Their conclusion: High tolls (up to $4) would bring in more than enough to pay for the state's $200 million commitment, but would lead to unacceptable traffic diversion. Meanwhile, low or no tolls would fail to pay for the $200 million portion of the tunnel that is supposed to be funded by tolls---and even low tolls would still lead to significant diversion of cars onto downtown surface streets.
The viaduct currently carries about 110,000 vehicles a day; without any tolling, the state anticipates the tunnel would carry around 22,000 vehicles a day.
The state looked at a number of different tolling options; all, in one way or another, came up short. In a scenario where tolls were $4 southbound and $3.25 northbound, diversion rates---the percentage of drivers who would decide to take alternative routes---would be 34 percent northbound and 60 percent southbound. In the opposite scenario, where tolls maxed out at $2.25, revenues would be just $80 million, far less than the state needs to finance the tunnel---and between 22 and 37 percent of traffic would still divert to other routes. In the middle scenario, with tolls maxing out $3.25, net revenues would be between $170 million and $210---theoretically, though not necessarily, enough to pay for the promised $200 million state contribution. And diversion in that scenario would range between 32 and 51 percent.
However, members of the committee acknowledged that their models didn't factor in people who decide to take the bus, change their travel patterns, or stay home instead of driving.
Nor did they consider any scenarios in which the maximum toll dipped below $2.25 each way---a very real possibility, given state legislators' reluctance to charge substantial tolls to drive through the tunnel. "The anticipation is that there will be more scenarios and more examples run at the end of October," Seattle Department of Transportation viaduct replacement program manager Bob Chandler said. Chandler said the committee plans to look at other scenarios---including tolls higher than zero but lower than $2.25---as it considers its options in the coming months.