Traffic congestion in Los Angeles has long been among the worst in the nation, defying decades of costly and often unsuccessful attempts at relief.

Now, the Los Angeles County Metropolitan Transportation Authority board of directors has approved an ambitious new proposal: a subway line running parallel to Interstate 405 through the Sepulveda Pass. The project is expected to serve more than 100,000 riders daily and could reduce commute times along the heavily traveled corridor by 20 minutes to as much as an hour.

Transit officials say the line would significantly improve access to major employment hubs that are bordered by affluent neighborhoods such as Bel Air, a dynamic that has forced many workers and students to endure lengthy cross-mountain commutes.

“Because of that jobs-housing imbalance, this corridor is where it can take an hour to go 10 miles,”Juan Matute, deputy director of the UCLA Institute of Transportation Studies, shared with Bloomberg that “having a much faster alternative is attractive.”

However, the projected cost of this project presents a challenge, as well as opposition from residents in Bel Air whose homes the subway will run under. Despite these concerns, supporters argue the subway could mark a transformative step for Los Angeles’ public transportation system.