Realizing that in normal circumstances we shouldn't be flying 'over' too many things, I was wondering what would be adequate distance above a vehicle to avoid any ground turbulence. For example, let's say a train was passing underneath at 40 mph. Obviously I don't want to be immediately above it, since it would surely knock me out of the sky. How far above should I be to be 'safe'? I know the larger and faster the vehicle, the more height I would want. But any rule of thumb? I'm sure we've all seen that racing video where the car zips underneath a Phantom and sucks it right out of the sky. I'd hate for that to happen!