SDOT Press Release:

Due to damage caused by a vehicular collision on northbound SR99 in the vicinity of S Massachusetts St, southbound traffic on the Alaskan Way Viaduct is blocked and northbound traffic is restricted. Seattle Department of Transportation crews are en route to the scene to undertake repairs, which are estimated to take four or more hours at this point. Drivers are encouraged to seek alternate routes north and south.

The vehicular collision created an oil spill and also caused a guardrail from a northbound lane of the viaduct to hang over the lower, southbound lanes. This hazard has forced the closure of southbound lanes until the hanging rail can be removed and the missing segment of rail can be replaced.

UPDATE: Traffic has now been restored to normal as of 11:00 p.m.