The enabling tech is battery density. Battery density (per kg) has been increasing 5-8% a year (doubling every 10 years). That seems slow compared to transistors (doubling every 2 years), but compound growth sneaks up on you. Enough that an electric car seemed dumb 20 years ago and obvious now.
Urban human quadcopters are unlikely, a car like number of these in cities is hard to imagine. Even with silent electric motors, the noise from take off and landing from air displacement exceeds safe noise levels, and is far above nuisance levels. So I'd guess 2-30 times as much traffic as current helicopters. Also they can and will fall down on expensive stuff and nice people.
The more plausible market is medium range 4 person air taxis, from small airport to small airport, 50 to 400 miles. That's NYC to Boston, SF to LA, Portland to Seattle.
This Boeing story says 50 miles. Pipistrel's Alpha Electro, a 2 seater plane, has a 93 mile range with another 45 miles as reserve (138 total).
In 2029 with similar battery improvement, that will be 231 miles with 45 miles in reserve. The regulatory challenges will be difficult, but in about 2025 you could launch a viable autonomous air taxi company somewhere in the world.
Existing advantages:
- Drone autonomy is already or easily solved, so you don't pay for a pilot.
- Electricity cost is low.
- Maintenance costs are much lower than gas engine planes.
- Even with maintenance, gas engines are less reliable.
Better battery tech unlocks these last three advantages and makes air taxies feasible.
[1] https://www.aopa.org/news-and-media/all-news/2015/october/pi...