The subject has come up before, but this is a somewhat different treatment...
I think I like it. Roads become more of an improvement to civilization
than to travel, which is different than the other proposals.
Hmm, could I build a road _into_ a province that had a HOSTILE garrison
in it? Sounds kind of farfetched in real life, but certainly possible
under your proposal. Getting around this feels kind of kludgy, though...
Either we require that both provinces have the builder's garrisons (making
roads something only the large-scale empire builder does), or have non-
hostile garrisons, or require the command to be executed twice, once in
each province, and only complete the road when both have ended successfully.