WASHINGTON — A District of Columbia police officer who is challenging the accuracy of his own department’s speed camera program said a discrepancy on a speeding ticket helped him beat the charge, and could lead to many other dismissals that would force the city to give back hundreds of thousands of dollars.

Sgt. Mark Robinson, who cast himself as a whistleblower, argued that the citation issued by a speed camera in Washington’s Third Street Tunnel was invalid because the camera should have been set to enforce a posted 40-mph construction-zone speed limit. Instead, the camera was set to enforce the 45-mph limit that is regularly in force there.

Even though the city’s police department is giving motorists the benefit of an extra 5 mph, it is violating the law because Title 18 of the D.C. Municipal Regulations requires the city to enforce the posted limit, Robinson said.

“Due to that technicality, the citation is invalid,” Robinson said Wednesday in a telephone interview. “We can only enforce what’s posted. We can’t say we’re going to forgive 5 mph. We have to be 100 percent accurate.”

Robinson said a hearing officer agreed with him and dismissed the ticket last month. The Washington Times first reported the story Tuesday.

Robinson said that if the decision were applied to other people who got tickets in the construction zone, the city would have to return as much as $1.8 million. Those numbers could not be immediately verified Wednesday.

City officials argue that the ticket was issued properly. Like other cameras set up near temporary construction zones, the unattended machine is calibrated to enforce the regular 45-mph speed limit because the machine cannot determine when workers are present and the District Department of Transportation’s construction site is active. It’s also not clear how long the stretch of I-395 will be a work zone.

“We have kept the photo enforcement speed limit at 45 mph since it was not clear when the construction would be finished and DDOT did not ask us to enforce at the 40-mph speed limit.

Any tickets issued have been issued for traveling in excess of 45 mph, not 40 mph,” police spokeswoman Gwendolyn Crump said in an e-mail.

Mayor Vincent Gray announced in November that the city would reduce some speeding fines, a move driven at least partly by complaints over the city’s reliance on the enormous revenues generated by automated traffic devices.

The city hauled in nearly $85 million in the past fiscal year, with a single camera on New York Avenue generating $11.6 million over a 23-month period. Gray said the adjusted fines would increase the fairness of a system that he also credited for dramatic improvements in traffic safety.

Robinson,who joined the force in 1990 and had been working with the automated traffic enforcement unit until recently, said problems have come up since sworn personnel were replaced with civilians. He said he has complained to superiors and other city officials about alleged inaccuracies in the speed camera program, but said nothing has been done.

Crump said in an e-mail that there is “no evidence to support” Robinson’s claim that there is a problem with enforcement.