Frequently Asked Questions

Most Popular Questions

In a hurry? Click a link to the most popular questions we hear. Otherwise, we recommended reading all of the questions and answers below.

How much area could be lidded?
How much do lids cost to build?
Who would pay for lidding I-5?
Will lids prevent a widening of Interstate 5?

What are lids (also called caps, covers, and decks)?

A lid is a bridge-type structure that spans over the gap of a sunken freeway or other transportation corridor. They are typically flat and built of concrete and steel. They can be designed to support soil, trees, people, vehicles, and buildings.

Why would anyone want to build a freeway lid?

In dense cities space is at a premium. Lids create new land that can be used for a wide range of public and private uses. In addition, lids over freeways make more efficient use of limited space while also helping to resolve social and environmental issues. Most other cities that build freeway lids use them for new park space, a clear benefit for adjacent neighborhoods as new space is created for relaxation and recreation. Lids also cut off the noise and sights of sunken freeways, transforming one of the least desirable places in the city to one of the most attractive.

Why is this campaign happening now?

Several key issues are motivating the Steering Committee to act today, even if lids may not be built for many years.

The economics of building lids and the lack of redevelopable and vacant land in the city center is gaining awareness among the development community. This may spur private developers to lid I-5 on their own initiative – this happening with projects like Capitol Crossing in Washington, D.C and Fenway Center in Boston. While future public-private partnerships may be desirable or even necessary for future lid projects, infrastructure of this scale and importance to Seattle’s future must be held in public trust. By acting now to secure public ownership of future lid projects we can ensure an appropriate balance of amenities for residents.

Seattle’s Center City is planned for at least 28,000 more homes and 55,000 new jobs over the next two decades (and we may reach those targets more quickly than anticipated). Population growth is accelerating the need for more public infrastructure such as a parks and open space, affordable housing, and other civic facilities such as schools or community centers. Lidding I-5 is a prime opportunity to create much-needed land for these important public uses.

Interstate 5’s structural elements such as overpasses and viaducts are over 50 years old, were not built to current seismic standards, and may require major rehabilitation. The freeway’s design is also outdated for current traffic patterns. Addressing I-5 is likely to become a major focus of the Washington State Department of Transportation (WSDOT) and the federal government as work is completed on the State Route 99 waterfront tunnel and the State Route 520 reconstruction over the next several years. By working now to build a community-led vision and establishing the feasibility and necessity of lids we are getting in front of any future corridor plans.

How much area could be lidded?

The Lid I-5 Steering Committee has performed a conservative analysis of the airspace between Madison Street and Denny Way (the City of Seattle’s 2019 feasibility study area). Excluding ramps, overpasses, and Freeway Park, the upper estimate is about 15 acres. This is equivalent to about eight city blocks.

We already have Freeway Park in Downtown over I-5. How does that fit into this?

Freeway Park is the original freeway lid in Seattle, the largest park in Downtown, and it recently celebrated its 40th year anniversary. It is an essential component of the campaign’s vision for a more connected and vibrant Seattle. In fact, this campaign is building upon earlier concepts for expanding Freeway Park (see the history page for more information about expansion plans).

The Freeway Park Association, in partnership with the Parks Department and the Metropolitan Improvement District, has experienced great success with public event programs and stepped up maintenance. However, the park still faces a number of issues that need to be resolved before other lid parks can be considered. Lighting, wayfinding, and handicap accessibility are examples of physical challenges that can be addressed with targeted reinvestment.

Freeway Park has great potential to be a more usable space for surrounding residents and businesses. More substantial upgrades like a coffee stand or restaurant and family amenities like a playground would help draw more people into the park. The Washington State Convention Center, which helped expand Freeway Park in the 1980s and which now uses the park as its backyard, needs to be a key planning and funding partner in these efforts.

Have lids been built in other places?

Yes, there are dozens of lids across the United States alone and more examples worldwide. Please see the case studies page for key examples and a work-in-progress map of all known freeway lid projects.

How much do lids cost to build?

The cost varies greatly depending on how large the lid is, what it is used for (parks or buildings), and whether it is built simultaneously or after the freeway. Based on other projects nationwide, a conservative cost estimate for lid parks is around $500 per square foot, or approximately $22 million per acre. Because soil is heavy the cost to support small buildings may be similar, but the cost to support taller buildings is likely higher.

The City of Seattle’s 2019 lid feasibility study will estimate construction costs and funding models in detail. The baseline cost in Seattle may be higher because of our local topography (other lids are generally built in flat areas), the need to design for strong earthquakes, and other factors.

Notably, Downtown land value is approaching or above $1,000 per square foot. This may mean creating new public land in the form of freeway lids is less than half the cost of acquiring private land, and this value difference may factor into funding opportunities.


Comparing the average cost of lid parks to the value of land in Downtown Seattle.

Who would pay for lidding I-5?

The campaign is not advocating a specific funding plan at this early stage and future projects will likely have a mix of funding sources. The City of Seattle’s 2019 lid feasibility study will help determine what options are available for both capital construction costs and ongoing maintenance.

Interstate 5 is owned by the state of Washington and partially governed by the federal government, so both the state and U.S. departments of transportation would be key partners. The state has recently built many other lids in the Puget Sound area and is planning more, including five lids over State Route 520 and another over Interstate 405 in Bellevue. Funding by the City of Seattle and King County are also options.

Private philanthropy may also be a factor. Seattle is home to businesses and individuals who may be interested in supporting such a transformative civic endeavor. Waterfront Seattle expects about 15 percent of the new waterfront parks to be funded with donations. About 48 percent of the capital funding for Klyde Warren Park in Dallas, Texas was achieved through private donations.

Is air pollution a concern with freeway lids?

No. Most freeway lids completely seal off the sights, sounds, and pollutants of the traffic lanes, making any pollutants undetectable on the surface. Even Freeway Park, which has openings to the freeway traffic below, has never been recorded to have harmful concentrations of air pollutants within the park.

Freeway lids over 800 feet long are considered tunnels by the National Fire Protection Association and may require mechanical ventilation systems. These systems usually do not vent normal traffic exhaust and are only activated for fire and smoke emergencies.

Do freeway lids have measurable public health benefits?

Yes. In one example, a 2018 American Journal of Public of Public study considered a hypothetical lid park above the Cross-Bronx Expressway in New York City. The authors found quantifiable public health benefits derived from: improved access to green space and exercise opportunities, reduced and filtered air pollutants, and reduction of traffic noise.

Can you really construct buildings on lids?

Yes. There are many examples of both private and public buildings being built over freeways to make efficient use of limited urban land. Locally we can see the Washington State Convention Center spanning over I-5. Other examples: a restaurant at Klyde Warren Park in Dallas; mixed use office, residential, and retail buildings at Capitol Crossing in Washington, D.C.; private development planned at Fenway Center in Boston, MA; and a city hall in Fall River, MA.

Would lids impact freeway traffic?

The lid feasibility study will determine where structural elements like walls and columns may be located in relation to traffic lanes. However, other parts of Interstate 5, Interstate 90, and State Route 520 already have multiple lids that traffic passes beneath without issue.

The most notable traffic disruptions would occur during construction, but the impact would be short term and could be done mostly on nights and weekends. The major structural work for Klyde Warren Park was only allowed 20 highway closures during low-traffic periods and was completed within a year.

Will lids prevent a widening of Interstate 5?

There are no plans to widen Interstate 5 in Seattle’s Center City, and it would be impossible anyway due to adjacent skyscrapers and retaining walls. The Seattle Times reports, “The highway’s west side is lined with buildings and businesses, some less than 100 feet from mainline traffic…retaining walls on I-5’s east side have steel and cement columns drilled 120 feet deep to hold up the hillside.”

The Washington State Department of Transportation (WSDOT) has publicly acknowledged this physical and financial reality multiple times, most recently in the 2017 State Facilities Action Plan (PDF) written in partnership with the Puget Sound Regional Council, saying:

“There may be places I-5 can be expanded, but in the core of the Puget Sound region I-5 is permanently constrained geometrically as it passes into and through Seattle. That constraint is the architectural limit for freeway expansion in the region.” In addition, “Funding is also limited, and maintaining aging infrastructure is a higher priority than expansion.”

WSDOT has maintained this position for some time. A 2005 Problem Definition Report summarizing a congestion relief analysis similarly said, “The study quickly concluded that it will be extremely difficult to widen I-5 within its existing right of way through the highly developed urban portions of central Seattle.” Going even further back, “studies since the late 1980s…indicate that expansion of the highway to carry more vehicles will be highly disruptive and extremely costly and will have many impacts.

What is being done to improve traffic on Interstate 5?

The region’s voters have decided to build a high-capacity transit system to ensure future mobility and provide the people-moving capacity needed for the region’s health. WSDOT is partnering on this buildout with light rail alignments and express bus infrastructure, along with managing demand and encouraging walking, biking, and carpooling.

According to WSDOT, “the Link light rail line has the ultimate equivalent people-moving capacity of an eight-lane highway running parallel to I-5” (2005 Problem Definition Report).

WSDOT is also making more efficient use of the limited I-5 footprint it has. For example, in 2019 a $20M project will add a northbound lane between Seneca Street and SR-520 by adjusting lane widths and moving barriers. A series of corridor studies have examined others ways to relieve bottlenecks and improve safety like converting left-side exits and installing ramp meters. Continued support for the incident response teams is also critical: according to The Seattle Times, “With an annual budget of $4.5 million, the teams are among the most cost-effective traffic reduction strategies that WSDOT has, far cheaper than adding lanes to the interstate.”

Who would own and maintain the lids?

As with other lid projects in the region, the Washington State Department of Transportation would maintain the under-structure of the lids and the local government (in this case, the City of Seattle) would maintain the surface.

Since lids will provide such large and unique new public spaces, a viable option for managing maintenance and operations would be the creation of a nonprofit or specialized City office to specifically oversee them on a daily basis. Such a management system would oversee daily programming like concerts and markets and hire custodians and security staff. This is the same model embodied by Friends of Waterfront Seattle and the Office of the Waterfront, which will manage manage Seattle’s new waterfront parks.

For example, the nonprofit management model is also used for at Klyde Warren Park (Dallas, TX). The foundation that runs the park has an operating budget of $3 million per year, funded through a local improvement district, vendor fees, and private sponsorships.

What’s the difference between the Lid I-5 campaign and the Seattle CAP by Patano Studio Architecture?

Patano Studio Architecture, a private firm, and Lid I-5 both want the same benefits that come with lidding the freeway, but they are focused on different time frames and scopes of work.

Patano Studio has a specific design for lidding I-5 from the Convention Center to Lakeview Boulevard, a distance of 1.2 miles and covering 45 acres. The proposal is pro-bono work and includes features like a major basketball/hockey arena, apartments, and a hotel. The firm has sparked a renewed public conversation about lids and attracted significant media coverage, which is beneficial to this campaign.

Lid I-5 is focused on an on-the-ground effort to achieve a vision established by the larger community. We are actively collecting the community’s ideas, working with elected officials, and pursuing funding for a comprehensive feasibility study and a prototype lid at Plymouth Pillars Park.

How can we address other parts of the freeway that are not sunken, like in the International District and Eastlake?

Lids are a response to the specific condition of sunken freeways and the opportunity to create new usable public land. For practical and economic reasons, lids are not applicable to other topographical conditions, such as where a freeway is elevated on columns. And generally, new usable land for housing and schools cannot be created below an elevated freeway due to physical constraints. Therefore, we are focusing primarily on where one or both edges of the freeway is below the level of the surrounding cityscape and where new connections can be made.

That said, we strongly support other solutions for the variety of other topographical conditions that I-5 faces in Seattle. New pedestrian bridges are feasible in many locations, as demonstrated by plans for the Northgate Pedestrian and Bicycle Bridge. The Bike Master Plan has conceptual ideas for new bridges over the freeway at Harrison Street near Downtown and 47th Street in the University District.

Many spaces underneath I-5 have already been converted to productive land uses. Between Eastlake and Capitol Hill the I-5 Colonnade offers 7.5 acres of mountain bike trails and pedestrian pathways, improving the area from a refuge for criminal activities to a recreation destination. Much of the space below I-5 in south Downtown and Roosevelt is utilized for commuter parking. We support citywide efforts to make these underpasses more active and inviting, such as by widening walkways, investing in colorful and interactive public art, and allowing new commercial uses like pop-up retail and food vendors. Click here for an inspiring example of an underpass improvement in California.