A chain reaction collision involving eight semi-trailers forced police to close Highway 401 at Mavis Road for nearly two hours.

All westbound lanes were reopened just after 1 p.m.

Ontario Provincial Police Const. Dave Woodford said the chain reaction collision occurred after a pick-up hauling an empty flat utility trailer stopped for traffic.

"It's one of those trailers that usually carries a sea-doo or something like that," Woodford explained.

A tractor trailer driving behind the vehicle could not stop in time to avoid it and as a result, ended up driving over the flat utility trailer.

Six other trucks driving behind collided as they tried to avoid the crash.

Woodford said it was a miracle a car was not caught in the collision.

"A car could have been crushed," he said.

Woodford said police were able to reopen the highway quickly because they decided against bringing in a reconstruction team to help them determine how the crash occurred.

One person is in hospital with a back injury. A number of other people suffered minor injuries.