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According to a new study by the state department of transportation (WSDOT), charging a toll of between $3.50 and $4 at rush hour would cut traffic on the proposed waterfront tunnel by as much as 39 percent below traffic levels that are anticipated without a toll. That finding could raise questions about whether a waterfront freeway is needed at all.

Because higher tolls provide an incentive for people to choose alternatives to driving (or combine trips, or avoid driving altogether), the higher the toll, the fewer cars there are on the road. Significantly, the study also found that tolling a new tunnel would not lead most drivers to divert to other routes, one of the more common arguments against tolling: "If a toll is charged to use the tunnel, traffic model analysis shows that some traffic would divert from the tunnel to local streets and Interstate 5, but travel times would stay the same or increase slightly," the study found.

"[T]olling would result in little or no change to travel times for trips to and through downtown Seattle. Due to the little or no change to travel times, WSDOT is not recommending mitigation for diversion from the tunnel."

Higher tolls also lead to lower revenues, but even the highest toll WSDOT is considering would fully fund the tolling portion of viaduct replacement.

The study, which looked at five different tolling scenarios, found that three of those scenarios would produce more than the $400 million the state legislature directed the department to come up with through tolling. Specifically, the study found that average daily tolls of $2.24 would produce revenue of up to $460 million; average tolls of $2.90 would produce revenue of up to $406 million; and average tolls of $2.58 would produce revenue of up to $439 million.

Of course, the fact that tolls reduce vehicle traffic so substantially raises an obvious question: If people don't actually need a new waterfront freeway (that is, if a modest toll makes people decide not to drive or to use other routes), why should we spend $4.2 billion building one?

I have calls out to tunnel opponent Cary Moon of the People's Waterfront Coalition and Mayor Mike McGinn, another tunnel opponent, to find out their answers to that question.