• West Approach Bridge North Project
• Floating Bridge and Landings Project
• Eastside Transit and HOV Project
• Pontoon Construction Project
• Project benefits
• Project funding

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.

West Approach Bridge North Project 

Completed December 2017

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 and the SR 520 Trail from the new floating bridge to the Montlake area of Seattle. 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.

Project timeline

• September 2014: Construction begins
• August 2017: New bridge opens to traffic
• December 2017: SR 520 Trail opens between Montlake and new floating bridge

Floating Bridge and Landings Project

Completed August 2017

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.

Project timeline

• Late 2011 - Pontoon construction begins in Tacoma
• Early 2012 - Anchor construction begins in Kenmore
• Spring 2012 - Bridge construction begins on eastern shore of Lake Washington
• Spring 2016 – Grand opening celebration and new floating bridge opens to drivers
• Early 2017 - Removal of old floating bridge complete

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.

Eastside Transit and HOV Project

Completed fall 2015

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 completed and improved the 2.3-mile high-occupancy vehicle system from Evergreen Point Road in Medina to 108th Avenue Northeast in Bellevue. The improved six-lane Eastside corridor now includes two general-purpose lanes and one transit/HOV lane in each direction. The Eastside project also constructed three community-connecting highway lids and two transit stations in the highway median, as well as the first leg of SR 520’s regional, cross-lake bicycle and pedestrian path. 

The new SR 520 freeway lid and median transit station at 92nd Avenue Northeast, on the Eastside.

Project timeline

• Spring 2011 - Construction begins
• Summer 2014 - Transit improvements open to drivers
• Early 2015 – SR 520 Trail opens on Eastside
• September 2015 - All major Eastside improvements complete

Pontoon Construction Project

Completed March 2015

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. (We built 44 additional pontoons for the new bridge in Tacoma, Washington.) Once built, each cycle of pontoons was floated out of the casting basin, inspected, and then towed to Lake Washington.

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.

Project timeline

• Early 2011 - Casting basin construction begins in Aberdeen
• Summer 2012 – First set of pontoons completed
• Spring 2015 - Pontoon construction completed

Benefits of the completed projects

Improves safety

• The old floating bridge outlived its design life and risked failure in a severe windstorm. The new floating bridge, designed to withstand a 100-year storm, is supported and stabilized by more than twice as many pontoons (77 vs. 33), with heavier anchors and stronger anchor cables.
• The new floating bridge’s roadway deck sits higher above the water than the old bridge’s roadway, so windswept waves no longer wash over traffic during storms. The raised roadway also gives WSDOT crews room to perform important maintenance and safety work from the pontoon deck beneath the roadway.

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.

• The new West Approach Bridge North, built to current seismic standards, is designed to withstand a once-in-a-1,000-year earthquake. Seismic isolation bearings beneath the bridge’s solid, concrete columns allow side-to-side bridge movement during an earthquake.
• Direct-access ramps at 108th Avenue Northeast for carpools and transit allow safer and easier highway access for buses and carpoolers. We’re also adding direct-access ramps to and from the transit hub on a new Montlake freeway lid.

Relieves congestion

• The new Eastside corridor and floating bridge added transit/HOV lanes to SR 520, which takes buses and carpools out of the general-purpose lanes and creates more vehicle capacity. When we complete the West Approach Bridge South in approximately 2023, the transit/HOV lanes will extend to Montlake Boulevard. When SR 520 is fully reconstructed by about 2029, the six-lane highway will have HOV lanes all the way to I-5.
• The rebuilt sections of SR 520 now have wide shoulders that allow disabled vehicles to pull off the highway and not block traffic.

Provides multimodal options

• The rebuilt highway extends the 14-foot-wide SR 520 bicycle and pedestrian trail across Lake Washington, providing a new cross-lake connection for both commuting and recreating. Eventually, the trail will extend from I-5 to Redmond.
• The addition of transit/HOV lanes and the cross-lake shared-use trail provide greater reliability and more travel options to accommodate growth in the region.
• The new floating bridge is designed to accommodate the addition of light rail – with a retrofit – if voters choose to fund that option in the future.

Enhances the environment

• The rebuilt highway and its fixed-column bridges capture and carry roadway runoff to on-land stormwater retention sites that naturally filter out pollutants and improve water quality in the Lake Washington watershed. The new floating bridge also captures runoff and screens out polluting sediments.
• The improved highway reduces carbon emissions by enhancing transit use and nonmotorized travel.
• The SR 520 Program supports a number of major enhancement projects in the Washington Park Arboretum, including removing the old SR 520 on- and off-ramps and never-used R.H. Thomson “Ramps to Nowhere” that ran through the park.
• SR 520 projects are enhancing parks, streams, wetlands, and other natural areas across the region to mitigate for the effects of project construction.
• The Eastside Project installed eight large, fish-friendly culverts to aid fish migration.

Project funding

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.