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Seattle is home to the world’s longest floating bridges

On 02 July 1940, the Lake Washington Floating Bridge opened to the public.

You want to build a permanent floating bridge? Made of “steel-reinforced concrete pontoons, linked together and covered by a roadway,” you say? (Unstated: you’re a “screwball”!)

I can imagine such a conversation in 1921, when Homer Hadley had the idea to build a bridge to connect Seattle and south Bellevue (with the help of Mercer Island) and thus retire the ferry.

After nearly two decades of sporadic consideration, strident opposition, and bureaucratic bumbling, in 1937 the state legislature created the Washington Toll Bridge Authority, which could issue revenue bonds to finance toll bridges and tunnels. Lacey V. Murrow (1904-1966), then Washington’s highways director, embraced Hadley’s concept, and construction finally began on December 29, 1938.


Critics labeled the plan Hadley’s Folly, but he quietly persevered through years of skepticism and rejection.

Lake Washington bridge
Lake Washington Floating Bridge, 1959, Seattle Archives

On 02 July 1940, Washington Gov. Clarence D. Martin paid the first toll to cross the Mercer Island floating bridge as part of a ceremony dedicating the bridge and the Mount Baker tunnel. In-between that vision and dedication: the Great Depression.

1940 toll booth
Toll booth, Lake Washington Floating Bridge, WSDOT

About 3,000 people celebrated the opening of the largest floating structure in the world, which cost $8,854,000 (~$184 million in 2022 dollars). The bridge spanned 6,561 feet and was comprised of 25 floating pontoon sections totaling 100,000 tons.

That floating bridge began with Hadley’s vision. Later, Hadley would convince Lacey V. Murrow, the second director for the Washington state highway system, that only a floating bridge could work.

Lake Washington is 214 feet deep, and the lake bed is too soft to support towers needed for a conventional bridge. A suspension bridge would have been too expensive because the towers “would have to be approximately 630 feet in height, nearly the height of the Space Needle.”

Murrow did the heavy lifting to move the project forward while local officials remained stymied.

Mt Baker Tunnel
East side of Mount Baker tunnel, looking west, Seattle Archives, 1939.
Construction, Seattle 1939
Gasless handling of concrete in tunnel paving [horse and cart]. Seattle Archives, 1940.

Construction began on 29 December 1938, financed in part through the New Deal’s Public Works Administration. The four-lane bridge was designed to carry 2,800 cars daily; in 2019, the (much expanded) bridge averaged 66,000 eastbound vehicles daily. Today the bridge is part of I-90.

Lake Washington Bridge
Arial, Mercer Island Floating Bridge, 2008, Flickr CC.

The legislature named the Mercer Island bridge the Lacey V. Murrow Bridge in 1967.

Aside: Murrow was the elder brother of Edward R. Murrow, the famous television journalist (“Good night and good luck“) for whom the Washington State University journalism program is named.

Recent developments

520 Floating Bridge
Arial, 520 Floating Bridge, 2008, Flickr CC.

In 1963, the Evergreen Point bridge opened, connecting Seattle with communities north of Bellevue on US Highway 520. That became the world’s longest floating bridge, at 7,100 feet (4,750 meters). The legislature named it for Homer Hadley.

At 50 years of age, engineers began refurbishing the Lacey V. Murrow Bridge (Mercer Island bridge) after a matching bridge had been completed in 1989. Over Thanksgiving weekend in November 1980, the original bridge sank. That meant building another bridge for eastbound traffic. It was completed in 1993 and (finally) directly linked I-90 with I-5.

The Evergreen Point bridge had its own refurbishment and re-opened in 2016 as both the longest and widest floating bridge in the world. The Mercer Island bridges are numbers two and three in global ranking.

In 2023, the East Link light rail project will expand service to Mercer Island, Bellevue and Redmond. It will be the first light rail system crossing a floating bridge.


#scitech, #society, #science  (163/365)
📷 Seattle Public Library
Daily posts, 2022-2023

By Kathy E. Gill

Digital evangelist, speaker, writer, educator. Transplanted Southerner; teach newbies to ride motorcycles! @kegill

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