Integrate I-5: Does Seattle Need Another Deep Bore Tunnel?

SR 99 Tunnel construction inside Bertha the boring machine's final resting place. (Credit: WSDOT)

Less than a year after the opening of the SR-99 tunnel, tunnel mania seems to be spreading across Seattle. An unfunded billion dollar price tag has not scared off supporters of deluxe tunnel options for Sound Transit 3 alignments in both West Seattle and Ballard. Seattle Subway’s newly released Sound Transit 4 vision map has gotten transit riders across the region dreaming of a comprehensive mass transit system where riders could hop on in neighborhoods like Georgetown and Greenwood currently cut out from light rail plans.

But dreams of burrowing beneath Seattle’s hills are not restricted to the topic of transit. A mysterious publication sent by mail to over a dozen of Seattle’s government, business, and community leaders has put forth a concept for another subterranean mega project: Why not replace the Downtown Seattle segment of I-5 with a deep bore tunnel that runs under Capitol Hill?

Self-published by mystery-person author Cheval Tepier, Integrate I-5: Reimagining Interstate 5 in Downtown Seattle makes the argument that a deep bore tunnel would allow for better integrating the highway into Downtown Seattle’s built environment and transportation systems than either the status quo or other options currently on the table, notably lidding I-5.

A depiction of Integrate I-5’s proposed 2.9 mile deep bore tunnel route running under Capitol Hill. (Credit: Integrate I-5)

With the SR-99 tunnel finally signed, sealed, and delivered, Tepier believes a tunneling project that removes I-5 from Seattle’s downtown core plays into the region’s expertise. “No one does tunneling through consolidated muck as well or as often as Seattle,” Tepier writes in Integrate I-5. “The whole blown-gasket tribulations of Bertha aside, it’s positively amazing how accomplished this region is at getting around by going deep.”

A new solution for I-5 in Downtown Seattle?

As emphasized in the Washington State Department of Transportation’s (WSDOT) recent I-5 Systems Partnership: A Call to Action, the entire I-5 corridor is need of updates that go far beyond a facelift. The cost of covering deferred maintenance, such as preserving pavement and seismically retrofitting vulnerable structures, is estimated at $2.5 billion.

Catching up with deferred maintenance won’t address the chronic congestion on I-5, which is estimated to cost local and regional economies about $5 billion annually, a figure that is projected to rise to $15 billion annually in 2026.

I-5 Systems Partnership: A Call to Action (Credit: WSDOT)

According to Tepier, a deep bore tunnel solution for I-5 would accomplish:

  • Replacement of a functionally obsolete link in the I-5 corridor;
  • A complete re-stitching of the urban fabric across the highway;
  • Provision for HOV lanes through the city in both directions;
  • Creation of at least 25 acres of urban parkland;
  • Restoration of up to 45 acres of real estate; and
  • A more affordable alignment for ST3.

Let’s dig in a little bit deeper to these selling points, beginning with the first one. It’s true that 2.9 mile segment of I-5 that runs through the downtown core is one of the biggest chokepoints for traffic in the entire corridor, in no small part because of the high density of entrance and exit ramps. Boring a new tunnel would give engineers the opportunity to start from scratch, but it presents its own complications as well, such as should the north and south portal sites be located? How could traffic patterns with I-90 and SR-520 be merged? How could access be maintained to the downtown core?

In reference to that last question, Tepier proposed building a new urban boulevard, dubbed Salish Way. While Tepier claims Salish Way would not be as “sprawling” as I-5, it would still function as arterial for surrounding streets. Does that truly constitute a re-stitching of the urban fabric? Think of Boren Avenue, another nearby arterial that slices through First Hill.

Tepier estimates that land dedicated to Salish Way and connector streets would take up about 25% of the approximate 100 acres that would be created by filling in the interstate. While that represents a sizable chunk of land, it does leave 70 acres available for development. Tepier argues for reserving the bulk of the land, 45 acres for developable land, would help defray the projects costs. The remaining 25 acres would be dedicated to green space that would enhance existing parkland, such as Freeway Park.

One of the plans most intriguing selling points is how it could fit into station locations for coming light rail. Tepier argues that using reclaimed land from I-5 would remove the need to dig trenches for three of five future stations planned for Downtown. But the stations suggested are admittedly described as “downtownish” in recognition that they would not be as centrally located as some of the station plans in the works right now.

Tepier’s proposed light rail station locations include:

  • International District under 5th Ave S and S Jackson St;
  • Marion/Madison and Salish Way (6th Ave)—built in reclaimed land;
  • Pike/Pine at the Convention Center—built in reclaimed land;
  • Thomas/John and Salish Way (Eastlake Ave)—built in reclaimed land; and
  • Mercer and Westlake.

Although the locations might not be ideal for Downtown access, Tepier argues that the cost savings and convenience resulting from use of the former I-5 right-of-way would make up for it.

What questions does a tunnel raise?

As intriguing as the tunnel solution is, there are some major questions to consider in response to the concept. A big one is whether or not it’s responsible to invest so much into infrastructure aimed at moving vehicles through Seattle. Does it lock in carbon emissions far into the future? Would the project itself be highly carbon intensive?

Tepier is honest about what such a plan could cost, candidly writing: “It’s going to be extraordinarily expense. We’re talking ‘begins with a B’ many times over.” Using the SR-99 tunnel for comparison, Tepier rounds the tunnel cost up to $4 billion to account for inflation, and then estimates that the I-5 tunnel would cost roughly two to three times as much, a whopping $8 billion to $12 billion.

High-speed rail is another consideration. WSDOT is currently exploring how to form and deliver a high speed rail system that would stretch across the state, as part of a regional system connecting from Oregon to British Columbia. Using I-5 right-of-way could offer a significant cost saving advantage for a future high-speed rail project. By advocating for use of light rail stations in the right-of-way, the Integrate I-5 plan is currently incompatible with high-speed rail.

A bullet train in Taichung Rail Station in Taiwan. (Credit: Issac Chang)

Finally, there’s the question of how Integrate I-5 compares to the concept of lidding I-5 in Downtown Seattle. Currently, an I-5 lid technical feasibility study is being undertaken by local firm WSP on behalf of Seattle’s Office of Planning and Community Development (OPCD). The technical feasibility study, which should be complete by April of 2020, is exploring the range of feasibility—technically and financially, as well as striving to create a framework to maximize benefits for all.

Lid I-5 Feasibility Study Update, June 11th. (Credit: OPCD)

While Integrate I-5 does create more land than current lidding plans would, it would do so at greater expense. While a freeway lid might not as dramatically vanish I-5 from the downtown core as a tunnel replacement would, the more affordable price tag of a lid could allow for a replicable model across Seattle in suitable areas of the interstate, whereas it’s hard to imagine that more than one deep-bore I-5 tunnel would ever be constructed, even in a city as tunnel crazy as Seattle.

Still Tepier gives themselves little credit by self-declaring as a “nutcase responsible for [an] insane plan.” The book is well-thought out and researched. For those of us searching for a solution to the problem of I-5, it makes for compelling reading.

Copies of Integrate I-5 are available for purchase at Elliott Bay Books in Capitol Hill as well as online.

The author is a member of the Lid I-5 Steering Committee. Her thoughts and opinions are hers alone and do not represent Lid I-5.

We hope you loved this article. If so, please consider subscribing or donating. The Urbanist is a 501(c)(4) nonprofit that depends on donations from readers like you.

Natalie Bicknell Argerious (she/her) is Managing Editor at The Urbanist. A passionate urban explorer since childhood, she loves learning how to make cities more inclusive, vibrant, and environmentally resilient. You can often find her wandering around Seattle's Central District and Capitol Hill with her dogs and cat. Email her at natalie [at] theurbanist [dot] org.

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The waterfront tunnel, for all its costs, accommodates only 4 traffic lanes, 2 in each direction. A deep-bore I-5 tunnel would need about twice that many lanes in a tunnel far larger than any built so far, or in 2 side-by-side bores like ST does for light rail. Add a couple zeros to the end of Mr. Tepier’s cost number.


The timeline with ST3 doesn’t make any sense. The new ST tunnel is starting EIS this years; trying to use I5 ROW would delay ST3 by years. Looking at HSR/Commuter rail usage is much more reasonable.

Also, the region will need the capacity provided by the 2nd tunnel to absorb all increase in transit ridership that would result in the removal of I5. Removing I5 without having the new ST3 stations would choke the city.

Otherwise, I’m a big supporter of an I5 replacement. I argued as much when the 99 tunnel open – it will be a good case study for the regional to see if we want another, much larger tunnel. Similar to 99, once major components need to be replaced (ship canal bridge, anyone?), the business case for a replacement on fresh ROW becomes much clearer.


Kind of a side comment, but whenever high speed rail comes up, someone suggests running it along I-5 similar to the light rail.

I don’t think this is possible. I-5 has way too much curvature for high speed rail. I think the light rail is already going as fast as it can given the curves in the route.

The main thing that makes high speed rail high speed is the very gradual curving of the track.

Aside from that, we will already have light rail running along I-5, so I don’t think there is space.


There will be space when we remove car lanes. There will definitely be curves that need straightening, but getting to use long stretches of an open concrete field will be helpful.


I’ve understood HSR on I5 as a “last mile” problem to get HSR into downtown, particularly from the north. Near the station, HSR doesn’t need to be designed for high speeds – as long as the train can accelerate to high speeds once it is a few miles from the CBD, it’s still good HSR. Also, that rail ROW would do well to support commuter rail, which won’t need the high speed either.


The last mile of HSR will probably need to be done via deep bore tunnel for the same reason that light rail uses deep bore tunnel past northgate. There is no extra space along the side of I-5 through downtown.

HSR will require new right of way. That’s why it’s so expensive. Probably a deep bore tunnel through downtown, and then elevated outside of it.

Joe Zagrodnik

Why not just get rid of I-5 from I-90 to 520 without a replacement? We already proved in January that we can survive with only one highway through downtown. Spend the money on a high-speed rail tunnel through Seattle.

Paul W.

Why not remove several miles of one of the most heavily-used stretches of the nation’s interstate highway system?

Because that is one of the most heavily-used stretches of the nation’s interstate highway system.

Little early to be all hopped up on that legal weed, Joe . . ..


Another author on this blog have proposed exactly that.

Brian Nelson

“Congestion costs $5 billion annually, and is forecast to increase to $15 billion by 2026”

Joe Zagrodnik & Doug Trumm: We should just remove I-5 through Seattle all together, problem solved right?

Joe Z

The congestion is there because of the highway. Closing I-5 in downtown would remove all thru-trips from that corridor (moving them to 405 or SR-99). Reduced thru-traffic would significantly ease congestion for local trips.

What economic benefit does Seattle get from cars that are driving through its core without stopping? As Brian notes, we’re losing money because of it. Cars should be encouraged to use the bypass as was intended by the original designers of the highway system!!!


Joe, among other things, I’m curious to hear your plans for freight/commerce that use I-5 to transport goods/services along the corridor.

The city isn’t losing $5b a year from congestion, industries reliant on our port operations (Seattle & Tacoma ports) are the losers. Removing I-5 would just reroute freight to I-405 amplifying traffic/lost cost.



“What economic benefit does Seattle get from cars that are driving through its core without stopping?” A vibrant regional economy built on trade and manufacturing? If we got rid of Boeing and the Port, I suppose there would be little need for I5.


Yes, “cars should be encouraged to use the bypass as was intended by the original designers of the highway system!!!” And which bypasses would those be, ones that have capacity to absorb I-5 traffic….?


To me at least the only version of an I-5 tunnel that makes sense is as an express downtown bypass. There is so much traffic that is passing through downtown not going to downtown but is caught in all the on/off ramp traffic downtown. So having an express tunnel that allows those travelers to skip all the downtown exits would make sense to me. Though the cost would likely be prohibitive and the question of where to place entrance exits would be complicated.

I’d agree that if a project like this were built it would make more sense to convert the current I-5 right of way to a high speed rail corridor. Though for the same price you could just build miles more of a high speed rail tunnel instead.


It would be much cheaper & straightforward to simply eliminate most/all of the downtown exits, convert I5 into an express only corridor, take 2 lanes for HSR use, and call it a day from a transportation perspective. Doing a deep bore is really about reallocating real estate, not transportation.

Paul W.

Referring to possible ST3 project tunnels as “unfunded” is incorrect.

Sound Transit’s board can choose to fund tunnels as part of the Downtown — Ballard and Downtown — West Seattle light rail extensions. The projected Sound Transit revenues are sufficient to cover expenses of that magnitude. The only limit on ST3 project budgets is available revenues, and the board can obtain as much revenues as it wants.

Stephen Fesler

It’s a correct statement. Sound Transit will not allocate a budget above that of the baseline Representative Project. Non-existent third party funding would be required to fund the West Seattle tunnel.

Paul W.

Stop lying — nobody told you that.

The board’s ‘phase gate’ budgeting process means any light rail extension capital project final budgets won’t be determined for decades. Whoever told you that “Representative Project” cost given to the ELG was any kind of limit on the revenues the board can allocate to the capital costs of the light rail extension from downtown to Ballard was full of doo doo.