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.
Actually the Spanish were ordered by the Viceroy of New Spain to use the established Indian trade routes. They were specifically told NOT to blaze any new routes. And yes there have been several studies done. Primarily by Herbert Eugene Bolton back at the turn of the 20th century,and more recently by folks like Al McGraw and John Clark. Back in the late 90's TxDOT published an extremely large study on the Camino REALS! Yes plural. It is still in print and still available!
The actual "San Antonio Road" as you mentioned were a series of routes that were totally dependent on the time of year they could be traveled. Weather conditions played in importantly. Remember the Spanish exploration and on into the 19th century was during a 500 year phenomenon known as as "The Little Ice Age", which many believe ended in the 1850's. Below San Antonio de Bejar, there have been identified three specific routes.
And several elsewhere between Bejar and Presidio Los Adaes in western Louisiana.
