A collision between a car and a tractor-trailer just after 5 a.m. on Wednesday morning closed all westbound lanes of Highway 401 near the Highway 6 south exit.
The road did not reopen fully for traffic until late Wednesday afternoon.
The driver of the car was taken to hospital with life-threatening injuries. The driver of the transport truck and a passenger in the car were not seriously injured.
The crash crushed the car and sent it into the ditch while the cab of the tractor trailer went up in flames.
"As a result of the collision, there was a fire that consumed the tractor portion of the tractor-trailer unit," Sgt. Kerry Schmidt said.
"There's a massive diesel spill across all lanes of traffic and that's the biggest concern right now for reopening because the diesel is eating into the asphalt and there's a chance that we may have to resurface the highway before reopening because it's so slippery right now."
The Ministry of Transportation has been called in to assess the roadway and the Ontario Provincial Police reconstruction team is also at the scene.
There was also a passenger in the car. That person is now in custody "for an unrelated investigation," Schmidt said, although he was unable to give specifics on that case.