8 Takeaways from Seattle’s Lid I-5 Feasibility Study

A park-like is shown over the freeway trench in this cross section.
A cross section of I-5 with a lid overhead. (WSP / City of Seattle)

The City of Seattle may have quietly released its Lid I-5 feasibility report late last year, but the results trumpet the benefits of lidding freeways in urban environments. Inspired by advocacy from Lid I-5 Seattle and bolstered by the $1.5 million lined up through the public benefits package from the $1.9 billion Washington State Convention Center (WSCC) Addition, the Office of Planning and Community Development (OPCD) commissioned the feasibility study, employing WSP as the consultant.

We had Lid I-5’s Scott Bonjukian and Natalie Bicknell as our January meetup guests. Both have connections to The Urbanist as Bicknell is our senior reporter and Bonjukian was our former events and programming director who also wrote about lids extensively. Bonjukian summarized the study, and highlighted some key nuggets in his visual presentation, which you can see for yourself in our recording.

If you were worried the financial crisis at the WSCC could imperil the lid project, have no fear: Bonjukian said the $1.5 million for the lid study had already been disbursed so there wouldn’t be direct impact. The partially completed WSCC Addition does border the proposed lid area.

I’ve adapted Bonjukian’s presentation and my own perusal of the 150-page study into eight takeaways.

The lid could support a new 10 acre public park

The test area was 17 acres stretching from Madison Street to Denny Way. Of that area, the study found that between 2.5 and 9.8 acres could be converted to new public park space. But that depends on the objectives of the project; the study looks at three test cases. Test Case 1 looked to minimize cost by not lidding the most challenging and difficult sites and focusing on public park space. Test Case 2 looked at the scenario of creating the most density and leveraging the most private investment. Test Case 3 was a hybrid seeking a balance of civic and private uses.

A WSP graphic shows the extent of the lid in each test case.
The three test cases studied by WSP. (Slide by Scott Bonjukian)

This could solve a major gap in park and open space access, since Downtown and First Hill sorely lack a major park, particularly in a central location. Seattle does not really have the equivalent of Grant Park in Chicago, but the lid park could serve that role.

Room for 4,500 homes

WSP did some structural engineering analysis to determine how much development could go atop the lid, and, while each section is different, the short answer is a lot. Bicknell’s 2019 article on earlier lid feasibility research had hinted midrise and some highrise development was possible. The final report estimated up to 4.7 million square feet of new housing is feasible–enough space for 4,500 homes–and between two and five million square feet of commercial/office space. WSP also estimated that Test Case 2, though driven by private development, would pay $150 million to $215 million in the City’s affordable housing trust fund via Mandatory Housing Affordability (MHA) payments. Test Case 3, on the other hand, set the target at 40% rent-restricted social housing on-site.

A map of the lid area showing the intensity of development. Most high intensity if on the edge of the lid.
The red shows areas that could support highrises (400 feet) and the blue is mid rise of up to 200 feet. The fuchsia denotes highrises up to 680 feet. (WSP)

Reconnecting the street grid

The lid could erase the street grid damage from plowing I-5 through in the first place. The damage to mobility downtown is massive. People walking and biking must detour rather than stay on their desire lines. Congestion abounds from the reduction in through streets mixed with the injection of traffic from a busy freeway (average daily traffic of about 270,000) with several on- and off-ramps in the vicinity. Reconnecting streets like Union and Howell and avenues like 7th, 9th, Terry, Minor, and Yale could make it much easier to get between First Hill or Capitol Hill and Downtown. Ideally we wouldn’t be adding much car space, but dedicated bike, pedestrian, and transit space could much improve mobility in the area.

Reducing noise pollution

A lid would dampen noise in area severely afflicted with noise pollution. “Ambient noise over 66 decibels qualifies as an affected area,” the study notes. “An environmental impact statement for a project in the corridor showed that existing noise levels at 10 short-term monitoring sites ranged from 70 to 78 decibels, depending on the proximity to I-5 and side streets in the area (WSDOT, 2020). A lid would act as a noise barrier in cases where it would interrupt the line of sight between a noise source (I-5) and a receiver (FHWA, 1974), and the noise reduction would depend on the material, size, and location.”

A cross section of I-5 showing light rail in a tunnel, the freeway trench, and a lid over top with the skyline in the background.
A lot is going on underneath the lid level with Link light rail tunneled underneath the freeway pilings. (WSP)

In other words, since I-5 went in during the 1960s and displaced about 30,000 Seattle residents, the neighbors remaining next to the freeway have suffered from noise pollution basically continuously ever since–the price of “progress” in a car-centric culture that waives off any social costs of car-centric mobility. The noise and polluted stench makes it miserable for people passing through, too–anyone who has walked, rolled, or biked the overpasses over the freeway can attest to how infernally loud and unpleasant it is. A lid could heal some of that scar, and there isn’t much else up to the task, short of freeway removal. Unfortunately, when electric cars are operating at highway speeds, they are pretty much just as loud as combustion engine cars.

A time-lapse night shot of I-5 facing north showing all the taillights bleeding together.
I-5 remains busy and load late into the evening. (Credit: WSP)

Reducing stormwater pollution

One finding that surprised me was how much a lid would help with stormwater and runoff management. “Approximately 30 percent of the Capitol Hill basin runoff could be treated or retained on the lid, reducing runoff and pollution to the waterways and reducing excess demand of the existing infrastructure and sewer system,” WSP said in the report. “An I-5 lid project could also explore the opportunity to treat currently untreated runoff from I-5 itself.” With recent revelations of how much salmon and other aquatic life are suffering from roadway runoff due to tire particles and other toxic compounds that leach into the water, this could be a huge environmental benefit.

Removing Olive Way on-ramp adds even more space

WSP mostly left the freeway interchanges as-is in their scenarios, but they did look at removing the Olive Way on-ramp and found that the space created would allow additional highrises. In general the space over the middle of the lid is not conducive to highrise development, but space created from the slopes and ramps on the edge of the freeway can typically support more weight without huge added expense. Removing the ramps would likely improve traffic flow on the street grid since the freeway interchanges tend to the pinch points and induce more car travel demand.

A cross-section schematic of removing Olive Way ramp to add more structures.
Replacing Olive Way on-ramp with a highrise. (Credit: WSP)

Lid would cost about $1.4 billion in hybrid scenario

WSP estimated (caveat: at 0% design) that Test Case 1 would cost $966 million, while they pegged the densest scenario, Test Case 2, would cost between $2.3 billion to $2.5 billion. The hybrid lid option would cost $1.3 billion to $1.5 billion. This would cover the structure of the lid, but the private development projects would have their own budget to build atop the structure. Bonjukian noted the study’s estimates assume an independent construction project rather than piggybacking on the I-5 repair and rehabilitation work that the Washington State Department of Transportation (WSDOT) thinks will soon be unavoidable, given concerns about the deterioration of I-5 structures and lack of earthquake readiness in some segments.

The lid wouldn’t be cheap, but it also would be cheaper than several other Downtown megaprojects including the SR-99 car tunnel and the convention center expansion. The public benefits may far outweigh the costs, and selling some of the parcels created could offset some of the cost, while intriguing financing options like a local improvement district could help cover the rest. In short, there are plenty of ways to make the lid work financially, too.

Next Steps

Bonjukian said getting some more money for studies and planning in the next state transportation package is the next major goal for Lid I-5. Studies they’re pursuing include a downtown street network study to look at the impact of reconnecting streets across the I-5 trench and the I-5 Corridor Scenario Analysis proposed by WSDOT and the I-5 Systems Partnership.

I-5 System Partnership graphic
Lid I-5 is watching out for or seeking these studies to advance the Downtown lid project. (Slide by Scott Bonjukian)

My dream scenario is still to remove I-5 from the central core of Seattle, but if we’re going to keep I-5 (and all indications are that state legislators are still trying to expand highways not remove them) then lidding I-5 is a must to mitigate the ill effects of such a busy freeway in a dense urban environment. Plenty of cities have covered their freeways and created parks and housing. It’s high time Seattle did.

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Doug Trumm is The Urbanist's Executive Director. An Urbanist writer since 2015, he dreams of pedestrianizing streets, blanketing the city in bus lanes, and unleashing a mass timber building spree to end the affordable housing shortage and avert our coming climate catastrophe. He graduated from the Evans School of Public Policy and Governance at the University of Washington. He lives in East Fremont and loves to explore the city on his bike.

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Lee (@Lee44572133)

I live on Hubbell Place, and bc I5 is open on the NB side for about a block or 2, under Freeway Park, the noise inside with all windows closed is AWFUL, oppressive at times. Definitely need to close that up!
Let’s house all residents AND lid I5!


Don’t know if you rent or own, but if you own the home, you should consider upgrading your windows. Standard window glass is designed only for thermal insulation and is very ineffective at sound blocking.

Rather than completely replacing the windows, an often better option is to supplement your windows with an additional layer of glass on the inside. While it’s not cheap, it really does make a big difference in the sound level, and it’s something that you can do unilaterally, without waiting for a billion dollar freeway lid. As an added bonus, the extra glass layer will also make your home more energy efficient, so you’ll at least some of the money back over time through savings in your heating and cooling bill.

John Gordon Hill

I love and support the concept, except we’ve already partially done it. Freeway Park was an internationally recognized urban amenity that is now a dangerous, filthy, cesspool. City government has abdicated any responsibility for the commons in the name of providing a miserable existence for the unhoused. If the existing Freeway Park cannot be restored to general public use, I see little public support for more Freeway Parks.


With the state looking to raise the gas tax again, the gas tax (maybe?) limited to freeway related infrastructure spending, and the gas tax our state’s best proxy for a carbon tax, wouldn’t it be best to incorporate all this lidding into the I5 rebuild, funded by state gas tax revenue? There might be a role for a LID to supplement the gas tax, given many of the benefits will accrue to the area immediately adjacent to the freeway, but seems like this would be best done within the context of WSDOT’s eventual rebuild of I5, rather than as a local add-on, for both financial and engineering reasons.

Just saying

With Seattle and the way they run the homeless situation, what you just described to me is more money the city doesn’t have to make a brand new 10 acre city park “ Homeless” camp area .. why would this benefit at all when I can’t even go to the existing parks with out dealing with needles and garbage.

Oh, and don’t forget this is the same city decriminalized so your and my property and now fare game to the same park problems.

Seattle should fix the current problems before spending money on a dream.


The world’s largest homeless encampment and shoot-up site. Cool.

If this ever happens, mixed use is the way to go.


This is probably my favorite section of the report:

The construction of I-5 severed nine of 18 east-west connections, particularly Minor, Yale, and Terry Avenues. The loss of these connections disrupted the traditional grid. The recent growth of the adjacent neighborhoods has made a reconstruction of the lost connections between Capitol Hill and Denny Triangle/South Downtown increasingly important (Figure 7-25).


This looks great. I really don’t have a strong preference for the alternatives. They all look like a huge improvement to me.

Reconnecting the grid is intriguing. I wonder what Metro could do with the new connections. It isn’t obvious, as many of the streets have awkward intersections. If nothing else, though, it would make it easier (politically) to get bus and BAT lanes. For example, there should eventually be a frequent bus from South Lake Union to Beacon Hill or Rainier Valley via Boren (https://goo.gl/maps/AckAWpngX4jYLR1b9). This bus would encounter plenty of traffic though. The traffic can be avoided by an alternate street (e. g. 9th) that would now go through. Or we simply add bus lanes on Boren, and drivers can always use an alternative (like 9th) — something they currently can’t do.

I’m excited about the idea of getting rid of the on-ramps. Even if you feel like I-5 is a necessary evil, there are too many ramps downtown. Various streets have congestion because of people going that way to get to I-5. For example, one reason Boren is so congested is because of drivers coming from First Hill use that as a way to get on northbound I-5: https://goo.gl/maps/ptnHBS8c57Avmf9L7. This goes away if the ramp is gone.

There are other benefits to closing those ramps. Many of the lanes on Olive are focused on getting cars to the freeway. This includes the buses. While it is great that buses get their own ramp, it is essentially turns Olive from a three lane road, to a one lane road. Without the ramps, you could have bus lanes on either side, along with the same one lane road (heading east). Drivers going up the hill are in the same boat, but now you have an eastbound BAT lane, and a westbound contraflow bus lane. A bus like the 10 would just stay on Olive all the way through.

There are a lot of advantages to this that I hadn’t though of. The value of reconnecting the street grid is huge, for bikes and buses especially. It makes sense to think of this as a transportation project (as opposed to a park or housing development project) even though it would benefit everyone in all those ways.


Yeah at minimum all the left-hand exits should be removed.

Given the age of I5, seems like it would be better to incorporate these lids with the rebuild of the freeway itself, rather than capping a freeway that is currently crumbling. Ideally I5 could simply be removed between I5 or 520, but if that’s not going to happen, keeping the freeway where it currently it might be better than a deep bore tunnel (like 99), as a capped trench would presumably be much cheaper than a new tunnel. If most of the downtown exits are removed, the freeway would be narrower, creating space for the Lid foundations, and perhaps creating space for HSR. This would be much like Boston’s big dig, except the freeway is mostly already in a trench.

More interesting is what to do with the segments in eastlake and the ID, where the freeway is elevated. Will those remain elevated, be converted to boulevards, or could they also be trenched & lidded?


Once Link is built out, will buses even need the Olive Way ramp to I-5 anymore? At first glance, it seems not. Literally every bus route that uses that ramp can/should be truncated to a Link station further out.


No, they won’t. But they will likely just change it to an HOV or general purpose on-ramp. Even if they got rid of the ramp, you still only have three lanes — one general purpose, one for the ramp, and one for the bus. The only way to dramatically change Olive is to get rid of *all* the ramps. Then you can run the bus both directions (in its own lane) and have the same general purpose lane going east.