The viaduct section of SR 99 is a main north-south route through Seattle, carrying more than 100,000 vehicles per day. Many commuters and industries, particularly the Port of Seattle, depend on this vital corridor as an alternative to I-5, since the stretch of I-5 through downtown Seattle is the most congested section of freeway in the state. Replacing the viaduct with a tunnel will ensure this capacity is available as the region grows. City street improvements and transit investments will also help accommodate future growth.

A major advantage of this bored tunnel is that it minimizes construction disruptions for businesses and the traveling public. It allows us to build the rest of the new SR 99 corridor while the remaining viaduct stays open to traffic. The SR 99 closures required during construction of a cut-and-cover tunnel or new viaduct would have required longer trips on detour routes through downtown Seattle for three to four years.