Just found this in a city traffic advisory:

The March of Dimes March for Babies will occur on Sunday, May 4, with as many as 10,000 walkers participating. The northbound lanes of the Alaskan Way Viaduct will be closed to traffic from 9 to 11 a.m. The march will begin at Qwest Field at 10 a.m., move south on Occidental Avenue South to South Royal Brougham Street, west on Royal Brougham to First Avenue South, north on First onto the northbound lanes of the SR-99 Alaskan Way Viaduct, and north on the viaduct to approximately the Western Avenue off-ramp. Then they will make a u-turn and move back to Qwest field along the same route.