I am asking the same as the others in previous posts. What is the danger here. I could possibly understand hanging upside down or sideways after an accident, but only if you were alone.
The advantages are way more. A lap belt in my roll over would have allowed injury. On top of that I already had a serious problem in my spinal cord in my neck. I was unaware of that, but a lap belt might have left me a quadriplegic on the spot.
As it turned out a month and a half later I did need urgent surgery on my neck to rectify the problem.
A lap belt is basically useless as evidenced by the racing/dakar drivers who use them
Cheers
Mike

