>>>300m is vertical separation . I'd be very scared if jets flying over my house where 300m apart laterally. I think lateral when approaching to land the minimum is about 3 miles.
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>Certainly many here will have landed at a busy airport and seen other aircraft flying much closer than 3 miles away. The point is that thinking in 3D, fixed wing aircraft can safely fly with only 300m separation or theoretically less if there's special airport control radar or automated landing. Helicopters and more maneuverable flying machines need a lot less. There's no scientific reason why highly maneuverable drones need to be 300m apart especially with the very high degree of automation/co-ordination proposed by Amazon. I nominated 5m separation, bearing in mind that road vehicles weighing tons and driven by tired humans with no transit co-ordination and slow biological reflexes can safely proceed with very little lateral separation on a 2D motorway (especially if it's the M5 at 5:30pm.) Allocating a generous 5m in all three axes for drones with split second reactions and 3 dimensions in which to avert collision, you get 40,000 simultaneous drone movements in each square km layer of space. That's a lot of Amazon deliveries. ;-)

Society may well impose conditions on drones that could be serious technical challenges. For example, I can foresee ordinances that say "thy drone shall not be noisier than a standard delivery van". With the latter becoming increasingly hybrid or even all-electric that might be tough for today's quadcopter-type drones to meet.

For high drone densities you'd need to look at the behaviour of flocks of birds or schools of fish but emulating those imposes some restrictions:

- uniformity of the individual units - difficult to achieve with drones from competing manufacturers
- uniformity of direction or goal - not what you want with 40k drones ;)

The mechanisms at work with those more closely resemble cellular automata than conventional air traffic control.