You are asking about jumping onto a moving truck/van/car?
How does that work? Just out of curiosity.
Are you jumping onto people's rides without them knowing?
I have hoped freight on the fly before, and honestly I think it's a bad idea. An old hobo rule of thumb is if you can count the 3 bolts on the spinning train wheel that it should be going slow enough to jump on, but that doesn't necessarily mean you are safe, as the story below elucidates:
once I was trying to hop on a unit on the fly, both my road dogs jumped on before me, and the train had sped up when it was my turn, I had a large pack and couldn't keep up, I grabbed the rail on the side of the unit leading to the little stairs up onto the thing, and the train just started dragging me, my feet and legs were being dragged across the rocks, it happened so quick that it took a minute before my brain could register what was happening, and I didn't think to let go until the very last minute. So, I let go and got rolled on the rocks, I jumped up and tried to catch the next car, but my ankle was fucked, I could barley walk. I had to stay in town until my ankle got better, then I had to hop out alone, as my road dogs were long gone. ( ...I haven't jumped a freight since I was 16 years old, and looking back its incredible that I made it through it all, specially as such a young kid. )
In the story above I had tripped, the train wasn't going that fast (thank God) but I lost my footing while my hand was still gripped onto the unit and got dragged for 100 feet or so... it's a miracle one of my legs didn't get caught under one of the wheels...