[DaveNP]

^^
A sheet of steel sounds horrific but I've been in a similar incident on the motorway which was 'just' an 8x4 sheet of plywood. The sheet of plywood was lifted off of a vehicle in front of me just by the force of the slipstream of air, fortunately for me it flipped to vertical and went down the side of my truck, it then flipped to horizontal and embedded itself in the nearside of the windscreen of the truck beside me, fortunately there was no passenger and the driver was the luckiest guy on the motorway that day. An 8x4 sheet of ply isn't light and the guy who lost it obviously chucked it on the back of his truck and thought 'well that ain't going nowhere', how wrong could he be?!