press release:
The following routes will be affected by the closure of the Alaskan Way Viaduct:

Routes 21 Express, 37 southbound, 54, 54 Express, 55, 56 Express, 113, 120, 121, 122 and 125

From Friday evening, October 21 at 7:00 PM through the end of service on October 30, Metro bus routes 21 Express, 37 southbound, 54, 54 Express, 55, 56 Express, 113, 120, 121, 122, and 125 will be rerouted off of the Alaskan Way Viaduct due to construction.

During the closure, the buses will travel via Fourth Avenue South in the SODO area and Third Avenue in downtown Seattle.

Bus stops on Columbia and Seneca streets at Second Avenue will be temporarily closed during this time.

Both northbound and southbound buses will stop on Fourth Avenue South at South Lander Street in SODO for convenient transfers to Link light rail and other bus routes.

Northbound buses will also stop on Fourth Avenue South at the South Jackson Street island stop; and southbound buses will not make any stops between Third Avenue at Columbia Street and Fourth Avenue South at South Lander Street.

Traffic congestion on the north end of the viaduct is also likely to cause delays for Metro bus routes 5, 15, 16, 18, 26 Express, 28 Express and 358.

More information about the closure and the affect on bus travel is available online.