I think this is an awesome idea and could franchise well if the liablility problems could be worked out. KK raised some good issues though. These things aren't cheap. Rental car companies charge huge premiums to people under 25 to rent a car. I have to think that mom, dad, and the kids taking these things out for the day or a few hours or whatever is the target audience. Do you need a drivers liscence to drive one? If you don't, how does the insurance company handle it when a 14 year old runs into the back of your Mercedes and messes up her pretty little face? Well, you have a great legal team so I'm sure you'll figure that stuff out.
How do these things handle on terrain other than sidewalks or paved surfaces? If they can handle grass and steep hills I can think of a great place I can put them in tomorrow.