I just got an email from Tim Marshall a well known storm chaser. Here is what he said on windspeeds and vehicles ending up on something other than their wheels:
1. Motorcycle: 50-100 mph depending on type, whether it is moving or not, and how good the kickstand is if parked.
2. Automobile: 125-135 mph according to wind tunnel tests conducted by Dr. Schmidlin.
3. Semi-Truck: 80-120 mph depending on type, whether it is moving or not.
4. Train: 125-175 mph depending on type and whether it is full or empty and/or moving.
The rocking motion of vehicles can also get them to flip if they reach a reasonant frequency with the wind.
