After a decade on the drawing board, construction begins this month to add a third northbound lane to Interstate 5 at Seneca Street, known as the worst bottleneck in Washington state.

The $27 million project, built on the existing elevated road decks, is expected to reduce the traffic jams that occur from Sodo to downtown Seattle when it’s finished in fall 2022.

“Particularly for I-5, it will result in less congestion. That’s very definitely the case,” said Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington.

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For instance, on days when traffic flows currently break down at 2 p.m., the new lane should delay those snarls until 3:15 p.m., Hallenbeck said. With fewer congested hours, traffic jams that currently extend back to Boeing Field might only reach as far as Michigan Street or Spokane Street after the project, he said. Trips at the peak around 5 p.m. likely won’t improve much, he said.

The project officially begins Monday, but the first overnight lane closures aren’t planned until sometime next week, said Bart Treece, spokesperson for the Washington State Department of Transportation. After that there will be a dozen or so full mainline shutdowns between midnight and 4:30 a.m., spread out over months.

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WSDOT will usually provide a detour by opening the left-side entrance to I-5 express lanes northbound. Sometimes, detour traffic will be funneled to the right, using the northbound collector-distributor lanes that also serve I-5 traffic arriving from I-90.

The project will convert the left-side Seneca Street exit to an optional offramp from the new left lane, as that lane continues north under the state convention center. Currently, the freeway there has two mainline lanes plus the exit-only lane at Seneca.

Whether through confusion or cunning, many drivers moved toward the yellow-marked Seneca exit and then veered at the last moment into through traffic — generating waves of slowdowns.

“You’re not going to have as much merging traffic or as many people lane jumping, so hopefully it will be safer,” said Sheri Call, vice president of the Washington Trucking Association. “I’ve seen it thousands of times.”

About 13,000 truck drivers a day use I-5 downtown and are often forced into sudden stops.

Instead, the drama will move to the right side. Drivers entering on the right from Cherry Street must quickly merge left — on a curvy downslope — before reaching the exit-only lane to Olive Way (formerly an optional exit).

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“There’s a lot going on in that stretch of highway,” said Hallenbeck. Severe weaving is already a problem, he said, as drivers from I-90 merge four lanes left between Cherry Street and Mercer Street.

Bob Pishue, analyst for the Kirkland-based INRIX traffic data and navigation company, said the project “will likely increase speeds through downtown.”

However, he wonders if congestion will worsen for Eastside traffic that arrives in Seattle via Interstate 90. That’s because incoming cars from I-90 and Edgar Martinez Drive at the stadiums onto the I-5 collector-distributor lane will be subject to new ramp-metering signals.

To make space for a third lane, workers will replace old side barriers and relocate the white lane stripes. That will leave a smaller shoulder that will taper from 4.4 feet to only 2 feet — too narrow to park a stalled car.

Motorists travel north on Interstate 5 toward the Seneca Street exit in Seattle on Friday. (Erika Schultz / The Seattle Times)
Motorists travel north on Interstate 5 toward the Seneca Street exit in Seattle on Friday. (Erika Schultz / The Seattle Times)

MidMountain Contractors of Kirkland won the prime contract, of which $9 million goes to ramp meters and new overhead lane-management signs, and $6.8 million covers road-deck expenses.

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WSDOT expects to remove homeless encampments next to I-5, records say. “No Trespassing” signs will be posted 72 hours before evictions and cleanup work, and notice will be given to advocacy groups.

When the freeway was designed in the 1960s, with two northbound lanes, planners had no clue how fast Seattle would grow.

“There weren’t people driving from Vancouver to Vancouver,” Hallenbeck said. “I-5 was designed for people to drive downtown, then go back out. It was a very big tidal movement. It made perfect sense, because so many people were going to get off downtown.”

I-5 traffic through downtown, from both directions, increased from 115,000 daily trips in the opening year of 1967 to nearly 210,000 trips by 2019.

A third through lane near Seneca Street has been suggested since 2007 in WSDOT lists of desirable I-5 improvements.

Six years later, the state sought federal money to engineer a third lane, calling this “the worst bottleneck in Washington state.”

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Legislators finally approved construction money in the 2017-19 budget. Gov. Jay Inslee declared a six-month delay to this and 25 other WSDOT projects in November 2019, while the state feared losing car-tab money to Tim Eyman’s winning Initiative 976, which the state Supreme Court later overturned.

Other major I-5 work lies ahead, including the replacement of 23 clanging steel expansion joints southbound through Sodo the next two years, and deck repaving on the Ship Canal Bridge.

Check the Seattle-area construction webpage and @wsdot_traffic on Twitter for updates.