OPP Sgt. Dave Woodford said northbound lanes of the highway have reopened after being closed for part of the morning at Highway 89.
Woodford said none was seriously injured but warned drivers to slow down as dangerous whiteout conditions remain in the forecast.
"It can happen so quickly and you may have to apply your brakes just as quickly so just slow down and keep a nice safe distance," Woodford said.
He says the treacherous conditions sent vehicles slamming into each other, careening into ditches and crashing into guard rails.
The CBC's Charlsie Agro said OPP had set up a command post in a commuter parking lot just off of Highway 400 where they were busy taking police reports.
The lot, she said, was full of "mangled" vehicles and tow trucks.
Environment Canada is warning that snow squalls in much of southwestern Ontario and cottage country are to continue and possibly intensify Saturday night before tapering off Sunday.
It warns the heaviest squalls could deliver 15 to 30 centimetres of snow in only 12 hours.