When I was a lad...
We went from Hampshire to Snowdon in one of them - overnight and very slowly. I would have been about 10 or 11. The other lads were (what I thought was) a lot older, up to 17/18ish.
I did a lot of things in my youth that I wouldn't do now, but they had a game that I wouldn't do then.
When hanging out of the back, hanging onto the 'entry rope', became a bit ordinary, a new task was started. The object of this was to exit the flap at the back and proceed along the side of the rear bed, using the odd straps that were flailing about in the wind, then finally entering the cab via the hatch, and returning to the back. About half-a-dozen lads achieved this goal.
All this happened as we progressed along the A5 on the overnight journey.
Amazingly, nobody was killed or even injured. A few car drivers were unnerved by the spectacle, but all the entrants used the nearside of the truck for their passage to the cab and back.
On the return journey, also overnight, we arrived back and there was no sign of me when the back of the truck we being emptied. I was discovered underneath a large crate full of equipment that had overturned onto me in my sleeping bag. It was too heavy to shift and I, an extremely heavy sleeper, just went back to sleep for the rest of the journey. When I was discovered, those who were supposed to be in charge were petrified that there "could be paperwork", but I was able to persuade them that I wasn't going to die - I wasn't even injured, just hungry.
One for a cameo scene..?