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General questions
Questions related to the proposed SR 99 bored tunnel
For more information, visit our central waterfront page.
What part of SR 99 will be replaced?
We will replace the double-deck bridge section of SR 99 that runs along Seattle's waterfront from S. Holgate Street up to the Battery Street Tunnel. This structure is called the Alaskan Way Viaduct.
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Why is the viaduct a safety issue?
The 1950s-era viaduct was already showing signs of age and deterioration before the 2001 Nisqually earthquake, but the earthquake heightened the need for its replacement. The major risk facing the viaduct is its seismic vulnerability. The viaduct stands on fill soil bounded by the seawall. Marine organisms have slowly eaten away parts of the seawall and weakened it. In an earthquake, the fill soil is subject to liquefaction, where a shaking motion causes the soils to turn into a quicksand-like condition. Another major earthquake could collapse the seawall and liquefy the soil, damaging the viaduct beyond repair.
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What happens if the viaduct, seawall or both are closed before they are replaced?
We worked with the City of Seattle to prepare an emergency response plan. In the event the viaduct is closed due to an earthquake or a traffic accident, this plan will help people move out of downtown safely and restore access to downtown as soon as possible.
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Is the viaduct still a safe structure on which to drive?
We conduct routine safety inspections and perform necessary maintenance to keep the viaduct and seawall safe for public use.
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Are there roadway restrictions on the viaduct?
In 2001, we imposed roadway restrictions that remain in effect today. Vehicles with a gross weight of more than 105,500 lbs. are prohibited. Each single axle is not to exceed 20,000 lbs., and tandem axles are not to exceed 34,000 lbs. Trucks and buses must travel in the right-hand lane only. Further restrictions could be imposed in the future.
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Is most of the traffic using the viaduct today going to downtown or through downtown?
The current viaduct carries approximately 110,000 vehicles per day just south of the mid-town ramps. Of this amount, approximately 17,000 vehicles enter or exit downtown at Columbia and Seneca streets, and 33,000 exit or enter at Elliott and Western avenues toward Belltown, Uptown, and neighborhoods along the 15th Avenue and Elliott Avenue corridor. The remaining 60,000 vehicles continue north through the Battery Street Tunnel, either exiting in the South Lake Union/Queen Anne area or continuing north across the Ship Canal.
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Where would the bored tunnel be located?
The proposed tunnel would be located several blocks east of the waterfront, mostly under First Avenue. The southern portal would be near Royal Brougham Way in the stadium area, and the northern portal would connect to Aurora Avenue N. near Harrison Street. View a graphic of the bored tunnel's route.
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Would the tunnel be safe?
The proposed tunnel would be safe for drivers in the event of an earthquake or accident. Structural engineers agree that tunnels can be one of the safest places to be during an earthquake. Seismic waves are amplified as they reach the surface of the earth, which causes a whiplash effect and subjects above-ground structures to stronger motions than a tunnel. The BART tunnel was inspected and re-opened the day after the Loma Prieta earthquake in San Francisco.
If an accident occurs in the tunnel, there would be room on the shoulders for emergency vehicles. The tunnel would also have emergency passages to safety, fire suppression systems, communication with vehicles from a central station, and emergency ventilation systems that meet federal standards. The tunnel would be monitored 24 hours a day by WSDOT, similar to the I-90 tunnel today. Visit bored tunnel safety for more information.
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Can we afford the tunnel?
The current estimated cost of the proposed bored tunnel is $1.9 billion. The total cost of the SR 99 replacement, including the S. Holgate Street to S. King Street viaduct replacement near the stadiums, would be $3.1 billion. Investments by King County and the City of Seattle in transit, city streets and waterfront improvements would bring the total to $4.2 billion.
This SR 99 replacement has $2.4 billion in committed funding from the state gas tax and federal funding. The remaining $700 million would come through tolls on the bored tunnel and a $300 million contribution from the Port of Seattle (still requires formal commission approval). The City and County are identifying local funding sources for their transit and city street investments.
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Would the bored tunnel be another "Big Dig?"
The “Big Dig” project was substantially larger and more complex than the proposed SR 99 bored tunnel. Boston’s project was eight miles long with multiple tunnels and bridges, including a signature cable-stayed bridge over the Charles River. The “Big Dig” also included a disruptive cut-and-cover tunnel through downtown and two sets of immersed tubes under the Boston Harbor to the airport. The project was built while traffic was maintained through the construction zone and businesses stayed open with heavy equipment operating outside their doorsteps. This approach required extensive traffic management and mitigation.
The SR 99 bored tunnel would be less than half the length of what was constructed in Boston. It would run 60 to 200 feet underground into stable soils. By boring under First Avenue, we would minimize traffic disruptions and impacts to the waterfront and downtown. Visit Big Dig comparison for more information.
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How is the bored tunnel different from the tunnel that was rejected in 2007?
The tunnel proposed to Seattle voters in 2007 was a cut-and-cover tunnel, which would have been constructed by excavating a 60-foot deep trench along the central waterfront. It would have required closure of the viaduct for almost four years, causing disruption to traffic and businesses.
The bored tunnel would be built primarily under First Avenue up to 200 feet below the surface, minimizing impacts on businesses and traffic. Traffic would be able to stay on the viaduct through most of the bored tunnel's construction. Visit tunnel comparison for more information.
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Is building a tunnel more risky than a new viaduct?
Constructing a tunnel is not necessarily any more risky - in terms of cost and schedule - than constructing a new viaduct. A new elevated structure on the waterfront would take between six-and-a-half and seven years to construct. It would also require working in difficult geotechnical conditions, maintaining access to businesses, and detouring traffic from the viaduct for three years during construction.
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Have similar tunnels been built elsewhere or is a new technology being used?
There are a number of successful tunnel projects with sizes similar to the proposed SR 99 bored tunnel, including:
- Shanghai Yangtze River Tunnel (China): Includes two bores, each about 5 miles long with a 50.6-foot diameter.
- Fourth Elbe River Tunnel (Germany): Includes a single bore with a length of about 2 miles and a 46.6-foot diameter.
- Madrid M30 (Spain): Includes bores with a length of about 5 miles and a 49.9-foot diameter.
Tunnel boring machines have been developing at a rapid rate with a major increase in diameter, better ground control, and improved reliability. These machines can now safely and efficiently excavate almost any type of soil, rock or groundwater conditions. Check out learning from other tunnel projects for more information.
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Would there be cost overruns on the tunnel?
The current cost estimate for the proposed SR 99 bored tunnel includes allowances for elements that are now at a low level of design. For example, the estimate assumes two ventilation buildings will be built – one at each end of the tunnel. The estimated cost of the buildings is based on other recently completed buildings. As the design advances, the buildings may be larger or smaller than assumed. To account for this uncertainty, plus other potential changes to scope, mitigation, right of way, issues during construction, risk (and opportunity) events, and escalation of costs, around $420 million “provisional risk cost” is included in the tunnel cost estimate.
Comparable tunnels recently completed around the world have been delivered for significantly less than the cost estimate per lane mile of the proposed SR 99 bored tunnel. Independent experts and contractors have reviewed our tunnel cost estimate and have found it reasonable for this early stage of design.
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Would the bored tunnel provide enough capacity for the future?
The tunnel along with the new waterfront boulevard that would connect to Elliott and Western avenues would accommodate the same number of vehicles as on the viaduct today. Initial estimates show approximately 85,000 vehicles could use the tunnel and 25,000 vehicles could use the waterfront roadway. This exceeds the number of vehicles that travel through downtown on the viaduct and either exit at the Elliott and Western ramps (33,000) or travel through the Battery Street Tunnel (60,000). Future growth would be mostly accommodated through investments in transit and improvements to city streets.
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How would northwest Seattle residents get to SR 99?
Residents from northwest Seattle would have two options to get to or through downtown Seattle with the bored tunnel. They could travel along Elliott Avenue, as they do today, and drive down a new street that will cross over the railroad tracks to a new four-lane Alaskan Way roadway. Alaskan Way would widen to six lanes in the south end and connect directly to SR 99 near Royal Brougham Way. If northwest Seattle residents want to use the SR 99 bored tunnel, they could take the new two-way Mercer Street to Aurora Avenue and enter the tunnel at Roy Street or Republican Street. They could also use any of the existing connections to Aurora Avenue north of Mercer Street. Connection maps are available on the central waterfront page.
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How would drivers from West Seattle get to downtown Seattle?
Residents from West Seattle would have access to downtown Seattle at new SR 99 ramps to the new Alaskan Way roadway near the sports stadiums and from a new Spokane Street Viaduct off-ramp to Fourth Avenue. Depending on their destination, some trips would get shorter while others may take a few minutes longer. Initial traffic analysis shows these new ramp locations would operate better than the current ramps because there would be more options for vehicles or buses heading into downtown. Connection maps are available on the central waterfront page.
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Would the tunnel work for freight?
The bored tunnel would maintain freight routes through Seattle and preserve I-5 for regional and state freight trips. It would also provide a route for vehicles through the city, which would otherwise use city streets or I-5.
Some freight trips destined for Ballard and the Interbay industrial area would likely use the new four-lane Alaskan Way, which would have a crossing over the railroad tracks to Elliott and Western avenues. Traffic signals on Alaskan Way would be operated to ensure through trips move efficiently along the waterfront. Connection maps are available on the central waterfront page.
Freight trips leaving Port of Seattle terminals will also have improved access to I-5 and I-90 as a result of SR 519 and Spokane Street improvements and a new grade-separated connection to bypass the railroad tracks near Terminal 46, which is included in the south end viaduct replacement. Visit related projects for more information.
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Would there be restrictions on freight using the bored tunnel?
Most freight would be able to use the bored tunnel. Vehicles hauling hazardous or combustible materials would be prohibited from the tunnel, similar to current restrictions in the Battery Street Tunnel and on the viaduct during peak hours. These vehicles would take I-5 or Alaskan Way, as they do today.
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Would the tunnel work if transit and city street investments are not funded?
New transit service is an essential part of the viaduct replacement program, because it would provide a reliable and efficient way for Seattle residents to get to and from downtown. Without the added transit service, drivers would have a more difficult time reaching the proposed tunnel on city streets that would likely be more congested. This would become more important as the city and region continue to grow. The city street improvements would provide critical access to downtown and the proposed bored tunnel.
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How would the bored tunnel and city street and transit improvements help the economy?
The proposed bored tunnel would support the economy by preserving capacity on SR 99 and I-5 for state and regional trips. Preserving capacity on SR 99 would also keep pressure off streets near the port’s container terminals. The city street improvements and transit investments that are part of the central waterfront recommendation would help accomodate future growth.
The viaduct could remain open during construction of the bored tunnel, minimizing disruptions to businesses and traffic. The SR 99 closures required during construction of a cut-and-cover tunnel or new viaduct would have required longer trips on detour routes through downtown Seattle for three to four years.
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Would the bored tunnel and city street and transit improvements help the environment?
The proposed bored tunnel would maintain capacity through Seattle, and improved streets and transit would handle future growth. Stormwater treatment would improve water quality in Elliott Bay, and traffic noise would be contained within the tunnel. New open space and public access would be created on the waterfront.
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How was agreement reached on the viaduct's central waterfront replacement?
Agreement was reached after an intensive public outreach effort. A 29-member Stakeholder Advisory Committee, which provided feedback on replacement options, met 16 times, and a majority asked that a bored tunnel replacement be considered further. Ten public meetings and more than 85 community briefings were also held. Comments from the public covered a range of topics, focusing on preserving capacity for the future and opening up Seattle’s waterfront.
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