Paging the Fat Controller...
•The drone's aerial environment is 3D-mapped to a high degree of granularity: similar, perhaps to the roads mapped by Google's self-driving cars.
Problem being the road environment is much less prone to change than the low-level aerial one. Even things like trees growing, sprouting leaves and blowing in the wind for example would be much more variable, not to mention similar temporary hazards like flags/banners, washing lines and other nice drone-snares.
•Every flying vehicle in that environment is controlled by the system. That's the only way possible, we think, to rule out controlled vehicles crashing into un-controlled ones.
Even leaving aside the uncontrolled elements like birds, quite how would control be achieved? Everyone knows quite how difficult it is to get a signal into built-up areas (sat navs vs urban canyons for example), plus the tussling between the requirement for overall control vs local autonomy so any drone could avoid the uncontrolled obstacles already mentioned.
If this kind of automated air traffic control was so easy, we'd already have it for our existing transportation like cars (ok, not air traffic control, but general control) and more integrated with that for normal planes or even small ones.
Plus to carry anything worthwhile (ie more than just a single small package over a very short distance) you'd need a large and powerful drone which would probably be getting to the realms of a small helicopter equivalent anyway.