What were high ways in pre-modern times?
In the 1790s the US Postal Service published standards to be met to qualify to be a postal coach stop. Among the first was a requirement that your establishment must be situated on 'an all season, all weather road.' By this, I believe they meant on a high road or high way. So what did that mean? What was a high road or a high way? In short, I believe it meant a road that passed streams high in their course where a passage was seldom if ever obstructed by flood and hewed as close as possible to ridge tops.
Highways probably shifted over time as transportation modes and road technology changed. For example, as draft animals grew ever larger in the 19th century so too did payloads. The increased weight tore up dirt roads so counties imposed tire dimension minimums; the heavier the cargo the wider their iron tires needed to be. And the greater the cargo weight the more restricted will be the options for fording streams; hence a route change.
As an example of a high impact road technology change, culverts will serve. Until the arrival of corrugated steel piping and half-piping culverts were made like mini bridges, wooden planks laid down over stacked rock piers. Like bridges, in the Southeast of the US, they rotted and needed to be rebuilt about every seven years if they didn't wash away in the meantime. The "Nissen hut" made such bridging obsolete.
During World War I a British officer named Nissen devised a portable easy to erect structure using a corrugated steel half pipe. It didn't take troops long to figure out that sinking a Nissen hut it in a trench and covering it with dirt or sandbags would shelter you from more than rain. Wartime demands increase production so that after the war there was a considerable excess of halfpipe around and it came to be a staple farm building in most of the British Empire.
Americans picked up on the value of corrugated steel during the war and it didn't take long before corrugated steel pipes and half-pipes replaced wood and stone culverts. The low cost and excellent use-life allowed culverting in places where it was previously impractical, especially along the base of a ridgeline. Ridge roads quickly slipped from ridge tops down to the ridge bases after World War I; hence routes changed.
Oh, and the Nissen hut was rebranded for American consumption as the ubiquitous Quonset hut in recognition of the site where they were built at Quonset Point in Rhode Island.