The state Department of Transportation is reviving the idea of converting a two-lane stretch of Interstate 5 to three lanes near the Seneca Street exit in downtown Seattle, for a relatively modest $23 million.

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For about $23 million, the state says it could add a third lane to northbound Interstate 5 entering downtown Seattle, relieving the two-lane pinch point between the Seneca Street exit and Freeway Park.

“The worst bottleneck in Washington state,” one Department of Transportation (DOT) staffer called it, in a recent grant application for engineering money.

Many of the 50,000 or so drivers passing through — and tens of thousands more stuck in traffic jams back to Boeing Field — would agree. At peak times, the average drive from SeaTac to Seattle took 26 minutes in 2011, twice as long as a 60 mph free flow.

Traffic on I-5 will become even more stressed as the economy improves and more people return to work, and drivers avoid tolls on a new Highway 99 tunnel in 2016. A proposed Sonics arena would attract 6,000 cars on game nights.

Over the years, the Legislature and transportation leaders haven’t funded a third lane, even though it would need a tiny fraction of the state’s current $8 billion, two-year budget for highways and ferries.

Clear answers are hard to come by, but DOT has hoped for a big infusion of tax dollars to ease the Seneca squeeze as one of many improvements throughout the Puget Sound corridor — in baseball terms, swinging for the home-run fence rather than slapping a single.

And earlier this year, the Puget Sound Regional Council (PSRC), a consortium of local elected officials, turned down DOT’s request for $2.4 million in federal grants to perform design work.

PSRC chose other projects that were “shovel ready” to create immediate jobs and results. However, the I-5/Seneca design funds rank first on the waitlist, if money is left over from other projects, or more federal cash arrives, said PSRC spokesman Rick Olson.

“This is a good project, but there are a lot of good projects that need to be funded,” said Josh Brown, PSRC chairman and a Kitsap County commissioner. For instance, the state needs to maintain ferries that often break down, he said.

In the grant application, the state estimated that it would take until 2018 to finish the third-lane project. Officials this week couldn’t confirm whether DOT really needs that much time.

As Gov.-elect Jay Inslee prepares to take office, the DOT is writing a new plan for I-5 spot improvements that include the I-5/Seneca re-stripe. This will replace concepts published in 2007 as part of civic debate over how to replace the Alaskan Way Viaduct.

“We’re very quietly working on one,” DOT Secretary Paula Hammond told the pro-highway Eastside Transportation Association in Bellevue. Beyond highway retrofits, potential strategies include variable-speed signs and tolling the express lanes, she said.

Lawmakers are being bombarded with new tax-and-build proposals that lean toward a nine- or 10-cent gas-tax hike, and car-tab taxes to aid local transit and roads. Two weeks ago, the DOT showed a statewide wish list to the PSRC, including $570 million for interchange and technology upgrades on I-5 from Everett to Tacoma.

One reason the Seneca widening is gaining momentum is a new strategy by DOT engineers that is less expensive than earlier versions, said Mark Leth, regional traffic engineer.

Instead of an exit-only lane at Seneca, the left lane would continue under the Washington State Convention Center. But that’s difficult because I-5 is actually on a bridge there, preventing a wider roadbed.

So workers would adapt by restriping the lanes and shifting them slightly to the right. Roadside barriers would be replaced by new, thinner versions, Leth said. And in spots, the shoulders would be only 2 feet.

There also would be changes off to the right, where drivers arriving on the collector-distributor road from Interstate 90 and the Cherry Street onramp are subject to ramp-metering signals. The signals would give drivers entering from Cherry Street some gaps in traffic to help them merge before their lane expires, as an exit-only at Olive Way. And rows of overhead variable-speed signs would be installed from downtown to Highway 520.

The risk is potentially worse congestion in the right lanes, if drivers have trouble escaping downtown — or if the queues stretch back to I-90 or Edgar Martinez Way.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom.