Doesn't tell all but PANSOPS has this to say:
1.1.2 Transition altitude
1.1.2.1 A transition altitude shall normally be specified for each aerodrome by the State in which the aerodrome
is located.
1.1.2.1.1 Where two or more closely spaced aerodromes are so located as to require co-ordinated
procedures, a common transition altitude shall be established. This common transition altitude shall be the
highest of the transition altitudes that would result for the aerodromes if separately considered.
1.1.2.1.2 As far as possible a common transition altitude should be established:
a) for groups of aerodromes of a State or all aerodromes of that State;
b) on the basis of an agreement, for aerodromes of adjacent States, States of the same flight information
region, of two or more adjacent flight information regions or one ICAO region; and
c) for aerodromes of two or more ICAO regions when agreement can be obtained between these regions.
1.1.2.1.3 The height above the aerodrome of the transition altitude shall be as low as possible but normally
not less than 900 m (3000 ft)
1.1.2.1.4 The calculated height of the transition altitude shall be rounded up to the next full 300 m (1000 ft).
Interesting study here recommending a single European TA (with some background to the current situation)
Eurocontrol
Don't know how current any of this is!
Isn't it 18k in USA?