Police say the woman died when her vehicle travelling northbound on Highway 400 hit a support of the Dunlop Street bridge and rolled over multiple times just before 2 a.m.
"The driver was partially ejected, other motorists stopped, pulled the driver out of the vehicle," said OPP Sgt. Dave Woodford. "The vehicle exploded a couple of times."
The driver was pronounced dead in hospital.
As of 6 a.m. Wednesday, the southbound lanes of the highway were open but the northbound lanes were closed from Dunlop to Bayfield for the police investigation.
The northbound lanes were re-opened to traffic just after 8 a.m.