I assumed when I was young that the high road was simply the road that was “higher”, e. g. the road that went over the hills rather than along the stream. But the high road is high in the sense of being the main road, or the legitimate road. It’s the road that is not a short cut or a back road. The high road is merely the highway of modern parlance.
Looking through Google Books, it seems the use of “the high road” to designate the moral ground is very new. For most of history the “high road” was just the shortest way.
No one actually knows what the low road and high road are in the beautiful 18th century ballad Loch Lomond. But it adds an interesting twist.
In the song as read by most, the high road is still the main road; the low road, however, is the route that conveys the spirit, a secret spiritual highway for the dead to find their way home.
Under this interpretation, the dead soldier says to his friends, walking defeated among the main road, that they are both returning to Scotland in their different ways, them on the high road, him on the low. And bittersweet — he will be home before them, but only returning to a world lost to him.
O ye’ll take the high road, and I’ll take the low road,
And I’ll be in Scotland afore ye,
Where me and my true love will never meet again,
On the bonnie, bonnie banks of Loch Lomond.