Lid I-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 structurally similar to overpasses and are built with concrete or steel. They can be designed to support soil, trees, people, vehicles, and buildings.

What is the purpose of a freeway lid?

In dense cities space is at a premium. Lids create new “land” out of thin air, allowing more efficient use of limited space while also addressing social and economic challenges. Lids can be used for a wide range of public and private land uses, ranging from open space to intense development. They also help reduce the local environmental impacts of freeways such as noise and air pollution.

What are the environmental benefits of freeway lids?

Lids can reduce noise and air pollution and restore neighborhood-scale connectivity, improving public health and environmental quality. According to the 2020 I-5 Lid Feasibility Study, lids over I-5 in Central Seattle also have the potential to treat up to 30 percent of the stormwater flowing off Capitol Hill and First Hill before it reaches our marine waters. The project could also reduce the urban heat-island effect and provide habitat for wildlife. There is also something uniquely symbolic about covering a visually blighted part of our city with green space, housing, and places of employment and community gathering that speaks to Washingtonians’ aspirations for a more environmentally gentle way of living. 

Future lid design can also be environmentally-friendly by not building any new parking spaces and instead inspiring walking, bicycling, and transit trips. High-speed rail (HSR) is being studied by Washington State as a solution to reduce aviation emissions, and an I-5 right-of-way alignment could make sense. A lid site is one of many options for a Downtown Seattle HSR station. 

Why is this campaign happening now?

Seattle’s Center City is anticipating at least 28,000 more homes and 55,000 new jobs by 2035. According to 2018 City data, Downtown, Capitol Hill, and First Hill are 3.5% of Seattle’s land area but are absorbing 29% of population growth, and at the same time are running out of land. Lidding I-5 is likely the only opportunity to catch up on much-needed affordable housing sites, public open space, civic facilities like schools and community centers, and other public and private infrastructure.

In addition, 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, which is occurring with Capitol Crossing in Washington, D.C and Fenway Center in Boston. While future public-private partnerships may be desirable or even necessary for lidding I-5, projects of this scale and importance to Seattle’s future must be held in the public trust. By acting now to secure public ownership of future lid land we can ensure an appropriate balance of economic development and community benefit.

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 levels (an average of 288,000 vehicles per day). Dealing with I-5 deficiencies 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.

Why not remove Interstate 5 entirely?

In our years of public engagement, collaborating with technical experts, and working with elected officials, removal has not arisen as a serious consideration. The I-5 corridor carries 288,000 vehicles daily through Downtown, so there would need to be viable alternatives to I-5 that address its role in carrying freight, transit, commuters, emergency response vehicles, and a myriad of other functions. People across the metro area travel here via I-5 because Seattle has the density of jobs, colleges, and hospitals they rely on and there are not enough reliable multimodal connections to the suburbs. We are unaware of such work being done to address these needs, so we have no further insights on the idea. 

We fully support removing urban freeways where it is warranted, such as low-traffic sections of freeways, inner-city loops and stubs, and duplicate routes – especially where communities of color have been disproportionally impacted. Regional examples of candidates may include Highway 99 in Seattle’s South Park neighborhood, I-705 in Tacoma, and I-405 in Portland. The Congress for the New Urbanism (CNU) maintains a biannual report, Freeways Without Futures, which identifies many such opportunities for freeway removal across the country. In 2021, lidding I-5 was identified by CNU as a visionary and appropriate project for our local situation.

How much area could be lidded?

The 2020 I-5 Lid Feasibility Study estimates a maximum lid area of 17 acres. This is equivalent to about eight city blocks. Comparatively, South Lake Union Park is 4.5 acres and Cal Anderson Park is 11 acres.

Have lids been built in other places?

Yes, there are dozens of lids across the United States alone and more examples worldwide. Please see our case studies page for the best examples and a 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 if a lid is built during or after the freeway’s original construction. Based on other freeway lids nationwide, a rough cost estimate for lid parks is around $500 per square foot.

The 2020 I-5 Lid Feasibility Study developed conservative, block-by-block cost estimates for lidding I-5 in Central Seattle at $2,000-3,500 per square foot, depending on the lid use (parks or buildings). This is higher than the nationwide average because it includes a very large risk factor of 50 percent, but also because I-5 in Central Seattle is unique with its curving lanes, sloping topography, and many ramps and structures. The figure is also inflated by conservative assumptions about parking and how to fill in Freeway Park (see more analysis on our feasibility study page).

Depending on the future scenario, ranging from a park design to private high-density development, the study estimates capital construction cost ranges from $1-2.5 billion. A moderate, mid-density hybrid approach would cost $1.5 billion. Compare this to the total $4.5 billion cost of the Alaskan Way Viaduct replacement and Waterfront Seattle program.

Who will pay for lidding I-5?

Research by Lid I-5 and the 2020 I-5 Lid Feasibility Study indicate the entire range of local, state, federal, and private sources are options. The Lid I-5 campaign is not advocating a specific funding plan at this early stage.

Both Washington State and the federal government will 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. The City of Seattle and King County may contribute local resources.

Several recent lid projects have been partially or entirely privately funded. Seattle is home to businesses and individuals who may be interested in philanthropically supporting such a major 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, TX was achieved through private donations.

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 (though not the first in the nation). It is 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 restrooms are examples of challenges that are being addressed in the Freeway Park Improvements Project, funded by the public benefits package of the Washington State Convention Center Addition.

The 2020 I-5 Lid Feasibility Study notes that a future lid in the block between Spring Street and Seneca Street will be a challenge to design because of the Freeway Park box gardens. This area must also be sensitive to Freeway Park’s recent addition to the National Register of Historic Places.

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.

In addition, lids should be located in low-income and minority communities that have faced historic under-investment. A study from the University of California, Irvine found that the limited freeway currently lids in Seattle (Freeway Park and Sam Smith Park) are more equitable than the many lids built outside Seattle.

Will lids prevent a widening of Interstate 5?

WSDOT has no plans to widen Interstate 5 in central Seattle and has publicly acknowledged it is not physically reasonable. The freeway is lined by deep retaining walls and blocks of high-rise development. 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.

Instead of widening I-5, WSDOT is making more efficient use of the existing footprint through traffic demand management, optimizing roadway design, and partnering on the buildout of regional light rail transit.

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 within freeway right-of-way, 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, the I-5 Northbound Seneca to SR 520 Mobility Improvements will convert an exit-only lane to a through-lane by adjust barriers and striping. The Express Lanes Connection Project will add an direct connection from South Lake Union to State Route 520 for carpools and buses. Multiple other improvements and maintenance work is planned under the Revive I-5 program. A series of corridor studies have examined 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.”

Would lids impact freeway traffic?

The 2020 I-5 Lid Feasibility Study confirmed that building lids over the current freeway is feasible, and could be staged with minimal traffic disruptions and would not require any long-term closures. Construction work could possibly be done focused on nights and weekends. The major structural work for Klyde Warren Park in Dallas, TX, was only allowed 20 highway closures during low-traffic periods and was completed within a year.

Once the lid is complete, traffic flows without any issues related to lids. This is shown with the current lids at Freeway Park and the Convention Center, and also on Interstate 90 and State Route 520. Tunnel lighting and other safety systems are provided as needed.

Who will 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.