May 21, 2021

WSDOT starts work to uncork bottleneck around Seneca exit

Photo by Benjamin Minnick [enlarge]

A segment of I-5 snakes down to two lanes after the Seneca Street exit. The elevated lanes to the right feed into I-5 from I-90.

On a typical work day, over 100,000 vehicles travel northbound on Interstate 5 through Seattle. While traffic is often thick, there's a 1,500-foot section running north from the Seneca Street exit that can really throttle the flow because there are just two through lanes here.

The state Department of Transportation this week kicked off a $27.48 million project that it hopes will alleviate backups by adding a third lane to this section. Space for the additional lane will be created by replacing existing barriers, rails and curbs with a thinner barrier, reconstructing asphalt shoulders with concrete and restriping the roadway. The Seneca exit will go from its current configuration as an exit-only lane to a through lane with an option to exit to Seneca.

The project also will add a second lane to the nearby Dearborn/Interstate 90 on-ramp to northbound I-5, with ramp meters for both lanes. Another ramp meter will be added to the Cherry Street on-ramp to northbound I-5. Finally, crews will install an advance signing system to warn drivers of incidents that block lanes and add sign bridges northward to the state Route 520 exit.

An animation of the new layout on WSDOT's project site shows northbound vehicles, except those in the far left lane, having to negotiate a slight rightward jog in the freeway after the Seneca exit.

Back in January, MidMountain Contractors of Kirkland was awarded a $15.75 million contract for the job. There were three other bidders and the engineer's estimate was $17.02 million.

WSDOT says the project will take about a year and a half to complete, and will require about a dozen closures of I-5 from midnight to 4:30 a.m. The early morning closures are planned to limit construction impacts on motorists. When possible, the agency plans to open the express lanes in the northbound direction during these closures as an alternative route. Closure dates haven't been announced.

Construction funding of $17.74 million is coming from existing federal and state funds. About $2.77 million for planning and design came from a Puget Sound Regional Council grant and existing funds.

WSDOT says the existing two-lane section of I-5 creates one of the worst traffic bottlenecks in the state, with cars often backing up to Boeing Field and many congestion-related collisions.