I-405 is a bypass auxiliary route of I-5, running along the southern and western parts of the Greater Los Angeles urban area from Irvine in the south to Sylmar in the north.

I-405, heavily traveled by both commuters and freight haulers along its entire length, is the busiest and most congested freeway in the United States.

[3][4] 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.

[4] It has played a crucial role in the development of dozens of cities and suburbs along its route through Los Angeles and Orange counties.

The freeway passes immediately south of the Irvine Spectrum Center mall before intersecting with State Route 133 (SR 133).

It passes South Coast Plaza before a partial interchange with SR 73, which serves as a partially-tolled bypass of I-405 between Costa Mesa and Laguna Niguel.

It passes near California State University, Dominguez Hills, and Dignity Health Sports Park, home of Major League Soccer club LA Galaxy.

I-405 then continues to roughly parallel the contour of the coastline as it passes through the South Bay communities of Torrance, Lawndale, Redondo Beach, Hawthorne, and El Segundo.

After cresting the mountains, I-405 descends into the San Fernando Valley, intersecting US Route 101 (US 101) in the Sherman Oaks neighborhood of Los Angeles.

The freeway then continues due north through the western part of the valley, passing east of Van Nuys Airport and California State University, Northridge.

The project included converting the existing high-occupancy vehicle (HOV) lanes, with them closing in November to test the tolling equipment.

[10][dubious – discuss] The freeway's West Los Angeles section is the subject of annual gridlock photos and videos taken during the Thanksgiving holiday weekend, most often from an aerial view.

[15] Despite four years of construction disruptions and billions of dollars of public money, Los Angeles Times commentary claims traffic with the lane expansions is actually just as bad or worse.

[18][19] Construction began in 1957 with the first section,[citation needed] mostly north of Los Angeles International Airport, which was completed in 1961 and initially signed as SR 7.

It was designed with a maximum grade of 5.5 percent and required the relocation of Sepulveda Boulevard and the Mulholland Highway, which was moved 1.1 miles (1.8 km) to the south along a new 579-foot-long (176 m) bridge over the freeway.

[25] 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.

[29] The Los Angeles Times on Sunday, July 17, 2011, featured comments and images of people enjoying the moment next to the I-405 freeway with the free-flowing traffic.

[28] The Los Angeles County Metropolitan Transportation Authority then had full closure of a 10-mile (16 km) stretch of I-405 on the weekend of September 29–30, 2012, while construction crews worked to demolish a portion of the Mulholland Bridge.