Whether you drive Highway 99 or not, the tunnel tolls starting Nov. 9 will disrupt your travel in downtown Seattle through higher costs, slower trips or more aggravation.

That’s because thousands of toll-dodging motorists will crowd city streets, rather than pay for a 2-mile tunnel drive between Sodo and South Lake Union. State officials warn as many as half of tunnel drivers might try other paths the first week — a worst-case scenario that’s unlikely to persist.

After the opening shock, around 35% of the current 75,000 daily tunnel users would still choose other roads, based on experience with tolls on the Highway 520 floating bridge, said Jennifer Charlebois, toll-project engineer for the Washington State Department of Transportation (WSDOT).

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That translates to 26,250 added vehicles. People are more likely to divert midday than at rush hour, studies predict.

“The alternate roads are saturated right now, particularly at the peak,” said Meghan Shepard, deputy downtown mobility manager for the Seattle Department of Transportation (SDOT).

The so-called shoulder of the peak, from 2 p.m. to 4 p.m., is the critical time that worries city traffic managers. Seattle’s wide arterials such as Mercer Street, Alaskan Way, or Fourth Avenue provide enough lane space to absorb a few toll avoiders. But as lanes fill, traffic will build into a wave of gridlock by 4 p.m. when an additional 1,000 to 2,000 cars per peak hour show up, also avoiding tolls, according to state predictions.

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WSDOT isn’t speculating about delay times. One clue appears in an old environmental study that predicted freight trucks that travel on the surface would lose three to six minutes because of toll-related spillover traffic, compared to a free tunnel.

A good guess is “several minutes,” but even that’s dicey, because the downtown road grid constantly perches on the brink of failure, said University of Washington professor Mark Hallenbeck, director of the Washington State Transportation Center. Toll diversion makes sudden traffic breakdowns more likely, he said.

“If it blows up, you see delays of 20 minutes or half an hour. If it’s [merely] slow moving, it will be two minutes or four minutes,” Hallenbeck said.

After exhorting the public through Carmageddons, Viadooms, Squeezes and fish-truck spills, Seattle has few traffic-management tools to employ.

Transit grew 50% in the last decade — the only U.S. region to gain riders — until boardings declined this year, as ride-hailing companies flourish and buses get stuck in detours. For instance, Sound Transit’s Seattle-Bellevue Route 550 lost 2,000 passengers since convention-center construction evicted buses from the downtown transit tunnel.

As drivers try their options, Charlebois expects a full year could pass before traffic settles to a “new normal” pattern.

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On the other hand, local history and the cheap tunnel tolls suggest the new normal might emerge much sooner.

Seattle’s best defense against spillover traffic is low toll pricing designed to keep motorists using the tunnel. Tolls will fluctuate by time of day, from $1 overnight and weekends, to $1.50 weekday mornings and $2.25 in afternoon peaks.

At those prices, tunnel drivers have less incentive to divert than do Highway 520 users, who paid up to $3.50 when tolling started in 2011 and now pay a maximum $4.30.

To the north, drivers filled the new I-405 Express Toll lanes by paying up to $10, weeks after they opened in 2015. Drivers pay an average $4.23, reaching $10 at least once on most days, to save an average 10 minutes. For a cheaper price, Highway 99 tunnel travelers would often gain an even bigger time advantage.

“If it takes forever, the people who are getting out of the tunnel will go back into the tunnel, because saving $2 isn’t worth losing 35 minutes,” Hallenbeck said.

Last winter, drivers entering Seattle reacted instantly to a three-week Highway 99 closure by going to work early, suffering longer on the West Seattle Bridge or I-5, and reducing trips. Bicycling and foot-ferry use spiked.

Motorists have used the tunnel for free since Feb. 4, so they’re acclimated to the four-lane tube.

Once tolls are charged, the turbulence from diversion commonly lasts three to six months, until drivers gradually return to the paid highway, said Bill Cramer, communications director for the International Bridge, Tunnel and Turnpike Association in Washington, D.C.

When tolls began on Highway 520 in 2011, traffic dropped from 103,000 to 68,000 daily vehicles, or 34%. People moved to the I-90 floating bridge or Highway 522 around the lake. Others took transit or stayed home. Highway 520 traffic today remains 17% lower than before tolling.

WSDOT found a surprise tracking 50,000 Good to Go toll passes it distributed free since May, in a tunnel promotion.

On northbound lanes, half of activated passes were used just once a week, and 20% twice a week. That suggests the tunnel provides a route for people traveling for work projects, shopping, entertainment, or other personal trips rather than fixed daily commutes.

The official toll-revenue study predicts travelers will average 26 to 29 mph using the tunnel at peak times, superior to stop-and-go surface streets.

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“The tunnel is going to continue to be the most efficient way to get through downtown,” insisted Heather Marx, SDOT downtown mobility director.

A smattering of block-by-block changes are meant to improve downtown’s people-moving capacity a bit.

Primarily, the toll launch was delayed until after Alaskan Way Viaduct demolition and a barricaded lane on Alaskan Way reopened. Builders of the new waterfront boulevard and parks must keep four lanes flowing there through 2023.

North downtown’s two-lane chokepoint through Seventh Avenue North should expand to four lanes by late November, as crews there finish rebuilding the street two months early. An additional bus lane each direction will open next year.

The city’s lower Columbia Street rebuild is due by January, to provide bus lanes between Alaskan Way and Third Avenue, and perhaps rescue 25,000 daily riders from Pioneer Square congestion.

King County Metro recently added 1,000 daily bus trips countywide. More bus lanes are painted red to discourage general traffic.

None of these moves, however, would offset 35% toll diversion, not to mention 50%.

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To tackle opening week, police will control nine downtown intersections at a cost of $100,000, said Marx.

Traffic-light synchronization offers little potential traffic improvement, after years of city refinements such as adaptive signals on Mercer Street that stay green longer as clumps of cars approach. No changes are planned, except in case of a blocking incident.

“If we make it easier for north-south traffic to move, east-west traffic will suffer, especially pedestrians,” Marx said.

A 10-week complication arrives in January, when Sound Transit trains must stop and reverse at Pioneer Square Station, while tracks are installed for the new Bellevue line that will open in 2023. Trains will be more crowded, and 30,000 daily riders must transfer to continue through downtown.

Many riders will temporarily avoid light rail, while others get off to call an Uber or Lyft, Sound Transit project staff has warned.

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Marx repeated her counsel from last winter: “Please be kind and patient with your fellow travelers, and remember that your commute is a community decision.”

SDOT invented a Muppet-like character, Sal the Salmon, who asks solo drivers in video and billboard ads to “flip your trip” for one day a week. “Driving downtown alone just doesn’t make sense. It’s like trying to spawn alone,” Sal explains.

Some 70% of downtown employees already forsake driving alone, yet congestion reigns.