> I should think that, so long as you didn't hit any air at any point, you're not going to hit the road. The basic deal here is that if the truck is travelling at 70 mph, so are you, and until another force moves to STOP you, you'll continue doing so. In other words, you would jump up, hitting no air, keep moving at 70 mph with the truck, and then land back in the bed of the truck.
>

Interestingly enough, I just spent the last school quarter studying motion and Newton's laws. This one is called the Law of Conservation of Momentum (different from the LoCo Energy), which states*, "In the abscence of an external force, the momentum of a system remains unchanged."

So, the only way you would land on the road with any measure safety would be to jump forward, away from the truck. To land standing still, you would have to jump off going 70mph (i.e. your net force being zero).

Oeras

*May be simplified for middle school students.