A glitch in the new tolling system on the Highway 520 bridge has caused some billing mistakes — an extra 25-cent charge for some drivers.

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The new tolling system on the Highway 520 floating bridge failed to detect windshield-mounted Good to Go passes on some trips — causing drivers to be charged an extra 25 cents.

After hearing a few customer complaints, the state Department of Transportation (DOT) examined the problem, then announced late Friday it will waive the 25-cent fees on all trips made Jan. 17-25, even those correctly billed.

The overbillings totaled somewhere around “a couple hundred” on some days, said Patricia Michaud, DOT tolling spokeswoman.

Tolling contractor Telvent has installed a patch to the flaw in the electronic system, so the problem should be solved now, she said. Engineers are investigating the cause.

Bridge customer Dick Tabbutt says he reported a flawed billing after looking through his online Good to Go account, “just to see if this thing is working.”

The Dec. 29 launch of bridge tolls began eight months late, for what some experts called the nation’s most technically challenging toll startup. Tabbutt noticed an extra 25-cent fee on a Jan. 12 trip.

“At the end of the day, a fee of a quarter is not an issue. But if the DOT’s system is simply not working up to snuff, they could be collecting a mountain of quarters in a very short time,” said Tabbutt, a medical-technology executive who crosses a few times a week for business meetings in Seattle.

Another driver showed a printout in which his account was overcharged three times and correctly charged 26 times.

Why 25 cents?

The state has three toll varieties: a variable rate of up to $3.50 per trip for Good to Go pass holders; an extra 25 cents for motorists who opt to register their license-plate numbers in a “Pay by Plate” account; or an extra $1.50 “Pay by Mail” fee on vehicles that aren’t enrolled in the toll system. Some drivers choose Pay by Plate because they cross the bridge infrequently, or don’t want to hassle with buying and mounting a transponder.

When people activate a Good to Go pass, they write one or more license-plate numbers on the enrollment form. Therefore, whenever the transponder was missed, a computer or toll-center employees still entered the characters from a license-plate photo taken at the bridge, so the plate is linked to the account — triggering the mistaken 25-cent Pay by Plate fee instead of a full $1.50 surcharge.

Tracking mistakes occurred intermittently, when an internal clock in the toll-detection devices went out of sync, Michaud said. So the electronic chip in a windshield-mounted Good to Go pass wasn’t recorded at the same exact time as the picture that toll cameras took of the same car, she said.

State toll director Craig Stone said he noticed last week a dip in the proportion of trips using Good to Go tags and wondered about the cause. Before startup, he said tests found 99.9 percent accuracy in reading the transponders.

“We expect to get that reliability,” Patty Rubstello, a DOT systems engineer, said Friday.

Telvent supplies the detection devices and cameras mounted above the east high rise of the bridge.

“We are working closely with WSDOT to fix the problem and will continue to monitor the situation to ensure the correction in place continues to work,” said a statement by Telvent Technology Director Darby Swank. “Telvent will work with WSDOT to determine any reimbursements.”

Another firm, Electronic Transaction Consultants, handles customer service, accounting software, billing and manual recording of vehicles that aren’t charged automatically.

DOT encourages bridge users to check their records, and to call the toll hotline at 1-866-936-8246 for questions or to report possible errors.

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