According to the UPI, Kevin McGraw and his wife were traveling along a stretch of freeway near Moncton, New Brunswick on a cold, snowy November 30. McGraw was passing a semi-truck in the left lane when a large slab of ice and snow lifted away from the top of the truck's trailer and smashed into McGraw's car. The icy missile shattered the car's windshield, showering McGraw's wife with bits of safety glass but thankfully not penetrating into the cabin. McGraw was able to get his stricken car across the freeway to the right shoulder while the trucker, blissfully unaware of the chaos he'd just caused, continued on his way. Thankfully neither McGraw nor his wife were injured, but things could have been way worse if the snow had been a little heavier or the car had been moving a little faster.
The American Automobile Association states in an Alexandria News article that drifted, compacted snow on top of a moving vehicle may weigh as much as 20 pounds per cubic foot. That's bad enough on the roof of, say, a Civic, but on the roof of a speeding semi trailer where multiple cubic feet of snowy sheet ice can peel off and fly into traffic it is a catastrophe waiting to happen. More than half of US and Canadian semi truck drivers surveyed in a 2009 study by the American Transportation Research Institute admitted that they "rarely or never" clear accumulated snow from their vehicles despite the fact that roughly 35% of truckers surveyed could relate a story about snow or ice falling from a moving truck and harming passing motorists.