The old Indian trade routes would be one hell of an interesting study but I don't know how it could be done or if anyone has ever attempted it.
Take the famous San Antonio road which is just south of me @3 miles.
It would be foolish to think that when the Spaniards first traveled it from Mexico into what is now Louisiana they just struck off across the country in some general direction. Dollar to a donut they were following Indian trails with local guides who knew parts of it.
All the rivers and streams that had to be crossed and there are only certain places to ford easily. If the Spaniards just hit a river, say the Brazos, willy nilly they would waste a few days trying to find a crossing. Indians knew where the fords were.
