Times-Standard as well as several local taxpayer groups supported the improved safety that upgrades would bring. The Redwood Empire Association also urged the State Highway Commission to not truncate US 101 outside state park boundaries.^[193]

Ridgewood Grade, located in Mendocino County and opened in 1954, was the first section of Redwood Highway upgraded to expressway.^[194] The following year, Assemblyman Frank P. Belotti introduced legislation that would allot $10 million ($120 million in 2025) from the state parks' funds towards a 23-mile (37 km) bypass through Humboldt Redwoods State Park. Governor Goodwin Knight later vetoed the bill, after which the two sides agreed to expand the bypass to 44 miles (71 km) around the park and only remove trees with heights less than six feet (1.8 m). This bypass was completed in the 1960s at an estimated $36 million ($426 million in 2025), while the old alignment, known as the Avenue of the Giants, became SR 254.^[195][196]

In Eureka, the Big Lagoon trestle was demolished in favor of embankments carrying a three-mile (4.8 km) expressway, which opened in 1959 at a cost of $1.84 million ($203 million in 2025).^[197] Additionally, the Arcata–Trinidad freeway located north of Eureka was completed by the early 1960s,^[198] as was a second freeway section from Hydesville to south of Eureka with bypasses at Fortuna and Loleta;^[199][200] the southern segment was later augmented by the Herrick Avenue interchange in 1984.^[184]

Elsewhere along Redwood Highway, a 14+1⁄2-mile (23.3 km) freeway upgrade was completed in Ukiah in 1965,^[201] then extended north through Redwood Valley in the late 1980s,^[202] and a $1 million ($10.7 million in 2025), thirteen-span viaduct opened across the Eel River in 1964.^[203] Additionally, numerous bridges were damaged during the Christmas flood of 1964,^[204][205] each of which was repaired or replaced by the following March, the last being a new construction that replaced Douglas Memorial Bridge over the Klamath River.^[206]

Los Angeles and the Central Coast

The last traffic signal on US 101 between Los Angeles and San Francisco, located in Santa Barbara, was removed in 1991.^[207] A lane was added in both directions of the Ventura Freeway in the 1990s, eliminating several bottlenecks,^[132] and new lanes and truck pullouts were added to the Cuesta Grade in 2004 as well.^[22] A four-phase widening project to add a lane in each direction in the Santa Barbara area began construction in 2008; its final phase began in 2020, at which point the phase was estimated to cost $700 million ($871 million in 2025) and be completed in 2027.^[208][209] Three interchanges were also added in Prunedale in 2014.^[154]

In 2024, the city of Los Angeles undertook a beautification project on 8.7 miles (14.0 km) of US 101 in Hollywood, the project emphasizing the movie culture in the area and featuring amongst other artwork a 22-foot (6.7 m) steel art sculpture depicting fluttering filmstrips in the freeway median.^[210] Construction of the Wallis Annenberg Wildlife Crossing also began in 2024; this crossing, a vegetated overpass spanning the Ventura Freeway in Agoura Hills, will connect the Santa Monica Mountains with the Simi Hills and upon completion will be the largest wildlife crossing in the world.^[211]

San Francisco bay area

In 1984, San Mateo and Santa Clara County residents voted to use sales tax funds to fund high-occupancy vehicle (HOV) lanes on US 101 in their counties.^[212] These lanes were extended through San Jose and into Morgan Hill by 2000^[213][214] and the Mountain View–Redwood City section was converted to high-occupancy toll lanes in 2022,^[45][46] then extended to San Bruno in 2023.^[47][215] Furthermore, US 101 was expanded to eight lanes between SR 85 in San Jose and Cochrane Road Morgan Hill in 2003 and a new interchange at Bailey Avenue, planned since the 1970s, opened in 2004.^[216]

The 1989 Loma Prieta earthquake caused severe damage to San Francisco's Central Freeway, prompting its immediate closure.^[217][218] In 1997, a narrow majority of residents voted to rebuild the freeway; however, another vote the following year saw the majority vote for it to be dismantled, and this vote was affirmed the year after that.^{[219][220][221]} Demolition was completed by 2005 and the vacated right-of-way was redeveloped into Octavia Boulevard.^[222][223]

In 2000, Caltrans and the Metropolitan Transportation Commission unveiled a joint plan to build continuous HOV lanes on US 101 throughout the North Bay.^[224] At the time, facilities were already existent in Marin County, having been converted from contraflow bus lanes in the 1980s,^[225] but a gap at San Rafael was not filled until 2011.^[226] HOV lanes opened in Sonoma County in 2022^[227][228] and the final segment between Petaluma and Sonoma County opened in 2025;^[229][230] these segments were also part of a separate 30-year, $1.5 billion ($1.5 billion in 2025) US 101 improvement project that was completed in 2025.^[231] As part of this project, HOV lanes were built between between Novato and Sonoma County, the entire route between Novato and Petaluma was upgraded to freeway standards, and new bridges were built across the San Antonio Creek and Petaluma River.^[229][232]

In 2009, a project began to replace US 101's Golden Gate Bridge approach through the Presidio. The new Presidio Parkway incorporated two sets of tunnels and opened in 2015,^[233] while the Presidio Tunnel Tops recreational area, located above the tunnels, opened in 2022.^[234] Also in 2022, San Francisco Municipal Railway built center bus lanes and landscaped medians on US 101's Van Ness Avenue segment as part of the Van Ness Bus Rapid Transit project.^[235]

Northern California

US 101 was truncated onto a new alignment through Prairie Creek Redwoods State Park from the late 1980s to early 1990s.^[184] A four-mile (6.4 km) bypass in Cloverdale was completed in 1994 at a cost of $40 million ($86.9 million in 2025); the project also added frontage roads and relocated Northwestern Pacific Railroad trackage.^[236] In 2009, a section of US 101 north of Leggett was moved across the Eel River via the Confusion Hill Bridges in order to prevent the infrequent but expensive winter closures that occurred along the old alignment.^[36] Additionally, a six-mile (9.7 km) bypass at Willits broke ground in 2013 and was completed in 2016, more than sixty years after initially proposed.^[237] This project, controversial because it re-routed US 101 through protected wetlands,^[238] cost $459 million ($616 million in 2025),^[239] included a 1.1-mile (1.8 km) viaduct across a flood plain,^[240] and re-located the US 101/SR 20 interchange from inside city limits to south of the city. As of 2025^[update], the remainder of the old route was still part of the state highway system, under the designation 101U (for unrelinquished).^[241]