The corridor is intended to relieve the heavily congested I-405 freeway through Sepulveda Pass and provide a fast public transit option on this north–south link.

[1] As of December 2024, the project is in the environmental review phase of planning and construction is expected to be funded via a combination of local Measure M revenue and applications to federal grants.

The proposed transit line will traverse the Sepulveda Pass largely parallel to the existing I-405 freeway, which connects US 101 to its North in the San Fernando Valley with I-10 to its South in the L.A. Westside.

[2] A five-year construction project to widen the I-405 at the cost of $1.6 billion was completed in 2014, but its impact on reducing congestion remains disputed.

[4] What is currently designated as phase 2 of the project is served by Culver CityBus lines 6 and Rapid 6 via Sepulveda Boulevard, with the latter only operating weekdays.

The Los Angeles County Metropolitan Transportation Authority (Metro) has $10 billion in funds available for construction planned to begin in 2026.

[8] The plan included in the Measure M transportation funding measure is to build improvements in three stages: additional lanes to be used for express bus service to open by 2028, an 8.8-mile (14.2 km) transit project between the G Line's Van Nuys Station and the D Line Extension’s Wilshire/Westwood Station by 2035, and a planned extension to LAX with a 2059 completion date.

Once the draft EIR has been completed and circulated for public comment, Metro staff will recommend a locally preferred alternative (LPA) to the board of directors.

[20] The rival design approach is a monorail as planned by LA SkyRail Express (LASRE), a consortium led by British infrastructure investor John Laing plc along with other partners including Chinese EV manufacturer BYD, Swedish construction firm Skanska, and French transit operator Keolis.

[31] The planned project has sparked a vigorous debate that centers on the two different proposed modes: the aerial monorail following I-405 or heavy rail in a subway tunnel under Bel Air.

A scoping process carried out by Metro from November 2021 to February 2022 showed a majority of the public favoring heavy rail over monorail, 93% to 7% respectively.

After describing details of both rail types were presented to residents, an HRT underground option stood at 71% approval, with respondents citing shorter travel time and fewer surface impacts.

[36] The heavy rail option is summarily rejected by a group of homeowners in the affluent Bel Air neighborhood, under which the subway tunnel would be built.

An organized effort against this option is primarily led by Fred Rosen, the former CEO of Ticketmaster and leader of opposition group Keep Bel Air Beautiful.

Jamie Meyer, president of the Bel Air Association, called the proposal a "mind boggling expensive and potentially dangerous subway under our community".

[37] Main concerns brought forth by the heavy rail opponents are worries about noise issues and traffic disruptions during construction, as well as vibrations during operation.

[39] Metro's July 2019 updated concepts for Phase 2 added a stop along Santa Monica Boulevard per public popular demand.

[24] All north-to-south routes from the E Line to LAX are: The following table shows all potential metro stations and the alternatives for which they apply: Alternative 5's concept for the Westside-LAX phase of the Sepulveda Transit Corridor Project would extend the Purple Line subway south down Centinela Ave along the same route as the other proposed Centinela Ave concepts (Alt 2).

This concept would provide a one-seat ride from the LAX Automated People Mover to Downtown Los Angeles but would require passengers from the San Fernando Valley to transfer at Westwood/UCLA station to travel further south.

Metro studies declined the LRT merge option and stated HRT would provide faster times and more occupancy on trains.

Phase 1 of the project is part of Metro's Twenty-eight by '28 initiative, which aims to complete its list of expansions in time for the 2028 Summer Olympics.