Interstate 10 in California

Time-lapse video of a trip on I-10 from Baldwin Park to its western terminus in Santa Monica, California

I-10 is signed as if it begins at the McClure Tunnel in Santa Monica, although it legally begins farther east at the Route 1 interchange at Lincoln Boulevard

A typical traffic jam on the Santa Monica Freeway, at 2:30 p.m. on a Wednesday afternoon near Robertson Boulevard

Downtown Los Angeles skyline as seen from the freeway. A slight (smaller than usual rush hour) traffic jam is ahead

The Dosan Ahn Chang Ho Memorial Interchange (I-110), as seen by traffic going westbound on the Santa Monica

Heavily defaced button copy sign marking an entrance to the Santa Monica Freeway in Downtown Los Angeles, 2005

Interchange with the Ontario Freeway (I-15) as seen by westbound traffic on the San Bernardino Freeway

Heavy traffic from downtown San Bernardino along the San Bernardino Freeway near the interchange with I-215

Cabazon Dinosaurs is a roadside attraction at the Main Street exit in Cabazon

I-10 near the SR 111, looking east with the San Gorgonio Pass wind farm in the background. Note, the overhead signs reading "Indio, other Desert Cities". Also note the signage for exit 112, since renumbered to exit 111.

I-10 eastbound near Indio

The California Streets and Highways Code defines Route 10 from:

(a) Route 1 in Santa Monica to Route 5 near Seventh Street in Los Angeles.
(b) Route 101 near Mission Road in Los Angeles to the Arizona state line at the Colorado River via the vicinity of Monterey Park, Pomona, Colton, Indio, and Chiriaco Summit and via Blythe.

Despite the legislative definition, Caltrans connects the two sections of the route by cosigning I-10 down I-5 between the East LA Interchange and the Santa Monica Freeway, negating a section of the San Bernardino Freeway west of I-5. This short section of Route 10 between Route 5 and Route 101, which was formerly defined as Route 110 (signed as I-110) until 1968, is signed overhead for I-10 eastbound and for U.S. Route 101 (US 101) westbound. This I-5/I-10 cosigning is consistent with the Federal Highway Administration's (FHWA) Interstate Highway route logs that such an overlap exists for the segment of I-10 in California.[8]

I-10 is part of the California Freeway and Expressway System,[9] and is part of the National Highway System,[10] a network of highways that are considered essential to the country's economy, defense, and mobility by the FHWA.[11] I-10 is eligible to be included in the State Scenic Highway System,[12] but it is not officially designated as a scenic highway by the California Department of Transportation (Caltrans).[13] The Santa Monica Freeway is Route 10 from Route 1 to Route 5, as named by the State Highway Commission on April 25, 1957. The section between the Harbor (I-110) and San Diego (I-405) freeways is also signed as the Rosa Parks Freeway, after the African American civil rights activist. The I-10 freeway is signed as the Christopher Columbus Transcontinental Highway in Santa Monica.[14] However, the removal of this designation may result in the eventual removal of this signage.[6][7]

The El Monte Busway is a grade-separated, shared-use express bus and high-occupancy toll (HOT) corridor running along the San Bernardino Freeway between Alameda Street near Union Station in Downtown Los Angeles and a point west of I-605 in El Monte. From Alameda Street to I-710, the El Monte Busway runs parallel to the north side of the freeway. After the I-710 interchange, these lanes merge back to the median of I-10. Eastbound buses exit the HOT lanes at El Monte Station west of I-605. Each vehicle using the HOT lanes is required to carry a FasTrak Flex transponder, with its switch set to indicate the number of the vehicle's occupants (1, 2, or 3 or more), regardless of whether they qualify for free.[22]

Plans are to extend the HOT lanes from I-605 to Ford Street in Redlands. This expansion is planned in four phases. As of 2022, the proposed segment in Los Angeles County between I-605 and the San Bernardino County line is under environmental review,[23] the segment in San Bernardino County between the county line and Etiwanda Avenue at the Ontario–Fontana city limit is scheduled to be completed in 2023, the section to Pepper Avenue in Colton is planned to break ground in 2024, and the segment to Ford Street in Redlands is still in the planning stage.[24]

I-10 after the 1994 collapse

What is now I-10 east of Los Angeles was generally part of the Atlantic and Pacific Highway, one of many transcontinental national auto trails. By 1926, when the United States Numbered Highways were assigned, the road across the desert east of Indio was unimproved, while the road from Indio west to San Bernardino (as well as various roads west to Los Angeles) was paved.[25] In late 1926, US 99 was designated along the section of road from San Bernardino to Indio, where it turned south along present SR 86 on the west side of the Salton Sea.[26] West of San Bernardino, US 99 ran to Los Angeles, concurrent with US 66 (via Pasadena) before turning north; this route to Los Angeles is north of the later alignment of I-10.[27] The piece of this between San Bernardino and Indio was defined in 1915 as Legislative Route 26. (It continued south from Indio via El Centro to Heber. A 1931 extension took it south to Calexico on present SR 111.)[28]

The route from Indio via Mecca to the Arizona state line near Blythe was defined in 1919 as pre-1964 Legislative Route 64. (Later extensions took LR 64 west along present SR 74; a 1931 cutoff bypassed Mecca to the north.) LR 26 was extended west from San Bernardino to Los Angeles in 1931, running along an alignment south of the existing US 66/US 99.[28] Neither of these was a signed route until around 1932, when US 60 was extended west from Arizona to Los Angeles, running along LR 64 to Indio, LR 26 (with US 99) to Beaumont, pre-1964 Legislative Route 19 to Pomona, and LR 26 to Los Angeles. (The original alignment of LR 26 ran roughly where SR 60 now is west of Pomona, but an alignment close to present I-10 opened around 1934).

Thus, in 1931, what is now I-10 east of Los Angeles had been defined as LR 26 from Los Angeles to Indio and LR 64 from Indio to Arizona. It was signed as US 99 from San Bernardino to Indio, and US 60 came along around 1932 from Los Angeles to Pomona and from Beaumont to Arizona. US 70 was extended west from Arizona c. 1936 along the whole route to Los Angeles, and, between 1933 and 1942, US 99 moved from US 66 to present I-10 between San Bernardino and Los Angeles, forming a three-way concurrency between Pomona and Los Angeles. Old alignments and names include Valley Boulevard, Ramona Boulevard, and Garvey Avenue.

I-10 holds the distinction of being the first freeway in Los Angeles. A four-mile (6.4 km) section of today's freeway was built between 1933 and 1935 at a cost of $877,000 (equivalent to $13.8 million in 2021[29]). The "Ramona Boulevard" highway linked downtown Los Angeles to the communities of the southern San Gabriel Valley. The roadway, which opened on April 20, 1935, was dubbed the "Air Line route", and was seen as a major achievement in traffic design.[30]

The route east from Los Angeles was added to the Interstate Highway System on August 7, 1957. It was assigned the I-10 number on August 14, 1957, and the short piece west of I-5 was approved as I-110 on November 10, 1958.[2] By then, most if not all of the San Bernardino Freeway had been completed, and I-10 was signed along the existing freeway along with US 70, US 99, and part of US 60. US 70 and US 99 were removed in the 1964 renumbering, while US 60 was removed in 1972, leaving only I-10.

The part west of downtown Los Angeles was pre-1964 Legislative Route 173, defined in 1933 from Santa Monica to downtown Los Angeles.[31] It was signed as SR 26 by 1942, running primarily Olympic Boulevard. It was later replaced by the Santa Monica Freeway, and added to the Interstate Highway System on September 15, 1955. It too was assigned the I-10 number on August 14, 1957.[2] It was completed c. 1964,[32] and became Route 10 in the 1964 renumbering.

Portions of the Santa Monica Freeway going over La Cienega Boulevard collapsed after the Northridge earthquake on January 17, 1994, and were rebuilt using new Seismic-Resistant bridge designs.[33]

The El Monte Busway was converted to high occupancy toll (HOT) lanes in 2013 as part of the Metro ExpressLanes project.[34]

On July 19, 2015, a bridge carrying the eastbound lanes of I-10 near Desert Center collapsed from floodwater from the remnants of Hurricane Dolores, trapping a vehicle.[35][36]

The legislative definition of Route 10 includes a spur from I-5 (the Golden State Freeway) west to US 101 (the Santa Ana Freeway) near downtown Los Angeles. This section of roadway, the westernmost part of the San Bernardino Freeway, was in fact part of the original San Bernardino Freeway, carrying US 60/US 70/US 99 long before the Golden State Freeway opened. It was added to the Interstate Highway System by 1958 as I-110, but in 1968 it was removed from the system, becoming a Route 10 spur.

This road is signed only for the roads it feeds into: US 101 northbound and I-10 eastbound. It has only two interchanges between its ends: a westbound exit off the spur at Mission Road immediately before merging with US 101 northbound, and the eastbound exit for State Street and Soto Street before it merges onto I-10 eastbound—this one is numbered (as exit 19).[1] There is no direct access from the I-10 spur to I-5.[16]

Exit list
The entire route is in Los Angeles, Los Angeles County.

mi[1]	km	Exit[40]	Destinations	Notes
0.0	0.0	—	US 101 north (Santa Ana Freeway)	Western terminus of San Bernardino Freeway; no access to US 101 south; US 101 exit 1D
0.1	0.16	—	Mission Road	Westbound exit and eastbound entrance; also includes access from Pleasant Avenue and northbound US 101 (via exit 1D) onto entrance ramp
0.6	0.97	19	State Street to Soto Street	Eastbound exit and westbound entrance
1.0	1.6	—	I-10 east (San Bernardino Freeway east)	No access to I-10 west; freeway continues as I-10 east
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Incomplete access       Route transition

There are three auxiliary Interstate Highways associated with I-10 in California:

• The Harbor Freeway between SR 47 in the San Pedro district of Los Angeles and I-10 near Downtown Los Angeles is designated as I-110. The rest of the highway, running along the historic Arroyo Seco Parkway to Pasadena, is instead designated as SR 110.
• The Foothill Freeway between I-5 in the Sylmar district of Los Angeles and SR 57 in Glendora is designated as I-210. The rest of the Foothill Freeway, between SR 57 and I-10 in Redlands, is instead designated as SR 210.
• The Long Beach Freeway between SR 47 in Long Beach and I-10 in Monterey Park is designated as I-710. Due to community opposition, a northern extension through South Pasadena was never constructed; the segment that was completed in Pasadena between California Boulevard and I-210 instead has the unsigned designation of SR 710.

One business loop of I-10 exists in California, running through Blythe in Riverside County.

•  California Roads portal
• Lloyd G. Davies, Los Angeles City Council member, 1943–51, urged rail transportation on the Santa Monica Freeway

Wikimedia Commons has media related to Interstate 10 in California.

• Caltrans: I-10 highway conditions
• Caltrans Traffic Conditions Map
• California Highway Patrol Traffic Incidents
• Metro ExpressLanes – includes toll information on the I-10 Express Lanes
• Interstate 10, Interstate-Guide.com
• Interstate 10, California @ AARoads.com
• Interstate 10, California Highways
• Juan Bautista de Anza National Historic Trail official U.S. National Park Service website