I know that in Portland there is a firm that purchases air rights over parking lots and builds new buildings; I am not an architect, but it seems like the same thing would be possible over your average downtown street. This would open up lots of new real estate in the city, for housing, offices, parks etc. as anywhere that there is open asphalt would be a potential construction site. I know that cities like Calgary and Minnesota have raised walkways over streets, so I'm just talking about something like that but a whole building. I have a vision of a city that has no visible streets at all, and is two stories up off the ground...any thoughts?