What had looked like a solution for the problem of setting toll prices on the tunnel will not raise enough money to cover existing debts, a new analysis shows. And raising tolling prices will only result in more cars diverting to already crowded alternative routes.

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In 2014, after two years of work and a number of false starts, the committee working to set tolling prices on the new Highway 99 tunnel in Seattle hit on a solution: $1.25 per trip during rush hours and $1 at all other times, day or night.

This solution, the committee calculated, would raise enough money to pay for pledged construction costs, with plenty left over for tunnel maintenance and even some for more transit service on the 99 corridor.

Three years later, drilling is done, but, upon further analysis, that solution doesn’t raise as much money as anticipated and doesn’t meet the bare minimum required by the Legislature, according to the Washington State Department of Transportation (WSDOT).

The initial results of an investment-grade analysis of tolling options, performed by a consultant for WSDOT, show that setting a price point for the new tunnel continues to be as tricky a proposition as it was for other highways in the region, where new tolls spurred howls of protests from angry drivers and led to worsened traffic on competing routes.

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Toll rates must find the sweet spot between two competing ideals. They must be high enough to raise the $200 million pledged to construction costs of the $3.2 billion tunnel and viaduct replacement. But they must be low enough that drivers will pay them, rather than veer off onto Interstate 5 or downtown surface streets, both of which are already at or near capacity.

The new consultant’s report analyzed two possible tolling options — the one favored by the prior advisory committee and one that’s more expensive and more complicated. Only one raises enough money, but both send significant numbers of cars onto alternate routes.

Back in 2014, the advisory committee had estimated that its recommendation would raise $1 billion over 30 years. The new analysis shows it raising about $900 million.

The second tolling scenario is more complex, envisioning four different prices depending on time of day.

Weekend and overnight tunnel trips would cost $1, daytime and evening off-peak hours would cost $1.50, morning rush-hour trips would cost $1.75 and evening rush-hour trips would cost $2.50.

This more expensive scenario would raise more than $1 billion, according to WSDOT, enough to cover the tunnel-construction debt.

(A third pricing scenario modeled by the consultant, with tolls as high as $6.80, is not under consideration and was only run as a baseline to determine maximum possible revenue, WSDOT officials said.)

Both scenarios considered would divert thousands of cars onto other routes, most notably Alaskan Way, downtown surface streets and I-5. The scenarios envision the number of cars on Alaskan Way tripling compared with current conditions.

The more expensive tolling scenario, when compared to the current viaduct, would cause nearly 25,000 additional cars per day to choose I-5 or downtown surface streets for their north-south trips.

Lars Erickson, a WSDOT spokesman, called that diversion rate “slightly higher” than what they’ve aimed for but said they did anticipate diversion to Alaskan Way, and more detailed analysis will be required to know where cars are diverted to and at what hours of the day.

The Highway 520 floating bridge saw a similar phenomenon when it first got tolls five years ago. The first months after tolling saw massive diversion, with about 40 percent of drivers using an alternate route. That led to speedy commutes for those who were willing to pay the toll and remained on 520 but also to soaring traffic on I-5 and Interstate 90. A year later, some 520 drivers returned, though there were far fewer crossings than before the tolls.

But in 2012, I-90, the alternative to 520, had capacity to absorb that added traffic. I-5 does not have that capacity.

WSDOT officials stressed that the new consultant’s report is part of an ongoing process, and they will continue to analyze alternate tolling prices.

Ultimately the decision on how to toll the tunnel lies not with WSDOT, but with the Washington State Transportation Commission, a seven-member group appointed by the governor.

That decision, following further study and public input, will occur sometime before the tunnel’s scheduled opening by 2019.

“We’re working on this more detailed analysis that’s going to lead into the commission doing their rate-setting process,” Assistant Transportation Secretary Patty Rubstello said. “This is just some information for them.”