Difficult one Dave. I think part of the problem is that they are trying to do two jobs. They aren't only instead of being able to walk short distance, round here they are used as an alternative to busses, pushbikes and cars for people who want to travel longer distance...not miles and miles but say a mile there and a mile back again.....but they aren't really safe on the roads and aren't safe for others on the pavement. I do agree about proper ID and insurance. Part of my job used to be offering advice to people about what they should buy, if anything and we always talked about the road/pavement issues and the importance of being properly insured. Maybe a way forward could be to control more tightly the larger more powerful road worthy vehicles and insist that they are used on the road only, and lay a lighter hand on the vehicles for using on the pavement but govern them down to a walking pace? In either case i think a test of controlling the vehicle would be a good idea. We used to "kind of" do that for our patients, taking them out in the kind of vehicle they were thinking of around a fairly tricky route around the hospital where the trial vehicles were stored. mostly of they weren't going to manage they scared themselves enough to decide not to buy, occasionally we had to have a "conversation" but not often.
Its also sad because round here they really do contribute to independent living for many people and without them they would have to leave their own homes.