As a design exercise to exclude the bloated contraptions that already EXCEED the permissible "footprint" for regular automobiles (car parking spaces and road width ) I would put the challenge of designing the smallest practical solo (single passenger) flying and road/footpath or pedestrian area capable 'vehicle'. the problem with any sort of vehicle is what to do with it after it has delivered the occupant to their destination and this fact puts a premium on the physical size of a vehicle that is intended to do the 'commute' function -where most people go to work or school etc on their own . Taking a 4000lb SUV to work is just stupid and unsustainable for any real fraction of the world's population --yet the Chines have transitioned from bicycles to SUVs in one foul step --we need a better way. I would point to Syd Mead's writings and renderings in this field as a starting point. My computer time is expiring --someone might be able to post Mead's artwork on the "Unipod" to give a conception perhaps (?) I think it self evident that any of the recent offerings for personal roadable vehicles are both too bulky,too expensive,need too much power by far and fail to deliver PERSONAL mobility --airports are definitely not personal attractors and all leave the user stranded to some degree -the ultimate freedom is a door to door or even better, 'desk to desk' device . (and the use of mechanical mobility WITHIN buildings has been pioneered by the disabled community -- look at it is giving a wheelchair wings, rather than getting a car to fly if you like -the segway arose from a device to allow the quadraplegic to walk and stand up (by Dean Kamen )--only afterwards was the wider application exploited'- but it is too limited by NOT being roadable and can't fly ("Codename Ginger" was written before it was revealed and speculation that "IT" might be a flying car topped the list ) _ I have an old flying friend who needs such a thing as this, he is quadraplegic and despite incredible efforts at self rehabilitation including getting himself flying again and using a hand trike plus driving faces huge problems in going from one to the other and being useful at each end of a trip --integrating the ground and air phases of a trip means more than just a bit more conveniance to many people . there you go Holden - a mission statement that will let you work out how to solve the hurdles it presents. single occupant, autonomous (no help in conversion) weather shielded, roadable (not highway - urban or sidestreet might suffice - not freeway speeds ) carry usual hand l,public transport accessible, etc etc -you decide. Market for millions of.