A man was taken to hospital with serious injuries Friday after the vehicle he was driving slammed into an Ontario Provincial Police cruiser on Highway 417 that had pulled over to stop another vehicle.

Ontario's Special Investigations Unit has been called in to investigate the crash, and traffic is badly backed up as the long weekend gets under way.

The highway's eastbound lanes were closed for six hours and detours were put in place. Traffic began moving through one lane after 6 p.m.

The crash happened at about 12:15 p.m. in the highway's eastbound lanes just west of Casselman, Ont.

OPP said a cruiser had pulled over a violator. The officer was out of his vehicle when the crash happened, and wasn't hurt. But the impact of the collision thrust the cruiser into the violator's vehicle, a white BMW.

The man and woman inside suffered minor injuries and weren't taken to hospital.

The reason for the crash isn't yet known.

OPP have been warning motorists to leave one lane of room between cruisers pulled over at the side of highways. Five officers have been killed in the line of duty while conducting traffic stops.

In Ontario, the Highway Traffic Act requires drivers to slow down and proceed with caution when passing an emergency vehicle parked on the side of the highway with its lights activated. If the highway has more than one lane, the law requires the driver to move over and leave one lane between their vehicle and the parked emergency vehicle if it can be done safely. Failure to do so can result in fines from $400 to $2,000 and three demerit points for a first offence.