Mile-based exit numbers are better because numbering sequentially causes problems whenever they wanted to build a new exit between existing ones 'cause you can’t just go around renumbering them all the time. Even resorting to adding letters (e.g. having exit 42A, 42B, etc.) only gets you so far (although they still do that with mile-based numbering, for multiple exits within the same mile).
The Interstates in my state used mile-based exit numbers but state routes do not, and I remember when GA 400 got renumbered because they added an extension with four extra exits on to the beginning of it.
















A “highway” is anything that isn’t a local street. Every state or federally maintained road* is a “highway,” whether it’s a controlled-access freeway or just a two-lane road.
(* except Forest Service roads, obviously)