Alternatively, you might build flying cars. They'll use more fuel/energy per passenger mile than cars on the face of it - but because they don't sit about in traffic jams and travel as the crow flies, you'll probably get an economy/emissions win straight off. (Yes, airliners sit in traffic jams: but this is because they need big runways and make a lot of noise, so they can only land at a very few locations. Everyone sharing a few big machines sometimes doesn't work as well as letting people have their own small ones.) Better still, with flying cars you don't need to build new roads to any serious degree, either in urban areas or rural undeveloped ones. You've saved a very large proportion of the road miles people would otherwise drive.
Overall, you save on environmental impact. The flying car in isolation doesn't appear to be very green, but in the bigger transport picture it's a pretty green idea. The undeveloped, car-deprived world might thank you, in fact, for allowing them to skip out the massive-roadbuilding stage of industrialisation and move directly to the point where people mostly fly. No need to lay ugly ribbons of energy-intensive, high-maintenance tar all over the planet, so that everyone can reach shops and hospitals and schools in a timely fashion; no need for everyone to suffer gridlock. Move the heavy freight on the surface if you will - though actually there may be no need to - but human beings should aspire to higher things. A clever, automated air-traffic infrastructure able to handle swarms of flying cars is surely a more elegant solution than another six lanes added to the motorways.
Of course, all this only works for flying cars which ordinary-ish people can afford and use. That means you need to make things as simple as possible for the designers. Building an affordable machine which takes off quietly in 100m is already very hard; screwing down on fuel economy or demanding that it run on batteries will make it impossible, certainly for a long time.
That would surely be a case of penny wise and pound foolish, if you actually care about saving resources and energy and green space - as opposed to merely enjoying a bit of pious, pseudo-green short-termism. ®
*And to a large extent, the rest of us too. We all fly Cessnas; and indeed last year's PAV challenge was won by a Czech-made aircraft flown by an Australian.
**Some contenders at the NASA event are actually very economical or even electric: Pipistrel motor-gliders can cruise at 50 miles per gallon, and there's a battery option - though this last can't stay up sustainably unless the thermals are in your favour. However, these airframes will never be short-takeoff, roadable PAVs. The planned NASA Tailfan would have been driven by a thirsty-looking V8 car engine. The Terrafugia Transition roadable is probably more representative of what could be expected, at 25 mpg.