The Washington State of Department of Transportation
has signed a contract earlier this month with Seattle Tunnel Partners to design and build the SR99 bored tunnel. The design-build contract will cost around $1.35 billion. If all goes well, construction will start this summer.  WSDOT had reviewed a few other alternatives as early as 2001 and finally the bored tunnel was picked as the preferred replacement for the central section double-deck viaduct. The project will be a two level, 1.7 mile tunnel running from S. King Street to Thomas Street. The tunnel is estimated for completion around late 2015. 

The City of Seattle also plans to build a new street and new public open space along the waterfront once the viaduct is removed and improve other city streets and replace the central waterfront seawall. More open space along the waterfront and the removal of the viaduct along Alaskan Way, will help increase the curb appeal or improve views for some of the condos like Waterfront Landings, HillClimb Court, Pomeroy, Fix Building, Florentine, and Our Home Hotel.

Bored tunnelViaduct picture

Looking at the pic, it feels like only having two lanes in either direction is a bit tight.  If something is gonna take 5 years, you'd hope it can support the needs for 50 but hopefully some people have thought about this and projected demand.