An armoured truck belonging to Montreal-based Garda World Security Services spilled two plastic bags containing about $5,700 in bills and coins from an unlatched door onto a Maryland highway, prompting a frenzy as drivers grabbed fistfuls of cash.

One witness of the Friday incident on Interstate 270 about 55 kilometres northwest of Washington, D.C., said she saw about 30 cars pulled over on the shoulders and people frantically collecting the money.

Police say the motorists grabbed almost all of it. Others kept driving.

Attorney Heather Kelly was driving to her office in Frederick when she passed through the surreal scene.

She didn't see the armoured truck but noticed the two clear plastic bags of currency along the road and people snatching the $1 to $50 bills wafting through the air and skittering along the highway.

"It was in the traffic lanes and on the shoulders and just generally kind of like a snow globe of cash," she said. "Some people had fists full of money, fists full of dollars, and other people were just still trying to collect."

Kelly decided it was too risky to stop, though no one was injured. She said she wouldn't have stopped even if it was a two-lane road with no other traffic.

"It was really unclear what was going on and I like to stay away from that type of thing," she said.

The truck belonged to Garda World Security Services Corp., a Montreal-based security and cash logistics company, spokesman Joe Gavaghan said. He said they're co-operating with state police investigators to find out what happened.

Maryland State Police urged people to return the money to the agency's barracks in Rockville, with no questions asked and no charges filed.