Transition Altitude and Transition Level will vary as a function of the terrain around the airfield.
So in UK for example, TA could be as low as 3,000 ft. (An exception is the London TMA where it is standardised at 6,000 ft).
Some countries have standardised TA on a countrywide basis. The USA has a standardised TA of 18,000 ft which I guess is the height of Mt St Helens + 1500 ft + 10% or some such formula. Therefore, you are safe changing at 18,000 ft wherever you happen to be in the USA. Certainly it keeps it simple.