When I was a boy, I had a friend who would delight in riding his bike behind one of these trucks and reaching out and hanging on to the back of the trailer and allowing the truck to pull him along.
One day, while he was doing this, the truck hit a bump, the tailend lifted off the ground and dropped back down over his handlebars. The handlebars were trapped underneath, there was no weight pressing down on them but he could not get them out from under the truck. He said it was a terrifying experience as he pictured himself tugged along at thirty miles per hour to heaven knows where and probably being unable to find his way home.
Luckily, after about half a mile, the truck hit another bump and it released him, but he never tried it again.