Interstate 405 (I-405, and colloquially referred to as "The 405" or the four-oh-five) is a major north–south Interstate Highway in Southern California. It is a bypass of Interstate 5, running along the western and southern parts of the Greater Los Angeles Area from Irvine in the south to near San Fernando in the north. The entire route is known as the northern segment of the San Diego Freeway.

I-405 is a heavily-traveled thoroughfare by commuters and freight haulers along its entire length and is the busiest and most congested freeway in the United States. The freeway's annual average daily traffic between exits 21 and 22 in Seal Beach reached 374,000 in 2008, making it the highest count in the nation. It has played a crucial role in the development of dozens of cities and suburbs along its route through the Greater Los Angeles area.

This route is part of the California Freeway and Expressway System.

Interstate 405 begins at the El Toro Y interchange with Interstate 5 in southeastern Irvine. It then runs northwest through Orange County to Long Beach in Los Angeles County. The freeway then roughly follows the outline of the Pacific coast, varying between five and ten miles (16 km) inland before crossing over the Sepulveda Pass in the Santa Monica Mountains. I-405 next travels northerly through the San Fernando Valley, before its termination with I-5 in the Mission Hills district of Los Angeles. Large portions of the route closely parallel Sepulveda Boulevard.

The freeway's congestion problems are legendary, leading to jokes that the road was numbered 405 because traffic moves at "four or five" miles per hour, or because drivers need "four or five" hours to get anywhere. Indeed, average speeds as low as 5 mph are routinely recorded during morning and afternoon commutes, and its interchanges with the Ventura Freeway (U.S. Route 101) and with the Santa Monica Freeway (Interstate 10) each consistently rank among the five most congested freeway interchanges in the United States. As a result of these congestion problems, it may take longer to pass through the entire Los Angeles area using this bypass route instead of merely taking the primary route I-5 through the city.

Commuters are known to despise the freeway. Steve Harvey of the Los Angeles Times once featured a personalized license plate with the text HATE405 in his column. While much of this gridlock has to do with the lack of alternate routes between many of the areas it connects â€” some of which, such as the Pacific Coast and Laurel Canyon freeways, were proposed but abandoned for political reasons.

There are a number of points of interest that I-405 passes by or connects to. For transportation, these include (in the order passed from south to north) John Wayne Airport in Orange County, Long Beach Municipal Airport and Los Angeles International Airport. With connections, it is also very close to the Port of Long Beach and the Port of Los Angeles.

The route also passes by or through many recreation and commercial destinations. These include more than ten California state beaches, several other beaches owned by counties and municipalities, many of the beach cities favored by tourists, as well as Century City and Marina del Rey.

Some of the educational institutions it passes include the California state universities at Dominguez Hills, Long Beach, and Northridge; the University of California at Irvine and UCLA, Loyola Marymount University, and Pepperdine University's West LA and Irvine campuses. I-405 also passes cultural facilities such as the Getty Center and the Skirball Cultural Center.

I-405 was approved as a chargeable interstate[clarification needed] in 1955. Construction began in 1957 with the first section, mostly north of LAX Airport being completed in 1961 (signed as SR 7) followed by sections west of Interstate 605 within the following few years. The final section covering most of Orange County opened in 1969. Construction required the already existing Mulholland Highway to be re-routed 1.1 miles to the south along a new 579-foot-long bridge, the Mulholland Drive Bridge, to span Interstate 405.

The freeway from present-day I-10 to I-5 near San Fernando was once known as the "Sepulveda Freeway" as it was named for Sepulveda Boulevard.

A section of I-405 was closed over the weekend of Friday, July 15, 2011 as part of the Sepulveda Pass Improvements Project. Before the closing, local radio DJs and television newscasts referred to it as "Carmageddon" and "Carpocalypse", parodying the notion of Armageddon and the Apocalypse, since it was anticipated that the closure would severely impact traffic.

In reality, traffic was lighter than normal across a wide area. California Department of Transportation reported that fewer vehicles used the roads than usual, and those who did travel by road arrived more quickly than on a normal weekend. The Metrolink commuter train system recorded its highest-ever weekend ridership since it began operating in 1991. Ridership was 50% higher than the same weekend in 2010, and 10% higher than the previous weekend ridership record, which occurred during the U2 360° Tour in June 2011. In response to jetBlue Airlines' offer of special flights between Bob Hope Airport in Burbank and Long Beach Airport, a distance of only 29 mi (47 km), for $4. a group of cyclists did the same journey in one and a half hours, compared to two and a half hours by plane (including a drive to the airport from West Hollywood 90 minutes in advance of the flight and travel time to the end destination). There was also some debate about whether the Los Angeles area could benefit from car-free weekends on a regular basis.

The Los Angeles County Metropolitan Transportation Authority announced full closure of a 10-mile stretch of I-405 for the weekend of September 29–30, 2012, while construction crews work to demolish a portion of the Mulholland Bridge.

The $1 billion Sepulveda Pass Improvements Project will add a high-occupancy vehicle lane and associated changes to freeway entrances, exits, and underpasses along a 10-mile stretch through the Sepulveda Pass between I-10 and U.S. 101/Ventura Boulevard. The project is to completed as a design-build in contrast to the traditional design-bid-build used typically in infrastructure improvement. This section of I-405 was closed for a weekend in mid-July 2011 to demolish the Mulholland Drive Bridge, and a 10-mile section was closed for the last weekend of September 2012 (See 'Carmageddon 2011' and 'Carmageddon 2012' above).

Proposed changes between the Manchester and Century Boulevard interchanges in Inglewood to provide a new southbound on-ramp and a new northbound off-ramp for Arbor Vitae Street, to reconstruct and widened the Arbor Vitae Street over-bridge and replace the Century Boulevard overcrossing structure. This work would reduce congestion on the approach to Los Angeles International Airport.[citation needed] The California Department of Transportation has not issued a start date for this work.

The Orange County Transportation Authority is currently studying a proposal to add a HOT lane between Highway 73 in Costa Mesa and I-605 in Seal Beach.

Following the 1966 UCLA-USC rivalry game, USC was voted into the Rose Bowl despite the UCLA team's having defeated the Trojans—with both teams having only one loss during the season. UCLA students protested by blocking the freeway's northbound lanes at Wilshire Boulevard.

While dangerous high-speed chases along the San Diego Freeway are not uncommon, perhaps the most famous chase in its history was also one of the slowest. On the afternoon of June 17, 1994, former football star O.J. Simpson, a suspect in the murder of his ex-wife Nicole Brown Simpson and waiter Ronald Goldman, took to the freeway in a white Ford Bronco (driven by former USC teammate Al Cowlings) with police in pursuit. A bizarre, widely televised low-speed chase ensued and ended hours later when Simpson returned to his Brentwood estate via the Sunset Boulevard exit and surrendered to law enforcement.

Ennis Cosby, the only son of Bill Cosby, was murdered along I-405 in Los Angeles on January 16, 1997, while fixing a flat tire.

I-405 was the location for the short film 405.

The opening lines to "Superman" by Lazlo Bane (featured in NBC's Scrubs as the opening theme song) are: "Out the door, just in time / Head down the 405 / Gotta meet the new boss by 8 AM."

Terra Naomi sings the following at the beginning of "The Vicodin Song" from her album Under the Influence: "You can drive you can drive you can drive/ down the 405/ to the 101 to my house/ and these highways are in so many songs/ I couldn't count them all/ I tried/ so much sad history described in a ride."

The Jimmy Eat World song "If you don't, don't" contains the line "Don’t you know I’m thinkin’, drivin’ 405 past midnight."

Lisa Marr's song "In California" refers to "another suicide on the 405". Neko Case covered "In California" and it is featured on the album Neko Case - Live From Austin TX.

The Death Cab For Cutie song "405" on their second studio album, We Have the Facts and We're Voting Yes, is often incorrectly associated with California's I-405. The song is actually referring to I-405 in Seattle, Washington. The band is from Bellingham, and is referencing the songwriter's drive south to Seattle to visit a girlfriend at the time of writing.

The Carbon Leaf song "Miss Hollywood" on their album Nothing Rhymes with Woman makes a reference to I-405. "But you can check out all the detours off the 405".

The Tupac Shakur song titled "Stay True" makes a reference to I-405 by saying, "Rollin down the 405, gettin high.."

The Gin Blossoms' 2010 album No Chocolate Cake contains the song "Dead or Alive on The 405".

Those Chosen recorded a rap song and music video as an ode to the closing of I-405 entitled, "Carmageddon"

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.