Why I-405 tolls drop by $7 when you cross the county line

Anne Hagel of Everett wonders about the math that’s used to calculate toll rates on the I-405 express lanes. Hagel is an I-405 commuter in a two-person carpool and feels the Snohomish County end is unfairly dinged at higher rates.

“For example, the other day my southbound morning toll was $7.75, but after crossing over the King County line, the toll for new drivers entering at that point dropped to 75 cents,” Hagel wrote.

In both sections, her drive included short slowdowns when the speedometer dipped below 45 mph. “The logic for the algorithm calculating a $7 difference isn’t apparent,” she said.

Basically, it’s all about too little space for too many cars.

When the state converted the old carpool lanes to express toll lanes, crews added another lane of pavement south of Highway 522 for a second toll lane. No pavement was added in the north end.

That leaves double the space to sell in the south end, while the north end fills up lickety-split.

That difference in capacity “has a big impact on price,” said Ethan Bergerson, a spokesman on tolling matters for the Washington State Department of Transportation.

“Additionally, the double-lane section is able to recover from a slowdown much more quickly than the single-lane section,” Bergerson added. “When a vehicle slows down, it causes all vehicles behind it to slow down, creating a shockwave that lasts as long as there is an uninterrupted stream of cars coming down the road. In the double-lane section of the express toll lanes this shockwave can dissipate relatively quickly as cars have the opportunity to drive around, but in the single-lane section this shockwave can last for the entire commute.

“Because the single-lane section is so sensitive to minor changes in volumes and speeds, the algorithm that controls toll rates needs to respond differently in order to manage travel volumes and keep traffic moving,” he said.

Traffic engineers already adjusted the algorithm for the north end once, soon after tolling started, to respond more quickly to rising traffic volumes.

A second toll lane in each direction at the north end is part of the state’s long-term plans for toll revenue from I-405.

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