Westbound lanes on the new Highway 520 bridge are tentatively expected to open April 11 and eastbound lanes are tentatively scheduled to open April 25, state officials announced Tuesday.

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The new Highway 520 bridge will open to traffic this spring, with westbound lanes tentatively expected to open April 11 and eastbound lanes tentatively scheduled to open April 25, state officials announced Tuesday.

At 7,710 feet, it will be the longest floating bridge in the world, surpassing by 132 feet the old Highway 520 floating bridge built in 1963.

The gap between opening the westbound and eastbound lanes is because of weekend work that will move barriers and finish other tie-ins between the floating lanes into the old 1963 bridge, which will continue to carry traffic from the Foster Island area to Seattle.

The new crossing provides two general-traffic lanes and a carpool-bus lane in each direction, subject to the same tolls already being charged on the old bridge. There is no carpool-bus lane on the current bridge.

Once the floating bridge is opened to traffic, cars will continue onto the old 1963 bridge just east of Foster Island, until more of the fixed Seattle bridge segment is finished.

That transition point, which narrows westbound from three lanes to two lanes, is likely to cause tricky merges and traffic bottlenecks.

The floating section is part of a nearly $4.6 billion project to replace the old four-lane highway with a six-lane corridor, from I-405 to I-5. Funding has been difficult, but state lawmakers voted last year to fund the final $1.3 billion, to reach the Seattle shore, using part of a new 12-cent gas tax.

The floating section was to open in late 2014, but was delayed, primarily to repair cracks, blamed on design errors by the state, that formed in the four large pontoons as they were built in Grays Harbor.

The bridge has 77 pontoons anchored by 58 cables.

Cost overruns have soared to almost $400 million, including increases for the section where the highway goes on land through the Eastside.

Before traffic gets moving, a party, with a fun run and walk, will be held on the new bridge Saturday, April 2. The next day, there will be a public bicycle ride on the bridge deck from the University of Washington to the Eastside, back into downtown Seattle, and up to the UW campus, organized by Cascade Bicycle Club.

The bridge includes a 14-foot-wide walk-bike trail, along with small viewpoints that extend several feet north, over the lake.

For the public parties, it’s worth keeping in mind that the floating bridge can withstand the weight of more than 60,000 people without sinking, said Suanne Pelley, a spokeswoman with the Washington State Department of Transportation. Crowds of that size walked across the second Tacoma Narrows suspension bridge during its grand opening party July 15, 2007, causing the crown to sag 1 foot, about the same as during thick afternoon traffic. More famously, the Golden Gate Bridge flattened in 1987 under the weight of 300,000 people, forcing an emergency decision to close off access.

Work continues on the westbound lanes near the Seattle shore, from Foster Island to Montlake Boulevard. Last week, a new exit ramp opened for drivers entering Seattle to turn south toward the Washington Park Arboretum and the Central Area.