Tolls might begin March 1, 2019, ranging from $1 overnight to a top rate of $2.50 for an afternoon-commute trip between Sodo and South Lake Union, under one leading option.

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A common theme emerged Tuesday, as a state commission launched its toll-rate talks for Highway 99:

Keep the rates down, so drivers choose to use the new tunnel under downtown Seattle, rather than clog other roads.

Tolls might begin March 1, 2019, ranging from $1 overnight to a top rate of $2.50 for an afternoon-commute trip between Sodo and South Lake Union, under one leading option.

Highway 99 tunnel tolls

Here’s one leading option. Tolls would vary by time of day, as on the 520 bridge. The lower the price, the fewer drivers would divert onto crowded surface streets, state officials assume.

• Overnight and weekends: $1

• Weekday mornings 5-6 a.m., midday 9 a.m. to 3 p.m., and weekday evenings 6-11 p.m., $1.50

• Morning peak weekdays, 6-9 a.m., $1.75

• Afternoon peak weekdays, 3-6 p.m., $2.50

Source: Washington State Transportation Commission

At prices like those, the misery of alternate commuting streets will outweigh the price of tolls, an adviser told the Washington State Transportation Commission.

“You’re not seeing a lot of traffic moving out of the tunnel, because the trip is worth it, because of the time savings,” said Steve Abendschein, senior planner for the Stantec consulting firm.

Instead, he predicted an “oscillating” pattern for travel demand.

Tolls rise for the afternoon peaks. Drivers divert. Downtown streets clog. Other drivers arrive and choose the quicker tunnel.

Commissioners won’t decide the toll rates until September 2018, after meetings and public hearings. State lawmakers delegated the thankless job of setting toll rates to the transportation commission, an appointed board.

Tunnel tolls are required by the state Legislature, to repay $200 million of construction bonds. Lawmakers in 2009 initially set the toll goal at $400 million, but then retreated in 2012 when they realized how unrealistic their numbers were.

Tolls would also cover $5 million a year in the tunnel’s maintenance and utilities costs, a state chart released Tuesday shows. The overhead cost of collecting tolls, roughly $8 million a year, could use up more than one-fourth of the income.

To avoid a mobility disaster, the state is certain to leave cash on the table.

If maximum income were the sole aim, tolls would reach $6.80 in afternoon peak, according to Stantec.

Years ago, Ron Paananen, former Highway 99 administrator, guessed the top rate would wind up somewhere between $2.40 and $4.

At the peak rate of around $2.50, most diversion to surface streets or I-5 would happen midday, because peak diversion is barely possible, said Abendschein. An earlier chart this spring showed about 51,800 vehicles using the tunnel per weekday in this scenario — which is 4,400 fewer than if the maximum rate were only $1.25. Some people would cancel their trips or use transit.

Commissioners discussed other minor variations, such as free overnight or weekend drives.

Some of Tuesday’s discussion revolved around trucks, which would make up an expected 7 percent of traffic in a toll-free road. Some members want rates that encourage freight to use the tunnel.

An overall $200 million toll contribution represents a mere fraction of the overall $3.2 billion replacement for the Alaskan Way Viaduct. The state might wind up spending more, if lawsuits go badly with contractors who want $600 million in extra payment for delays and overruns.

Retired state Rep. Larry Seaquist, D-Gig Harbor, told commissioners to suspect that the state would inflate its costs to maintain and operate the tunnel systems, and he questioned whether tolls will actually support the $200 million target, based on his observations at Tacoma Narrows Bridge.

During the 2019 tunnel startup, central Seattle will be choking on other construction, he added. The projects include a waterfront boulevard rebuild, a First Avenue streetcar, bus detours and a convention-center expansion.

“The tunnel is going to create a mess. The people are not ready. The facilities are not ready,” he said.

Seaquist urged commissioners to speak up about that.