Middlesex OPP closed down the eastbound lanes of the 401 at Wonderland Road after a multi-vehicle pile-up Friday afternoon.
It happened around shortly after 12 p.m.
Between 15 and 20 vehicles were involved in the crash.
Five people were transported to hospital by ambulance, one of them suffering serious but non-life threatening injuries.
It's unclear how the crash happened but road conditions were not optimal at the time, thanks to blowing snow and flurries.
The investigation continues.
