Seattle’s I-5 squeeze finally not so tight as new lane opens

Drivers on Interstate 5 will endure less weaving and fewer waves of brake lights entering downtown Seattle, after the state finally widened the original northbound two-lane mainline to three lanes at Seneca Street.

Instead of the old left-side “exit only” lane, the new layout stretches the lane past Seneca more than a quarter-mile to continue straight, while still providing a short left-turn pocket to exit from I-5 toward Seneca Street and downtown. By the time drivers reach the Seattle Convention Center, there are five lanes total.

The improvement commenced without fanfare over the weekend.

An I-5 drive from Columbian Way to Mercer Street took only 10 minutes Monday morning. Then again, Monday tends to bring the lightest or second-lightest volumes for urban highways since the pandemic started, due to work-from-home.

The former two-lane constriction, loathed by two generations of commuters, was built into the south downtown freeway segment when it opened in 1967.

The city was already developed, and space was so scarce that First Hill residents held a protest march in 1961 against plans to carve a roadbed through their neighborhood. Back then, fewer people drove and were expected to end trips within downtown. Nowadays more than 200,000 daily trips pass through I-5 in the area.

The Washington State Department of Transportation had listed the I-5/Seneca project in its plans since 2007 before lawmakers finally approved funds in the 2017-19 budget. The $38 million conversion began in 2021.

After all that struggle, WSDOT opted for a “soft open” that the agency heralded in an early Sunday news release. There wasn’t much hype because some features aren’t finished yet, spokesperson Amy Moreno said. The state’s announcement also couched Seneca as merely one piece of its decadelong “Revive I-5” series of upgrades from Federal Way to Marysville.

“Opening a third lane at what has traditionally been a bottleneck on northbound I-5 in Seattle should help improve traffic flow,” said a statement by Engineering Manager Ed Kane. “We don’t expect this will solve every issue in that area, but it should make it a little easier for people to move through there.”

Some drivers could find longer delays, because ramp-metering lights will operate in the collector-distributor lanes, where traffic from Interstate 90 merges into I-5 right of the mainline, as well as metering from the Cherry Street onramp.

Another potential factor is “induced demand,” the phenomenon of increased traffic that clogs the new lanes because people are more willing to drive when lanes are added.

WSDOT hopes this renovation merely unclogs traffic, rather than creating more. Travelers should save minutes because fewer drivers will cruise to the left in a now-defunct Seneca exit lane, through confusion or conniving, and then suddenly veer into traffic queues. Instead, downtown-bound drivers can stay in the through lanes before using the short, optional turn pocket at the Seneca exit.

Crews painted a new double-white lane stripe, and marked it illegal to cross, in hopes that motorists in the far-left lane — which pours into the reversible express lanes — won’t use that space to snake around fellow northbound commuters when the express lanes point south each morning. Monday morning at about 8:05 a.m., a couple drivers cruised that way, veered late into the mainline and then exited at Seneca.

WSDOT doesn’t have any recent modeling or traffic predictions available for this long-developing project.

The ramp-metering signals and signs to the right haven’t been unveiled yet and should begin this fall, Moreno said. A new white sign, over the Sodo mainline, advises drivers to use Seneca’s left exit to reach the convention center. Under downtown, the right lane from Cherry Street to Olive Way is now an exit-only lane.