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To date, the SR 520 Bridge Replacement and HOV Program's completed projects are the West Approach Bridge North Project, the Floating Bridge and Landings Project, the Eastside Transit and HOV Project, and the Pontoon Construction Project.
The West Approach Bridge North is the most recently completed segment of the SR 520 Bridge Replacement and HOV Program. Opened to traffic in summer 2017, the new approach bridge extended SR 520’s three westbound lanes (including a transit/HOV lane) and the SR 520 Trail from the new floating bridge to the Montlake area of Seattle. The new bridge, built to modern earthquake standards, also added a full shoulder to this stretch of the roadway. We are building a parallel West Approach Bridge South, for eastbound traffic, as part of the current SR 520 Montlake Project.
Note: To enable the removal of the old, structurally vulnerable west approach bridge and make room for constructing the West Approach Bridge South, we've temporarily shifted all traffic over Union Bay onto the north bridge, with two lanes in both directions.
The new West Approach Bridge North passes over Union Bay and Foster Island before reaching
Seattle’s Montlake neighborhood.
After more than a half century of use, the old SR 520 floating bridge was showing its age. The bridge's pontoons were vulnerable to windstorms and waves, and its hollow support columns were vulnerable to earthquakes. Additionally, the aging structure had only two lanes in each direction, no shoulders and no bus/carpool lanes.
The SR 520 Floating Bridge and Landings project replaced the 53-year-old span as the “world’s longest floating bridge.” With more and bigger pontoons and heavier anchors, the new six-lane bridge across Lake Washington is built to withstand stronger winds and waves. The bridge opened to traffic in April 2016. To learn about how the new bridge was constructed, check out our SR 520 floating bridge booklet (pdf 6.8 mb).
The new SR floating bridge, the longest floating bridge in the world, connects Seattle with King County's Eastside communities.
Noise from new bridge
After the new SR 520 floating bridge opened to traffic in April 2016, some lakeside residents complained about the noise from vehicles crossing the large expansion joints on the highway's east and west high-rises. In response, WSDOT committed to working with the city of Medina and the expansion joint manufacturer to explore options for reducing the noise.
Studies in 2017 and 2018 by WSDOT and a University of Washington engineering team found that noise levels from the new bridge's joints were lower than on the old SR 520 floating bridge, the I-90 floating bridge and the Tacoma Narrows Bridge. Their conclusion was that quieter, specially grooved pavement on the new bridge made the noise of cars running over the new joints more pronounced.
In 2020, the Legislature included $556,000 in the state transportation budget for WSDOT and the UW team to test potential options, and costs, for reducing the floating bridge’s expansion-joint noise. A report is due back to the Legislature in December 2021.
SR 520 is one of only two urban highways across Lake Washington that connects Seattle and the Eastside’s steadily growing communities, including Bellevue, Kirkland and Redmond. Population and employment have grown dramatically in these communities in recent decades, leading to increased traffic and transit demand. This increase in congestion – and a roadway designed for the 1960s – led to frustrating commutes and unreliable transit travel times.
The Eastside Transit and HOV Project improved the 2.3-mile stretch of SR 520 between Evergreen Point Road in Medina and 108th Avenue Northeast in Bellevue. The reconstructed Eastside corridor includes the following upgrades:
The new SR 520 freeway lid and median transit station at 92nd Avenue Northeast, on the Eastside.
The Pontoon Construction Project broke ground in February 2011 at a 55-acre site in Aberdeen, Washington. WSDOT and contractor Kiewit-General built a 4-acre casting-basin facility to stage construction of 33 pontoons for the new SR 520 floating bridge.
Pontoons are the foundation of a floating bridge. The largest pontoons built in Aberdeen were the 21 longitudinal pontoons, later joined end-to-end on Lake Washington to form the backbone of the new floating bridge. Each of these pontoons – the largest ever built in Washington – is 360 feet long, 75 feet wide, nearly 30 feet tall, and weighs 11,000 tons.
We constructed the Aberdeen-built pontoons in six cycles. Once built, each cycle of pontoons was floated out of the casting basin, inspected, and then towed to Lake Washington. We built 44 additional pontoons for the new bridge in Tacoma, Washington. These were the smaller, "supplemental stability" pontoons that flank both sides of the larger, longitudinal pontoons. At the peak of construction, more than 650 workers – steelworkers, carpenters, concrete specialists, welders, crane operators, and others – were on the job building pontoons between the two locations.
Learn more about the pontoons by reading our online SR 520 floating bridge booklet (pdf 6.8mb)
Tugboats guide a longitudinal pontoon through the Lake Washington Ship Canal and past the
Fremont Bridge en route to Lake Washington for assembly of the new SR 520 floating bridge.
Improves safety
A damaged, hollow column supporting the old SR 520 west approach bridge across Union Bay,
prior to its repair. WSDOT is replacing the structure with a new, solid-column bridge.
Relieves congestion
Provides multimodal options
Enhances the environment
The four completed SR 520 projects were funded by a variety of state and federal sources, including SR 520 tolling that began in December 2011. Final project costs, including preliminary engineering and right of way costs, are as follows:
West Approach Bridge North Project: |
$264 million |
Floating Bridge and Landings Project: |
$849 million |
Eastside Transit and HOV Project: |
$455 million |
Pontoon Construction Project: |
$509 million |
Visit the SR 520 Budget and Performance page for additional information about SR 520 program funding.
West Approach Bridge North
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Floating Bridge and Landings
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Eastside Transit & HOV Project
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Pontoon Construction Project
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