Puget Sound Transportation Projects
SR 520 Floating Bridge
The SR 520 Bridge Replacement project will replace the SR 520 floating bridge, plus expand and rebuild 520 between I-5 and I-405. The current bridge built in 1963, was never designed for the amount of traffic it now handles and is reaching the end of its life. With a large storm or a major earthquake the bridge could collapse, break apart and sink.
This is the world’s longest floating bridge at 7,500 feet. It is two lanes in each direction. The middle of the bridge opens up to allow boat traffic, mainly shipping to and from the Boeing plant in Renton. This section is also opened in sustained winds over 50 miles per hour to reduce pressure on the bridge.
Why a floating bridge? Lake Washington in this area is 100-200 feet deep. Even below that there is about 100 feet of muck until you hit bedrock. Normal bridge supports would be prohibitively expensive, and were past or pushing the limits of available technology at the time. A tunnel would also be too deep to be useful.
With the growth of Microsoft and other tech companies on the Eastside, the bridge has become an important commuter route. It actually has a reverse commute than in other cities: out of Seattle in the mornings, into Seattle in the evenings. The bridge now handles 115,000 vehicles a day, but was designed for 65,000.
Current plans are to replace the bridge with a 6 lane version, supporting a carpool lane in each direction. The new pontoons, however, will be able to support more lanes and/or high capacity transit in the future. On the East side of the bridge, 520 will also be widened to support 6 lanes, with some transit-only ramps integrated as well.
Work is now underway to cast the new pontoons. They will be stored for the project, or to be used in case the current bridge is damaged or destroyed. Work is also beginning on the stretch from I-405 to the bridge. Work has been completed on SR 520 from I-405 into Redmond.
The cost of the project will be covered by tolls. An innovative tolling system using transponders and cameras means there will be no stopping at toll booths on the bridge. This has only been used in rare cases, such as with the Golden Ears bridge in Canada. The tolls, however, are controversial, especially since they are being implemented before the bridge itself is completed.
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Not having grown up in Seattle I never used the bridge until I started a contract with Microsoft. Living with a relative in Seattle I used the bridge to get to work daily. I eventually moved to the Eastside, however, because of the enormous traffic on the bridge. Unfortunately this also increased my housing costs and limited my activities.
Seeing the large amount of people who work at employers like Microsoft and live in Seattle this bridge is a critical transportation link. Losing this link would result in almost constant gridlock for the area. I am pleased to see that this project is finally getting underway.
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Photo: © Peter Svensk via Flickr