Up to 9,100 drivers could divert onto downtown Seattle streets to avoid a Highway 99 tunnel toll at afternoon peak times, a traffic model says.

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A new study confirms the public’s worst anxiety about future Highway 99 tunnel tolls — that thousands of cars will avoid the tunnel and cram into downtown streets, if toll rates are set high enough to support the project’s budget.

For instance, with tolls of $3.50 southbound and $2.50 northbound in 2017, an extra 9,100 cars would spill into downtown Seattle during the afternoon commute, or 42 percent of cars that would use an untolled tunnel.

Such predictions might seem a good reason to charge lower tolls. But the state is depending on $200 million in revenue to help support the $3.2 billion replacement for the Alaskan Way Viaduct.

The Highway 99 tunnel traffic numbers raise a question about whether the state must siphon money from other projects again, or hope for a future tax hike, if the tunnel is to accomplish a fundamental purpose of preserving traffic mobility.

But project administrator Linea Laird emphasized in a briefing that no other money is available to the state Department of Transportation at this point.

Tough questions remain. How high can tolls be without causing downtown gridlock, or safety hazards? How low can tolls be without breaking the budget?

“This is not easy,” Laird acknowledges.

Unlike past estimates about diverted traffic, these latest predictions were prepared for a government advisory committee that will help set toll rates. But they are still preliminary. Actual toll proposals aren’t due until year end. The advisory group isn’t scheduled to discuss revenues until July 11, said tunnel spokeswoman KaDeena Yerkan. Maud Daudon, co-chair of the toll advisory committee, said the group is early in its work, and traffic models remain at “a very gross level” of accuracy.

Daudon said she doesn’t know yet if a toll plan can be found to meet the goals. “There are many ways to skin a cat, in terms of moving things around to make things work,” she said, for instance by tweaking specific toll rates at certain times of day.

“The whole thing is a balancing act between revenue generation and diversion,” she said.

The finished four-lane tunnel is to open for traffic, and tolls, at the end of 2015.

Other traffic factors

Any significant traffic diversion, on the scale of what’s happened with new Highway 520 bridge tolls at similar prices, would increase the pressure on an already strained transportation network.

In the coming years, Seattle will welcome its second Amazon office campus. Streets will absorb some transit buses that now use the viaduct, and buses that will be displaced from the downtown transit tunnel as light-rail trains increase. The city is also looking to add streetcars, bike lanes, retailers and perhaps a third Sodo sports venue.

Tunnel tolls are expected to vary by time of day, similar to the 520 bridge, so drivers pay more at peak commute times and less at other hours.

Tolls originally were supposed to fund a potential $400 million in construction-bond sales toward the tunnel, in the state’s tunnel legislation of 2009.

But this winter, DOT and the Legislature slashed the target to $200 million based on advice from the state treasurer, and conservative traffic estimates. And to satisfy cautious bond markets, DOT no longer assumes tolls can rise every year with inflation, Laird said.

The state will backfill the $200 million Highway 99 shortfall by earmarking some yearly federal grants for the viaduct replacement — cash that otherwise would pay for bridge improvements around the state.

Toll scenarios

Here are some highlights from the new traffic models:

• Southbound drivers in the afternoon are more likely to avoid a toll tunnel than northbound drivers, because there are more southbound street lanes to choose from, said DOT traffic engineer Mark Bandy, so the solution might be to set tolls that differ by direction.

• At the example $3.50 peak toll described above, the diversion is so heavy that Interstate 5 reaches its saturation point. In other words, I-5 clogs as if someone tried to dump an entire bag of rice into a narrow funnel.

• A lower-priced toll scenario, still under consideration, aims to cause only a 10 to 20 percent diversion. The peak rate would be $2.25 in late afternoon southbound, or 75 cents midday.

• During certain off-peak times, such as 1:30 to 2:30 p.m., the diversion rates don’t vary much by toll price. Planners expect an extra 350 to 450 vehicles that hour on waterfront Alaskan Way. Bandy said officials need to consider how added traffic affects new parks, trails and boulevards that would be part of a waterfront redevelopment.

• Current tunnel models don’t include ideas such as tolling I-5 express lanes, or widespread tolling on several roads entering the city.

Workers are now reinforcing and excavating a launch pit west of CenturyLink Field, where the giant 57-foot-diameter drilling machine will begin its journey next year to South Lake Union.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.