Mike Rapoport wrote:

Pretty tough to see how you could keep the 172 farther from the 747 than
the 747 is from the 172.

Why must distance be a symetric relationship? I'm sure that a privatized
ATC would do away with such silly assumptions.

More seriously: that's not quite what I meant. I'm thinking of a "bubble"
of a certain size that must be kept clear. I'm sure there's an official
term, but I don't know it.

I'd imagine that this "bubble" needs to be larger around a fast-mover than a
slow-mover. In other words, a sky of 172s could be permitted to be more
densely packed than a sky of 747s.

At least, that's my assumption. I've no idea whether or not it's correct.

- Andrew