But now many drivers say they follow all those rules and still find unfair charges on their FasTrak accounts.
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Drivers say it's more annoying than costly.
They find extra charges on their FasTrak bills -- anywhere from 50 cents to $10. But they say those charges add up and it's not easy to get FasTrak to correct the mistakes.
Nancy Andre of Pleasanton lets her two kids drive to school in Concord every morning. She also drives a carpool back home. "Both girls are on the softball team and today they don't have practice so I get to come pick them up after school," she said as she drove her daughter and a friend home from Concord.
With FasTrak, they get to use the I-680 express lanes - and the ride is toll-free.
VIDEO: Lawsuit says FasTrak charged unfair penalties: Here's the result
Lawsuit says FasTrak charged unfair penalties
At least, it's supposed to be.
"Literally every statement I get from FasTrak, there are errors on it. So the one I got a couple days ago, they charged me an additional $10.50..." she said.
Her daughter Claire confirms it. "I feel like every month there's, like, multiple charges for the carpool lane."
Andre sets her FasTrak toll tag to indicate either "two people" or "three-plus people" in the car, which tells the electronic sensors they are a carpool -- entitled to a free ride. On I-680, on the "Livorna Road" stretch, as few as two in the car qualifies as a carpool. Yet Andre's FasTrak statements are sprinkled with extra charges -- 50 cents here, $9 there even though she was driving as a carpool in the express lanes..
"So I spend a lot of time corresponding with FasTrak, unfortunately," she said.
Disputing the charges can take weeks.
RELATED: FasTrak has the wrong stationery, sends 'toll violation' notices instead of invoices
"I would send copies of my statements with my written explanation. And I would hear nothing..." she said. "It's just, I am playing by the rules, and I'm being punished. And I just wonder how many other people are getting charged $9.50 for this and $9.50 for that, that adds up fast."
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It happened to Scott Linn of Alamo, too. "Usually for me, you know, typically it's between $6 and $10,'' Linn said.
He used the I-680 express lane while driving his daughter's soccer-team carpool -- yet he kept getting charged the full toll.
"After the third time, I thought there's got to be something wrong with the toll reader," he said.
He was sure the electronic sensor near Stone Valley Road could not read his toll tag. He says the tag worked everywhere else.
"I said to the lady, 'I just want you guys to, like, fix it.' 'Well, there's really nothing we can do about it.' And I'm like, 'Well, that's, I mean, that's just, that's terrible,'" he said.
WATCH: Why you may already be a part of the FasTrak class action lawsuit
Why you may already be a part of the FasTrak class action lawsuit
FasTrak kept telling Linn he probably mounted his toll tag wrong.
"They're really tough about it, when you, when you go to fight it, to a lot of times... they'll say, well, you know, your reader should be in the right location..." Linn said. "Sometimes they would give me credit, sometimes not."
Finally, Linn just stopped using the express lane altogether.
"I don't go over there. I don't want to deal with it," he said.
Linn and Andre each contacted 7 On Your Side. We asked FasTrak if there may be problems with electronic sensors.
FasTrak sent a team to investigate. So far, no answers.
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RELATED: FasTrak 'secret' policy revealed: No more penalties for drivers it cannot contact
FasTrak said: "Nothing yet would definitively point to an equipment failure... Where we identify toll errors, the error rate is extremely low."
Still, with millions of drivers, there are thousands of complaints.
The latest data show FasTrak received 1,967 disputes of toll charges on that corridor, in just three months, at the end of last year.
"If people are complaining, there's probably a real problem," Linn said.
"Now it's become a game. How much have they overcharged me this time?" Andre quipped.
FasTrak says many drivers may need to trade in their old toll tags for new ones. Toll tags run out of battery power over time, making it hard for some sensors to detect them. If you get an unfair charge, file a dispute -- and let 7 On Your Side know about it too. We can help.
Take a look at more stories and videos by Michael Finney and 7 On Your Side.
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