BROOMFIELD — Some motorists mistakenly hit with toll charges on U.S. 36 between Boulder and Denver are receiving refunds after a glitch was discovered by the managers of the newly minted highway, Plenary Roads Denver.

About $3,600 in total charges were levied mistakenly against motorists in the past 13 days, said Terry Ostrom, vice president of Plenary Group. The tolls were charged to carpoolers who are allowed to travel the new express toll lanes for free if they use a switchable HOV transponder.

Carpooling without a toll is allowed for two-occupant vehicles — a driver plus one. Three occupants — a driver plus two — will be required to travel for free no later than 2017.

The charges were all generated from the toll station at Sheridan Boulevard toward eastbound traffic, Ostrom said.

The station “was billing as if you were tolled customer,” said Ostrom.

“If you switched to HOV mode, our reader sees that, and it throws out the transaction,” Ostrom added. “In this case, it wasn’t reading when it was in HOV mode.

“We got the system fixed and it won’t happen again.”

Those tolls will be waived and carpoolers’ accounts will be credited. Most of the tolls amounted to about $1 apiece, Ostrom said, and involved about 3,600 transactions.

Tolling began on the first phase of the U.S. 36 remake in July. A tolled express lane was built in each direction of the highway, in addition to the two free general purpose lanes.

The project also is replacing several bridges, building a commuter bikeway and adding improvements for a Bus Rapid Transit system that starts next year, said CDOT spokeswoman Amy Ford.

The first phase extends from Federal Boulevard to 88th Street in Louisville/Superior. The second phase, from 88th Street to Table Mesa Drive in Boulder, opens in 2016.

So far, this was the only glitch to the tolling portion of U.S. 36 that directly cost motorists.

“The number of users who were accidentally charged represent a fraction of 1 percent of all toll users during that time frame,” Ostrom said.

Monte Whaley: 720-929-0907, mwhaley@denverpost.com or twitter.com/montewhaley