I saw this happen about 2:30 PM as I was driving on the M40, the motorway between Oxford heading towards London. A family in a Volvo were towing their full size house trailer (caravan) on their way to the continent for their summer holidays. The left wheel on the caravan came off and the left side "anchor" started to leave sparks flying. I was following about 200 feet behind when I saw it come off. Before we all stopped on the edge (the verge) of the motorway, I saw the wheel (still at about 50 MPH) roll up a 50 foot high slope where it took a leap and a bound another 20 feet higher over a fence and into a farmer's pasture. When I got out, the driver of the Volvo came back to look at the drum on his trailer. This is when he told me that he had just changed the wheels 30 minutes before as he was getting ready to leave home. He assumed that he must have forgotten to tighten the wheels nuts sufficiently.
Then he turned and looked all around for his missing wheel, as he had no spare. He asked me if I had seen where it had gone. After I told him, he went up the hill and over the fence. I'm sure he thought I was joking, but that's where he eventually found it. If I hadn't stopped to tell him, he would never have found it in a donkey's age.