Overnight detours are in place on Sunset Boulevard and the San Diego (405) Freeway through Wednesday as part of the widening of the Sunset overpass.
The heavily traveled section of freeway through the Sepulveda Pass is the final link in a billion-dollar project that will complete a northbound carpool lane that runs from Orange County to the San Fernando Valley.
Construction is expected to be completed in 2013.
The burst of construction, which started Friday, should wrap up early Wednesday, but knocking down and rebuilding the 54-year-old overpass is expected to take nearly two years.
"The longest detour is 2 miles max," Dave Sotero of the Metropolitan Transportation Authority said.
"It's not going to be that large, and it's going to happen in the middle of the night."
The overpasses at Skirball Center and Mulholland drives also will have to be widened, and work on them will continue apace of the Sunset overpass.
Sunset Boulevards motorists will be routed around the work on Sepulveda Boulevard and Church Lane.
Northbound freeway traffic will exit at Montana Avenue, turn north on Sepulveda, then get back on the freeway at Moraga Drive.
Southbound freeway traffic will exit at Sunset, take Sepulveda south, then get back on the freeway at Wilshire Boulevard.
Overnight Tuesday and Wednesday, Sepulveda will be closed between Montana Avenue and Sepulveda Way, and traffic in both directions will be diverted to Veteran Avenue.