The world’s longest floating bridge, connecting Seattle with the neighboring city of Bellevue across Lake Washington, opened to westbound traffic yesterday, reported the Seattle Times. The 7,710-foot-long span replaces the old Albert D. Rosellini Bridge, built in 1963 and long understood to be seismically unsound. Because of the lake’s depth and silty bottom, solid supporting columns are nearly impossible, requiring bridges across it be supported by pontoons.

The new six-lane bridge includes features intended to improve safety, decrease pollution and encourage multimodal transit in a region where only 4.2 percent of commuters use public transportation, and more than 50 percent told pollsters last year that nothing could stop them from driving alone.

An HOV lane headed in each direction is marked for mass transit buses and carpools of at least three people. An illuminated 14-foot-wide trail will make way for cyclists and pedestrians when it opens in 2017. The new road deck raises traffic 20 feet above lake level, as opposed to the former 13 feet. At that height, waves often slammed onto the bridge — and into cars — during intense storms. Each of the 26 football-field-size pontoons are equipped with internal sensors that will alert maintenance crews if a leak is detected. New drainpipes have also been installed to capture oil, brake linings and pulverized tires, which have been dribbling into the lake for the last 53 years.

Concerns still remain about the project — which hit a few roadblocks along the way — including the $1.9 billion in loans and bond debt the state took out to pay for three-fourths of the $2.7 billion in work already completed or underway. WSDOT is contractually obligated to repay those costs, even if it means less funding for future projects and maintenance. And maintenance will be key with high use projected. The old bridge carried 103,000 vehicles per weekday before the imposition of tolls shrank that number to 70,000. A 2007 study projected that 130,000 vehicles per day will travel the new bridge. That number could be even higher as traffic capacity on the floating I-90 bridge, just south, is reduced next year to allow the construction of the world’s first light-rail line over a floating bridge.

Pedestrians and runners got a sneak preview of the new 520 bridge last weekend. Eastbound lanes will open in two weeks.