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.

Now, if you suddenly hit air the instant you jumped, you would find considerable friction to stop you from moving. Of course, you're not guaranteed to miss the bed of the truck (it would depend on how high you jumped, where you jumped from and the length of the truck bed). If you DID miss the truck and hit the ground, yes, you would be hurt because you just run into a stationary object while you were travelling rather quick.

Ste "Not 100% certain, but I think that's correct" phen