You're on a highway with an 80 km/h or higher speed limit, divided into clearly marked lanes, with two or more lanes travelling your direction. Two snow plows are working side by side (in echelon), each occupying a lane. Under HTA Section 148.1, when are you allowed to overtake them?

Correct answer: B. Only if the lane you'd use is entirely clear of any part of the plows, including attachments and blades

Section 148.1 specifically targets snow plows working in echelon (side by side) on higher-speed, multi-lane divided highways: you may not overtake them unless the lane you'd use to pass is entirely clear of any part of either plow, including attachments and the plow blade itself. A plow's blade or wing can extend well past the truck's body, so a lane that looks open from a distance may not actually be clear.

Source: MTO Driver's Handbook: Highway Traffic Act, R.S.O. 1990, c. H.8, s. 148.1 — No overtaking a snow plow