World's Longest Floating Bridge Opens in Washington

Part modern marvel and part practical mobility solution, the new State Route 520 floating bridge enables multiple modes of land-based transportation to simultaneously cross the state’s second-largest natural lake. On April 2 and 3, HDR helped the Washington State Department of Transportation unveil the new bridge, giving the community a firsthand look at this innovative project and the technology that made it all possible.

"This incredible feat was made possible by our strong partnership with WSDOT," HDR Project Manager Larry Kyle said. "The technical innovation is extraordinary but equally impressive is the community-building power of the bridge, and the way communities came together to create something useful and uniquely aesthetic. It’s been a privilege to be a part of this project and I’m excited about what lies ahead. The grand opening was the culmination of a remarkable collaborative effort."

The bridge's grand opening attractions included interactive exhibits, a 10K fun run, in which about 13,000 runners participated; and a 20-mile bicycle ride, sponsored by the Cascade Bicycle Club, in which about 6,000 cyclists participated. There were expert Lego-builder demonstrations, local food trucks and much more. Fifty organizations donated time, staffing and funds to make the weekend fun, and educational, for the entire family. About 30,000 people were estimated in attendance, not including the runners and cyclists.

With depths as great as 214 feet, Lake Washington can not be traversed using traditional bridge design, creating the need for innovative design. In 2006, WSDOT selected HDR to serve as general engineering consultant (GEC) on this multi-phase project because of HDR’s comprehensive infrastructure experience and reputation for innovative solutions. HDR and WSDOT staff co-located to a central project office, and from this location, led efforts related to all aspects of design and construction collaborating with120 sub-consultants.

The project included replacing the existing, 52-year-old floating bridge and re-constructing the rest of the SR 520 corridor, from Interstate 5 on the west side of the lake to Interstate 405 on the east side. Requirements for the new bridge included greater capacity and more multimodal options.

The result is a bridge that, at 7,708 feet (about 1.5 miles), has earned a place in the Guinness World Records as the longest floating bridge in the world. It is 130 feet longer, has a bridge deck 56 feet wider and uses 44 more pontoons than its predecessor. 

The new bridge accommodates a 14-foot-wide, shared-use pathway for cyclists and pedestrians. It provides six travel lanes for vehicular traffic, including a dedicated transit and high-occupancy vehicle lane in each direction. The pathway features five belvederes, or overlooks, that give cyclists and pedestrians a place to rest and enjoy the scenic north side of glacier-carved Lake Washington.

The GEC team led by HDR comprises a wide range of consulting firms, including numerous small businesses, among them, businesses certified as Disadvantaged Business Enterprises, Minority Business Enterprises and Women Business Enterprises. For additional information and to check out all of the grand-opening events, visit the Washington State Department of Transportation’s website or

About HDR’s bridge experience

We have a passion for all types of bridges, and offer every bridge-related service you can think of and a few you might not. We work on some of the largest and most complex bridge programs in the United States, including leading the Hoover Dam Bypass design team, winner of the prestigious 2012 ACEC Grand Conceptor Award; serving as lead designer for both the New NY Bridge replacing the Tappan Zee and the Pennsylvania Rapid Bridge Replacement Program public-private partnership; and co-managing the Oregon Transportation Investment Act III Bridge Delivery Program.

About HDR

HDR has partnered with clients to shape communities and push the boundaries of what's possible since 1917. We specialize in engineering, architecture, environmental and construction services. With nearly 10,000 employees in more than 225 locations around the world, we think global and act local.