Commuters who use the express lanes on I-85 in Gwinnett County saw a puzzling sight Thursday morning: toll rates that seemed unusually high, given the light traffic.

The State Road and Tollway Authority tells The Atlanta Journal-Constitution the rates were a mistake, and commuters won’t be charged the high rates.

A little background. SRTA operates the growing network of toll lanes on metro Atlanta highways. With the opening this year of the Northwest Corridor Express Lanes in Cobb and Cherokee county and an 10-mile extension of the I-85 lanes, there are now about 68 miles of such lanes across the region.

The tolls fluctuate along with the traffic. The worse the traffic, the higher the toll. The minimum toll is 10 cents a mile. There is no maximum toll.

Given the light holiday traffic this week, some commuters were surprised to see tolls of $9.30 to get from Old Peachtree Road to Shallowford Road on I-85. By comparison, the toll was $1.55 mid-morning Friday.

So the AJC asked: What’s going on?

SRTA spokeswoman Deidre Johnson responded late Thursday:

“Our system uses dynamic pricing to determine the toll rate and dynamic pricing functions in two main ways to set the rate:

1) Reaction to existing congestion

2) Anticipation of incoming congestion

The system parameters set for today were those used for a typical traffic day in the I-85 corridor, instead of those used for the lighter traffic we expect during a holiday week. This resulted in unusually high rates being displayed given the relatively low traffic volume. Upon observation of the higher-than-necessary rates, the system parameters should have been adjusted sooner to bring the posted toll rates down. In order to address this situation, all trips in the I-85 Express Lanes that occurred between 6:00 AM – 8:30 AM will be charged at a rate of 10 cents per mile. Drivers will not be charged at the higher rates that they saw during their commutes this morning. We apologize for any inconvenience.”

Johnson  said no one at the agency can recall a similar incident happening previously.

Bottom line: If you used the lanes at the height of rush hour Thursday morning, you’ll pay the minimum toll of 10 cents a mile, not the higher rate you saw during your commute.

You can learn more about Georgia's plans to use toll lanes to combat Atlanta traffic here.

To use the lanes, you'll need a Peach Pass. You can get that here.