A group dedicated to warning motorists of photo radar locations in Winnipeg had a run-in with police Tuesday morning.

Larry Stefaniuk of WiseUpWinnipeg.com said a member of his group, James Cotton, had set up a sign on Ness Avenue, informing drivers of a radar trap in a nearby school zone. Shortly afterwards, police officers approached and warned him he could be charged with obstruction of justice.

Cotton said he parked his car with the warning sign half a block behind a Winnipeg Police mobile radar truck.

He said three cruisers pulled up and officers accused him of obstructing justice. They threatened to charge him if he did it again.

"I wasn't obstructing justice," Cotton told CBC News.

"If I was in front of the van or preventing the unit from giving tickets that would be a different story, but there was no way I was preventing them from giving tickets. I was just warning people to slow down," Cotton said.

Stefaniuk said the obstruction threat doesn't make sense.

"From what I can understand, that would only come into play if somebody was standing in front of the radar unit, preventing it from actually performing its duties or say, if an officer was set up doing radar and somebody was acting in a manner that would prevent them from taking radar readings or laser readings," he said.

Stefaniuk can't figure out why police reacted that way when they maintain they want people to know where the radar locations are so drivers will slow down.

No obstruction: lawyer

Winnipeg criminal defence lawyer Jay Prober said he doubted if an obstruction of justice charge would stand up in court given that Cotton was trying to stop people from breaking the law.

"They're actually preventing — well I don't know if its a crime, but it's an offence under [Manitoba's] Highway Traffic Act, so there they are trying to prevent people from speeding," Prober said.

A Winnipeg police spokesperson said if Cotton filed a complaint against the officers they would investigate it.