Fortunately, the chances you cite are not criteria for NAS design.

In engineering a workable NAS I would prefer that the designers employ
methodologies that _insure_ separation of air traffic, not merely
reduce the _chances_ of a MAC. Anything less is irresponsible
negligence.

In any system, there's always a small probability that a catastrophe will
occur. Aircraft certification rules and separation standards acknowledge this
and are established to keep the risk acceptably low. For example, for lateral
separation of two aircraft traveling at the same flight level on parallel
routes, the Target Level of Safety (TLS) set by ICAO (with FAA participation)
is 5 x 10^-9 per flight hour. That is, loss of lateral separation should
lead to no more than one accident every 200 million flight hours. The TLS is
not zero. Some people don't like to accept this, but it's just not realistic
to insist on zero risk.

Barry