A crash on Highway 401 eastbound during the Friday afternoon rush backed up traffic through Guelph and Cambridge.
Before 4 p.m., a tractor-trailer flipped over on its side while driving the highway, between Highway 6 North and Highway 6 South.
Traffic quickly slowed, with gridlock reported through Cambridge and into Kitchener
Two eastbound lanes were closed initially, one of which reopened around 6 p.m., as was the eastbound off-ramp to Highway 6 South.
Charges are expected to be laid, OPP told CTV News.
