You know I just don't understand the mentality of anyone who would attach a mattress or sheets of drywall to the roof of their vehicle and then proceed to get onto the highway. They have no idea what force a 100kph blast of wind will have on the load. If it will not fit inside your vehicle you should cough up the $$ for delivery.
Whenever I see anyone with anything tied to their roof, I move out from behind them as quickly as possible, whether I am riding or in my truck.