Washington state proposes to combine I-405 and Highway 167 into a vast, 40-mile network of both free lanes and toll lanes, and use future toll income on projects up and down the suburban crescent.

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State leaders are thinking beyond the Interstate 405 bottleneck at Bothell, and they might add Kent and Puyallup on Highway 167 to the list of places where a torrent of toll dollars could fund additional road lanes.

A new proposal would combine the two freeways into a vast, 40-mile network of both free lanes and toll lanes around the suburban crescent. The future toll income might be pooled to support $710 million in bonds for projects, beyond the planned Renton-Bellevue I-405 widenings already funded through gas taxes.

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By borrowing money this way, the state could fund Bothell-area expansions by the mid-2020s — three to six years sooner than a pay-as-you-go method.

That’s a drastic change in state policy, which was to build toll lanes as a way to manage traffic flow and give drivers a quicker travel option if they were willing to pay. Instead, toll income would become a foundation to expand highways.

Such rosy assumptions are possible because in their first 27 months, the I-405 toll lanes from Lynnwood to Bellevue have collected $34 million beyond operating cost — or triple what the state originally planned.

Thousands of drivers north of Bellevue routinely choose toll lanes at an average $2.40 per trip, and even when variable prices spike to $9 or $10. By law, surpluses must be reinvested in I-405.

For the time being, the state assumes south-end drivers will be equally willing to buy their way into the toll lanes and that trips will increase, so net income quadruples to $68 million in 2026 after the network is done, says a preliminary, 12-page forecast.

Lawmakers are just starting to hear about this idea, and they would make no decisions until at least the 2019 legislative session.

Of the toll-funded projects, the most ambitious would be a second I-405 Sammamish River Bridge at Bothell, to carry northbound cars by perhaps 2024. Southbound lanes could then occupy the entire 1970 bridge.

That would allow the state to add a second express toll lane in each direction between Highway 527 and Highway 522. The existing bottleneck is especially bad southbound where the single toll lane moves as slowly as 23 mph on average around 7:35 a.m.

But the draft plan also would add southern projects, such as a general lane through Kent on Highway 167. Express toll lanes would be added between Puyallup and Auburn, where solo drivers could pay for a faster trip among carpools and buses.

These are in addition to $1.2 billion worth of toll lanes to be built between Renton and Bellevue, already funded by a statewide 11.9-cent gas-tax increase the Legislature approved in 2015. That stretch of the highway will wind up having two general lanes and two express toll lanes each way by 2024, plus short exit-only lanes for general traffic southbound.

Doug Vaughn, chief financial officer for the Washington State Department of Transportation (WSDOT), unveiled the toll-funding concept Monday in Kirkland to an advisory group of elected officials.

Already, the state is forming the 40-mile corridor with a new ramp at Renton, funded by gas taxes, that connects the bus-carpool lanes of Highway 167 with I-405.

To sell bonds using combined toll income, state law must be changed to designate the two highways as a single corridor. An investment-grade financial study is also needed, Vaughn said.

The push to integrate the two highways started with Rep. Judy Clibborn, D-Mercer Island, who is House Transportation Committee chairwoman and will retire in December.

Politically, this integration should improve the Legislature’s potential to form coalitions between the Eastside and South King County. A shared program over 40 miles and 25 years should avert rivalries by spreading the payment burdens, so neither area overly subsidizes the other.

Sen. Guy Palumbo, D-Maltby, said he has opposed toll lanes but believes they will continue, so a combined project list from Lynnwood to Puyallup deserves a closer look.

Palumbo hopes combined tolls will fund a direct bus-carpool ramp from Highway 527 to I-405, currently unfunded.

“We probably can’t wait another 10 years for the next gas-tax package,” he said.

Strictly speaking, the bonds wouldn’t be toll-backed because national experience shows tolls can be a financial gamble and boost borrowing costs.

Instead, WSDOT would sell top-grade “triple-backed” bonds that pay investors using gas taxes, and that are ultimately backed by the full credit of Washington state, with toll money compensating the fund.

Vic Bishop, a Bellevue traffic engineer representing the pro-roads Eastside Transportation Association, warned elected officials that going into debt would force WSDOT to create perpetual traffic congestion to assure drivers pay top dollar to use quicker toll lanes.

Sen. Phil Fortunato, R-Auburn, said the state should try a study to “set everybody free” by making toll and carpool lanes open to all.

Kim Henry, the state’s project director for both highways, said toll lanes often carry 35 percent more cars per hour than general lanes. That’s because rising prices prevent toll lanes from clogging.

Fortunato remains unpersuaded. “There are (toll) lanes with very few people in them. Most people I know use the general lanes,” he said.

He went on to say, “Tolls are the new opioid for transportation” because state toll experiments on I-405 and Highway 167 have triggered cravings for more and longer toll corridors.

WSDOT calls I-405 toll lanes a success since their September 2015 startup. They carry 57,000 vehicles per day. Drivers who go the whole 15 miles from Lynnwood to Bellevue can save an average 12 minutes at peak times compared to general lanes, a report said.

But the system missed its goal to move express toll traffic at the ideal speed of 45 mph during 90 percent of commute hours. To achieve that goal will require spending the toll surpluses.