From ICAO 4444 CH 4.10
Note 1.— Unless otherwise prescribed by the State concerned, the lowest usable flight level is that flight level which corresponds to, or is immediately above, the established minimum flight altitude.
So regardless of separation with aeroplanes, avoiding the ground is the reason for the lowest usable flight level/transition level.
I think it's 17,000 ft TA in the US and Canada, making FL180 usable...
No, they have a far less complicated system there, and have dispensed with most of the silly titles for things. AIM 7-2-2 says use QNH below 18,000 and set standard when climbing through 18,000. Set QNH when descending through the lowest usable flight level, (which they don't define, just list).
http://www.faa.gov/airports_airtraff...7/aim0702.html It blathers on later about how to fudge a lowest usable flight level when the minimum altitude is above 18,000ft. VFR can fly at 17,500FT there, so get it right!