Ben Carson appears never to have ridden handcuffed in a paddywagon. Unless the wagon is fully loaded with prisoners, sitting sideways, there is no way to stop yourself from sliding around or off that shiny stainless steel bench seat unless someone is next to you. If you slide off, you have no balance whatsoever. You are surrounded by nothing but sharp corners of cold hard steel.
If you have a back problem, you are SOL>
This isn't the old style paddywagon. This was a panel van divided into individual compartments. From TV it lookes like you can sit down in there and not much else.