The Texas Department of Transportation says it's fixing a billing system that erroneously charged more than 41,000 motorists.

The department told the Dallas Morning News on Monday that the effected motorists drove a new toll road in the Dallas-Fort Worth area during a free period. The department billed for more than 192,000 transactions between May 27 and July 6.

It's unclear what caused the errors.

The North Texas Tollway Authority has an agreement with TxDOT to bill customers who use state-developed toll lanes. An authority spokesman says his agency will credit all TollTag users who were wrongly billed. He says that amount was about $171,000.

He says bills for drivers without TollTags won't be mailed out.

Department spokesman Tony Hartzel says the agency is "taking steps to rectify it."