SAN FRANCISCO — Washington plans to reimpose tolls on the busy Seattle-area State Route 520 floating bridge to back almost $2 billion of bonds that will be issued to finance its replacement.

The Washington State Department of Transportation has a budget of $4.65 billion to build a replacement six-lane floating bridge, which links Interstate 405 on the eastern side of Lake Washington with Interstate 5 in Seattle.

The state has authorized $1.95 billion of bonds to pay for the 520 corridor project, according to the treasurer’s office.

Chris McGann, a spokesman for the office, said the state is still in the planning stages about how to finance the 520 project.

“We have a lot of options we are weighing right now,” he said.

At least some of the bridge bonds will likely be backed by a “triple pledge” of toll revenues, an excise tax on vehicles and special fuels, and the general obligation of the state, according to a debt affordability study the treasurer’s office released in January. Such debt is expected to carry Washington’s high rating, now double-A-plus across the board.

The tolling, which is expected to bring in $1 million a week, will be mostly used to pay off debt service on 30-year and 40-year bonds sold to fund construction of the new floating bridge, according to the DOT.

Janet Matkin, a department spokeswoman, said a firm date for tolling start needs to be set before a final bond financing plan is in place.

Originally set to start in April, the tolling has been delayed until later this summer to work out glitches.

“We are in the process of creating the final plan that will go to the ratings agencies,” Matkin said.

The DOT said it needed to run a series of tests to make sure the system set up by vendors would work properly.

The department said the delayed tolling will extend bond repayments out a few months at the end of the bond period at a minimally higher financing cost.

Tolls for cars will vary from nothing during low traffic times to $3.50 each way during peak congestion hours, with higher tolls for trucks and other larger vehicles.

Drivers will be charged through transponders attached to their car, and if they lack the electronic pass, a camera will record their license plate and they will be mailed a bill plus a $1.50 surcharge.

The state has also studied setting up tolls on I-90 to help pay for the parallel 520 bridge. While no decision has been made, the suggestion has spurred opposition from anti-tax crusader Tim Eyman, who has started an anti-toll initiative drive.

Mary Ellen Wriedt, an analyst with Standard & Poor’s in San Francisco, said it’s a credit positive when a new toll road already has a sense of demand, making it easier to project revenue and debt service coverage.

According to the DOT, 115,000 vehicles use the current bridge each day. It was designed to handle 65,000 vehicles per day.

However, she said it all comes down to how the debt is structured.

“The structure is what is key,” said Wriedt, who does not cover any credits related to the bridge.

Washington set up tolling on the original 520 bridge on the day construction finished in 1963 until its debt was paid off in 1979, generating $60 million.

Matkin said the state has so far funded 12 bridges through toll financing.

Washington has revived tolls on a bridge before to fund an expansion project. In 2007, it did the same to pay for the upgrade of the Tacoma Narrows Bridge.

So far, the DOT has identified $2.62 billion in funding for the 520 bridge: $550 million from the state, $70 million in federal funding, and $1.85 billion marked for future tolling and federal funding.

The project still needs more than $2 billion in funds to make its proposed budget.

To date, the department said it has spent $377 million on replacing the bridge, though just $59 million of that was spent on construction,.

The new bridge, which is expected to be open to traffic by July 2015, will expand to six lanes from four and will include space for pedestrians.

In addition, it will be built with the capacity to add light rail.

In November 2008, voters in the Puget Sound region approved a measure to fund light rail across Lake Washington and bus rapid transit on the 520.

The bridge, which will be 115 feet wide with the roadway 20 feet off the water, will use 77 pontoons to support four basic traffic lanes and two transit and high-occupancy vehicle lanes split in both directions.

The current 60-foot-wide floating bridge is supported by 33 pontoons and the roadway is 13 feet off the water.

Floating bridges are typically made of large, watertight concrete pontoons connected end to end.

One of the bridge’s pontoons — 28 feet tall, 75 feet wide, and 360 feet long — weighs more than 11,000 tons, equal to 23 Boeing 747 jets, according to the DOT.

The pontoons will be built on land near the lake and then floated to the bridge site and held in place by steel cables connected to anchors in the lake bed.

Washington State has more floating bridges than anywhere in the world, and the four longest and heaviest floating bridges.

They are the current 520 bridge, the two Interstate 90 bridges across Lake Washington, and the State Route 104 Hood Canal Bridge.

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