The MoPac toll lane’s first full-fledged work day produced some bobbles, based on reports from commuters who found themselves stalled to stop-and-go for awhile in the toll lane north of U.S. 183.

The culprits: human nature, tollway officials theorize, and a balky computer algorithm that for about an hour was lowering the toll rate when it needed to be raising it to discourage drivers from entering the toll lane.

"People slow down in curves," said Mike Heiligenstein, executive director of the Central Texas Regional Mobility Authority, referring to the wide right-hand swing in northbound MoPac Boulevard near the Domain. "And we didn’t raise the rates fast enough.

"It’s going to take some of these heavy-traffic days to really figure it out."

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The backup in the toll lane occurred between 6 and 6:30 p.m. Tuesday, while drivers in the free lanes whizzed along at high speed, according to drivers caught in the toll lane slowdown.

The front of the backup in that northbound toll lane was somewhere north of the Duval Road overpass and extended south to near U.S. 183, Heiligenstein said.

During the peak hour of 4:30 p.m. to 5:30 p.m., the toll rate for the north section of the toll lane, directed by the mobility authority’s computer software, ebbed from 83 cents to 64 cents. Under the theory of how the toll lane and driver psychology is supposed to work, that had the effect of feeding more traffic into the toll lane and setting the stage for the backup near 6 p.m.

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At that point, Heiligenstein said, agency officials observing all this from a control center in Cedar Park stepped in and overrode the computer. The toll rate for the north section was raised to $1 at 6:09 p.m. and then $1.12 five minutes later. By 6:47 p.m. the crunch had ended and the toll rate returned to its minimum 25 cents.

The mobility authority said that it saw 14,312 toll transactions on Tuesday, close to its opening-year prediction of 14,900 transactions per day.