It seems reasonable, but at least one study[1] concluded that during peak hours in 2004/5 90% of northbound and 77% of southbound traffic on SR99 entered the viaduct from one of the downtown exits. WSDOT's own analysis from 2011 found almost 50% of the viaducts traffic entering or exiting downtown [2].

To my knowledge there is no plan to improve surface streets to accommodate this traffic.

[1]: http://cnu.civicactions.net/resources/publications/alaskan-w...

[2]: http://www.wsdot.wa.gov/Projects/Viaduct/Faqs