press release:
The contractor working for the Seattle Department of Transportation on the Spokane Street Viaduct (the elevated roadway between the West Seattle Bridge and I-5) will close the viaduct to westbound traffic between I-5 and SR 99 on Thursday and Friday nights, March 15 and 16 from 10 p.m. each night until 5 a.m. the following morning. This will close direct access to West Seattle via the Spokane Street Viaduct from both northbound and southbound I-5. (Motorists traveling southbound on SR 99 will still be able to access the off-ramp to the West Seattle Bridge.)

Southbound traffic from I-5 wishing to reach West Seattle can detour by exiting I-5 at S Forest Street, turning right (northbound) onto Sixth Avenue S, then turning left (westbound) onto S Lander Street, turning left again (southbound) at First Avenue S, then turning right (westbound) at S Hanford, turning left onto E Marginal Way S, and finally turning right (westbound) onto the lower S Spokane Street roadway and across the swing bridge into West Seattle. Northbound traffic from I-5 wishing to travel to West Seattle should exit the freeway at the Sixth Avenue S off-ramp, and then follow the same detour route outlined above. Detour signs will help motorists follow the designated route.