Police are still trying to locate the wheel that came loose from a vehicle travelling on Highway 400 on Wednesday and smashed into another car.

The runaway wheel struck the 29-year-old Barrie woman's windshield during the morning rush hour.

She was airlifted to a Toronto trauma centre. Police say she remains in hospital with a broken nose and facial fractures but is expected to recover.

The wheel came loose from a vehicle driven by a 56-year-old off-duty Toronto police officer.

Provincial police say they expect charges will be laid.