The new Highway 520 floating bridge will open this month on Lake Washington, just in time for crews to remove the old one, instead of Mother Nature downing it with an earthquake or windstorm.

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After nearly 31 years of arguments, planning, construction, and even some cracked pontoons, the people of Washington finally get to walk Saturday morning on their new Highway 520 floating bridge, and drive on it by mid-April.

This is the world’s longest floating bridge, at 7,710 feet, surpassing by 132 feet the old Albert D. Rosellini Bridge built in 1963.

The effort began as a traffic remedy and evolved into a safety project as the bridge reached the end of its life span. The new Lake Washington crossing arrives just in time for crews to remove the old one, instead of Mother Nature downing it with an earthquake or windstorm.

This weekend’s bridge party

More than 20,000 people are expected to party on the new Highway 520 bridge from 10 a.m. to 5 p.m. Saturday, during a closure of both the old and new bridges. The old bridge reopens for traffic at 2 p.m. Sunday.

Saturday

The state’s “Go Long” festival, to mark the world’s longest floating bridge at 7,710 feet, will include:

Virginia Mason Heart Institute 10K run and walk, 7:30 a.m. start at Husky Stadium.

• Opening ceremony, 10:30 a.m., with speeches and a Guinness World Records award. Gov. Jay Inslee will attend.

• Food trucks.

• Activity stations such as a Lego model, a “build a bridge” area to touch heavy equipment, and a Pacific Science Center interactive display about bridge technology.

• Shuttle service on the bridge deck will pick up passengers only on the east and west ends.

• Pets, skateboards and bicycles will not be allowed.

• There is minimal drive-up access. Free shuttles will run from the University of Washington light-rail station, Bellevue Transit Center and the South Kirkland and Houghton park-and-rides.

Sunday

The Emerald City Bike Ride (sold out with 7,000 riders), a Cascade Bicycle Club fundraiser, will cross the new bridge, the I-5 express lanes and arterials, between 7 and 11 a.m. There is no other pedestrian or bike access to the bridge that day.

Source: Washington State Department of Transportation

“This is a phenomenal safety increase,” said House Transportation Committee Chairwoman Judy Clibborn, D-Mercer Island.

Tougher components should make it resilient against gusts and waves. The road deck is high above the lake, so waves won’t slap cars on the highway.

Six lanes instead of the old four should improve commutes by allowing transit and carpools to zip past traffic slowdowns. And illuminated towers are intended to make the bridge not just a landmark, but a ceremonial entrance.

It’s a massive structure, including 23 jumbo pontoons, each as long as a football field. Yet it’s intricate enough that hundreds of cells within the pontoons are monitored for watertightness, by computers in Medina and Shoreline.

Floating bridges are a novelty in all but a handful of countries. Washington state operates the four longest and heaviest, including two at Interstate 90 and the Hood Canal Bridge. Others exist at Canada’s Lake Okanagan, in Guyana and in Norway, which has two.

Floating pontoons, a spinoff from military technology, are needed on Lake Washington because its 200-foot depth and silty bottom make it nearly impossible to build solid columns underwater.

Years of arduous, repetitive public process preceded this spring’s milestone, adding features at every step to protect neighborhoods or serve traffic, leading to a $4.6 billion series of projects on Highway 520, from Interstate 405 to Interstate 5.

Elected officials first worried about the bridge becoming a bottleneck in 1985, just before Microsoft issued its first stock. A Translake planning committee formed in 1997, and so-called stakeholders took 10 years to compromise on a six-lane roadbed, rather than four lanes sought by environmentalists, or eight lanes by Eastside automobile supporters.

The time took its toll on the old bridge. Pontoons have sunk 1 foot, and sprouted more than 30,000 feet of cracks that maintenance teams have repaired. A barge rammed and crumpled a hollow column. Storms closed the bridge many times, including a few weeks ago, when the drawspan gear was briefly damaged.

The state missed former Gov. Chris Gregoire’s goal of a completed floating section by late 2014, and taxpayers will absorb $342 million in cost overruns. Cracks formed in the first batch of pontoons because of state design errors, requiring a year of fixes. Soil problems on the Eastside raised the cost of noise and retaining walls.

Such problems aren’t unique to this era. Two pontoons being built for the Hood Canal Bridge sank off Harbor Island in 1958, and several others cracked. A mudslide on north Capitol Hill delayed a portion of the initial Highway 520, said Gerald Purdum of Camano Island, a retired engineer who worked on that project.

In a car-free preview, people are invited to walk the deck of the new bridge Saturday, which the Washington State Department of Transportation (WSDOT) calls a once-in-a-lifetime opportunity. A similar party on the 2007 Tacoma Narrows suspension bridge attracted 60,000 people, while 3,200 people said goodbye to the partly torn-down Alaskan Way Viaduct in 2011.

Traffic will be phased in. At 5 a.m. Monday, April 11, westbound drivers can begin to use the new floating bridge, following a weekend closure. Two weeks later, at 5 a.m. Monday, April 25, eastbound drivers can use the new bridge, also following a weekend closure.

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Here is a look at features of the new six-lane bridge:

• High-occupancy lanes. The left lane each way will be marked for transit and carpools of at least three people. Someday, a bus stop will be built on a future Montlake lid.

• Walk-bike lane. The illuminated trail is 14 feet wide, or 4 feet wider than the Interstate 90 trail. The outer steel railing is convex, curving out toward Kenmore, which reduces the risk of snagging bike handlebars. But bicycles can’t cross the entire lake until 2017, when the connection to Montlake is finished. Trails will cross above the highway to the Arboretum, and, someday, under the highway to continue into Seattle on the south flank of the future concrete-girder bridge, over Portage Bay.

• Belvederes. The trail includes protruding rest areas, benches and interpretive displays about Native American history, bridge construction and geology. A plaque at the east high-rise honors Joe Arrants, a 34-year-old carpentry foreman from Burien who died in a fall when his cable broke in March 2015.

• Sentinels. Spires on the east high-rise are topped by white light spheres. Other lights glow within the towers, where bulb colors can change for special occasions, such as another Seahawks title or Pride weekend.

• Noise reduction. Concrete walls at the east high-rise will reduce the roar for Medina homeowners. Concrete pavement is grooved, to reduce tire noise to drivers and boaters. On the bridge’s underside, boxlike panels should contain clanging and thumping noises from expansion joints, which have occurred sometimes at the Narrows and Aurora bridges.

• Leak detection. Every cell within the pontoons contains a lightweight sensor that floats like the ball inside a toilet tank. If 3 inches of water seep in, alarms will go off inside the maintenance shop.

• Navigation. This bridge has no drawspan. Boaters will find 70 feet of clearance at the new east high-rise, 44 feet at the west high-rise.

• Stormworthiness. The road deck is 20 feet above lake level, instead of the old 13 feet, and it stands on columns over the primary pontoons. Waves will wash under traffic instead of hitting vehicles. Smaller, supplemental pontoons, which extend to the sides, will dissipate wind and water force. It’s rated to withstand 89-mph winds.

• Drainpipes. Oil, brake linings and rubber-tire specks have trickled into the lake for 53 years. The new bridge ought to capture most pollutants.

Wastewater will flow in pipes from the high-rises toward the center. Mesh screens within the drainpipes will capture solids.

Pipes empty into lakewater ponds built within certain eight-cell pontoons, like the middle square of a tic-tac-toe game. Solids will disperse deep below the surface, because the pontoons sit 20 feet deep. Oil sheens will float on top, to be skimmed away. Cormorants have divebombed the new pools to snatch fish but couldn’t fly out of the tight space. At least one died and a half-dozen were netted and rescued. Contractors added yellow plastic ramps, where a bird can walk to deck level and attempt a low-angle takeoff.

Debt and traffic

Beyond the celebration, a few sobering problems remain.

First, the state coped with tremendous costs by taking out $1.9 billion in loans and bond debt, to pay for three-fourths of the $2.7 billion in work already finished or under way.

WSDOT made contractual commitments to repay, even if that means less money for future projects and maintenance.

Lawmakers relieved that pressure somewhat, and pulled the 520 project out of limbo, when they approved 11.9 cents per gallon in gas-tax increases last year, and earmarked $1.3 billion to finance the stretch from Montlake to I-5.

Second, traffic projections. The old bridge carried 103,000 vehicles per weekday. That shrank to 70,000 when tolls were imposed four years ago, causing people to detour, cancel trips or switch to buses.

A state-commissioned study in 2007 projected 130,000 vehicles a day by 2030, though even WSDOT has recently flattened its statewide predictions for car use.

That many cars crossing the lake would screech to a halt at an increasingly jammed I-5. Traffic growth also stymies the effort to reduce global warming, as well as the region’s investments in greater transit ridership. Electric and self-driving cars may offset pollution and congestion.

Clibborn thinks rapid growth will create more traffic and more transit demand. Trips on the six-lane bridge could surge as soon as next year, she says — when light-rail construction on nearby I-90 reduces its car capacity, making the 520 toll bridge more attractive.