The Seattle Department of Transportation has been working to reinforce and seismically retrofit the Fauntleroy Expressway (that's the stretch of road that is raised at the west end of the West Seattle Bridge).
Their press release states:

Closures began the week of December 12th and will continue regularly until the project’s completion in June 2012. Future closures may also occur on weekends, though none are scheduled at this time.

Closures will begin at 9:00 p.m. and reopen the next morning at 5:00 a.m. and are planned for the following evenings:
Tuesday, December 27th through Thursday, December 30th
Note – December 30th has been added as a work night since the previous notice
Tuesday, January 3rd through Friday, January 6th

During this week of evening closures the following detours will be in effect:
· Westbound motorists traveling to West Seattle via the West Seattle Bridge will be directed to use the Delridge Way Southwest off-ramp.

· Eastbound motorists on Fauntleroy Way Southwest will be detoured at Southwest Avalon Way and will be directed to the Southwest Spokane Street entrance to the West Seattle Bridge under the Fauntleroy Expressway.

For information on changes to bus service, look for Rider Alert notices at bus stops, visit Metro Online at www.kingcounty.gov/metro or call (206) 553-3000.
Completing this project is necessary to maintain a safe and reliable transportation corridor. This project is funded by the City’s Bridging the Gap program, a nine-year levy passed by Seattle voters in November 2006 for transportation maintenance and improvements.

For more information on this work, visit SDOT’s project site at: http://www.seattle.gov/transportation/bridge_rehab_fauntleroy.htm.