2 p.m. update:
One person has been taken to hospital with minor injuries.
At least six vehicles are involved in a collision including a transport truck, which has smashed through a guardrail.
Heavy tows are on scene
Traffic is backed up for miles as a transport is blocking all northbound lanes of Highway 400 south of Mapleview Drive.
OPP also report via Twitter that Highway 400 northbound has one lane open at Innisfil Beach Road.