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It will create 18,000 construction jobs.
"This is an important project that will improve mobility while creating thousands of much needed, high-paying jobs," Villaraigosa said. "With this shovel-ready project, Los Angeles is proving once again that we're putting the pedal to the metal when it comes to revitalizing our economy, reducing traffic and rebuilding our city."
Officials say the 405 is one of the most congested freeways in the nation with nearly 280,000 commuters and freight haulers per day.
The new carpool lane, which will stretch from the Ventura (101) Freeway to the Santa Monica (10) Freeway, is the last gap in the freeway's carpool lane system and should save drivers an average 20 minutes, according to the mayor's office.
Crews will also realign existing on- and off-ramps, build 18 miles of retaining walls and improve conditions on adjacent city streets.