Interstate 5: It’s ugly, ubiquitous, and almost universally hated. It’s a wall that separates one side of Seattle from the other. It’s noisy and certainly not new. I-5 through downtown Seattle was finished in 1967. Fast forward over 50 years to the present day, and it’s fallen into a state of disrepair. The Washington State Department of Transportation (WSDOT) has been deferring I-5 maintenance for decades–road maintenance isn’t the shiny new project that gets politicians elected. At this day and age, a full rebuild is likely necessary if we want to keep I-5 like it is now.

But considering just how much money is on the table, sticking with the status quo and rebuilding the miles of I-5 suddenly seems less appealing. Perhaps we could spend our money on something else, something better than a freeway slicing through our city. 

Something like we did with SR-99: burying the road into a tunnel and redeveloping the land where the road once stood.

The sheer size of I-5 through Downtown Seattle is staggering. (Google Maps)
The sheer size of I-5 through Downtown Seattle is staggering. (Google Maps)

And removing I-5 would be an absolute godsend for Seattle’s urban renewal. I-5 is a block wide–about as wide as the Central Library and the Columbia Center. Demolishing I-5 from SR-520 to I-90 would free up more than 50 blocks of land that could be redeveloped into much more useful things. A half-block parcel of land in the Denny Triangle recently fetched $60 million, meaning that an entire block of land could theoretically sell for $100 million apiece or more. (See also: the Mercer Megablock land sale.) Burying I-5 wouldn’t just be an urban awakening, it’d be a fiscal awakening.

The money would easily be put into the bored tunnel–something Seattle seems to love. Politicians in particular love a new grand project to brag about. Our city’s hilly terrain has made it the home to so many bored tunnels: the Link Light Rail tunnels under Capitol Hill and Beacon Hill, the Downtown Transit Tunnels, the Mount Baker tunnels… the list goes on. And the new I-5 tunnel could just be like the SR-99 tunnel we just built, meaning that we already have the research and development for the boring machine done. WSDOT could even take the lessons it learned the first time and apply them the second time around (e.g., don’t leave your metal pipe in after soil tests so that your contractor tries to blame you for its tunnel boring machine breaking down, delaying your project several years, and piling up cost overruns).

The new tunnel would be the same size as SR-99 (read: Bertha) and run from just south of the interchange with SR-520 to just south of the interchange with I-90. Taking into account the geography of our city, the tunnel would traverse Capitol Hill, dip down under S Dearborn Street, and then reappear somewhere near Beacon Avenue S, near where I-5 is right now.

A deep-bore tunnel would allow us to straighten the path of I-5 and remove the noise, eye-sore, and pollution from the surface, as shown above. The black line should interchange options to exit or enter the I-5 tunnel. (Google Maps edits by author)
A deep-bore tunnel would allow us to straighten the path of I-5 and remove the noise, eye-sore, and pollution from the surface, as shown above. The black line should interchange options to exit or enter the I-5 tunnel. (Google Maps edits by author)

At the northern and southern portals of the tunnel, I-5 would connect to expanded Eastlake Avenue and 4th Avenues. There could even be an exit somewhere near Madison Street, as the buildings on Capitol Hill aren’t nearly as tall as those in Downtown, meaning that the I-5 tunnel could run much closer to the ground. Nevertheless, traffic patterns would need to change drastically. The tunnel’s four lanes wouldn’t be nearly as numerous as the monstrous 16 lanes I-5 has now. Future traffic levels are a valid concern.

On the other hand, lidding I-5 is a proposal that would largely leave traffic patterns untouched. It’s been gaining traction lately and won a feasibility study via the public benefits package from the Washington State Convention Center. On paper, lidding I-5 sounds like a great idea–and it is. Miles of land between Downtown, First Hill, and Capitol Hill, would be freed and would serve as a much-needed extension of Downtown. The loud, polluting cars and trucks would be hidden away from view and be replaced with natural flora.

A rendering of a design concept for an I-5 lid between Madison and Thomas Streets. (Credit: Central Hills Triangle Collaborative / lidi5.org)
A rendering of a design concept for an I-5 lid between Madison and Thomas Streets. (Credit: Central Hills Triangle Collaborative / lidi5.org)

But it’s a conservative approach. Only the section of I-5 running below-grade between Thomas and Madison streets is capable of being lidded. The rest, such as portions in Eastlake, run atop elevated viaducts. Also the vast majority of the on-ramps and off-ramps remain the same, dumping cars in all the neighborhoods of central Seattle.

Map view of proposed I-5 lid (lidi5.org)
Map view of proposed I-5 lid (lidi5.org)

Lidding I-5 would leave everywhere else unchanged. The International District, an area historically neglected by the city, would still be left with a giant, block-wide viaduct running overhead. Eastlake would still be cordoned off from Montlake. And we would have to pay for both the lidding project as well as the rehabilitation of the existing freeway.

While planning the new urban spaces for Seattle, we must not forget the 274,000 vehicle trips on I-5 each weekday. By building the tunnel, we will provide a bypass option of central Seattle, potentially improving long-distance interstate trips while nudging people taking shorter trips to shift modes, whether transit, biking, scooting, walking, or rolling. The loss of several on- and off-ramps will be one of those nudges.

Bye bye, I-5 on-ramp through the Seattle Municipal Tower--what a metaphor that is. (Photo by author)
Bye bye, I-5 on-ramp through the Seattle Municipal Tower–what a metaphor that is. (Photo by author)

Regarding traffic concerns, having less capacity isn’t necessarily a cause for more traffic. It’s the law of induced demand: once drivers notice the new lanes and increased capacity, they’ll start making more and more, sometimes unnecessary trips, while commuters who previously took transit might even get back into their cars. Traffic levels will soon return to normal, even with the new lanes. 

Further applying the principle of induced demand, it may seem that removing I-5 altogether would also be a viable option for urban renewal. But a large chunk of traffic on I-5 are cars travelling through the city, such as from Everett to Kent. I-5 is their primary link to all destinations north and south of Seattle. The SR-99 tunnel links to local roads and SR-509 and doesn’t provide the direct link for long-distance travel that I-5 does. Diverting traffic onto other highways such as I-405 would only exacerbate the existing congestion on those roads.

Moreover, I-5 is essential for truckers to efficiently get where they’re going. “WSDOT estimates an average of 16,000 trucks carry goods through Seattle on I-5 every single day, moving more than 81 million tons of cargo annually in Seattle alone,” Lester Black reported in a recent Stranger feature on the future of I-5. Not all of those trips can end up on SR-99 or other northward routes from the Port of Seattle.

The lowered capacity of the tunnel will discourage excessive travel; people will actively begin to look for alternative ways to get downtown–and we will have those in droves. Sound Transit 3 and the ongoing expansion of light rail service will play an essential role. King County Metro also has an ambitious “Metro Connects” plan to add RapidRide service, and Community Transit has big bus rapid transit plans in Snohomish County. All in all, the tunnel will be there for those who need it and those who don’t will have other options.

Above ground, the removal of the current I-5 will spark an urban renaissance. It opens up acres of land for green development. The Cascadia high-speed rail could run through cut-and-cover tunnels constructed where the sunken freeway used to lie. New apartments can be built between South Lake Union and Capitol Hill. Skyscrapers can stand between Downtown and First Hill. Parks can rejoin the two sides of the International District that have been split apart. Streets that have been severed for 50 years could finally be connected again. Affordable housing can finally have public land to thrive. Where there was noise and pollution there would be life and nature.

But today I-5 is still crumbling. We have the chance to shape our city’s future. Burying I-5 connects people from afar while bridging together those near. Together, we can build a Seattle that we will be proud of in the next 50 years.

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14 COMMENTS

  1. If we put I-5 under ground, we should use the current route for HSR corridor incl a downtown station. We can lid it and put parks and houses above.

  2. Great post. I think the comparison to 99 is spot it – it’s nearly the same case study, at a much larger scale. Maintain through traffic, but do a lane and exit diet on the new freeway.

    Two thoughts

    1. I think you can do without the downtown on-ramps. For traffic coming from the south (5 & 90), expanding 6 & 7 to be a 1-way couplet of similar scale as 2nd & 4th should be sufficient for flowing vehicle traffic in & out of downtown. For traffic to the north (5 & 520), you can either do with a (separate) tunnel or a rebuilt boulevard between 520 and Mercer. TVR is right – by eliminating direct downtown access you actually improve throughput for the tunnel, particularly at peak.

    2. IMO, the I-5 Bridge is actually a bigger piece of infrastructure that will need to be replaced. It’s less of a scar on the urban fabric, but it’s the single piece that’s most likely to push WSDOT towards needing to replace I5, rather than continue to rehab & extend life. And once WSDOT is looking at rebuilding the I5 bridge, then the tunnel becomes appealing because, just like with 99, by building the tunnel before taking down the old infrastructure you leave with city without critical infrastructure for weeks rather than years.

  3. I know there was more immediate imperative to redo sr99 downtown, but now we’ve got the tunnel i5 needs somewhere not i5. Thanks for validating my thoughts

  4. I found this phrase to be interesting, we must not remember the 274,000 drivers that take I-5 each weekday. Was this intentional, or did you mean we must not forget the 274,000 drivers…

  5. Do both. Build a two-lanes-in-each-direction tunnel as a downtown bypass (call it the RH Thompson Bypass just for kicks), and rebuild the above ground freeway with smaller capacity and lid as much as possible.

    Let’s not forget when they dig the tunnel to make room under the road-deck for a double tracked high speed railway.

  6. Great article!

    One additional point regarding capacity, the removal of on/off ramps downtown would greatly simplify traffic flows. Thus, the reduction in lanes would not necessarily mean a proportional reduction in throughput capacity. Including a dynamic toll, too, would go a long ways towards combating the perceived loss of capacity. While not as innovative as a full removal concept, such a compromise could achieve similar benefits at lower political costs (albeit greater financial costs).

  7. Should also note in regards to freight that not all freight that is allowed on the current surface I-5 would be allowed in a tunnel (i.e. anything explosive, highly combustible or nigh impossible to put out) as an accident involving them could endanger the structural integrity of the tunnel itself.

    As noted the severe lane reduction would make this a non-starter. You’d have to build two Bertha size tunnels (8 lanes total) to even get people to consider it. The interchange with I-90 is fairly critical as well, it needs to connect directly to I-5 for smooth transition of N/S travel to E/W travel.

    The lose of over 50% of the lanes not to mention numerous on/off ramps make this a non-starter of an idea. It would be political suicide for any politician to back it.

        • I mostly use I-5 to bypass downtown Seattle, so a tolled tunnel with no on-ramps or exits into downtown would be massively more convenient for me.

          I’d hazard a guess that most people using I-5 on any given day are bypassing downtown.

          • If most people are bypassing downtown, then more than most are exiting and entering I-5 using the downtown exits and on ramps. Mercer Street exits and on ramps are prime examples people entering and exiting I-5 at high volumes everyday.

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