Why we look at dirt first, then paper.
Muscle powered transportation is my definition of "pre-modern" and those are the defining technologies for the old traces the Trading Path Association seeks. But (and this is a major but) muscle powered transport persisted well into the industrial age and the farther into that age it persisted the farther the roads deviated from their original course. For example, railroads caused wholesale rerouting of local roads and extinguished many a town that thrived until snuffed by cessation of wagon commerce. So, by the time scheduled bus service (stagecoaches) entered the backcountry there is no knowing where the original roads went. This is one reason that I pretty much reject the normal process of history which consists of looking for documents and then finding whatever was referred to in the documents.
First, the historic document set is very incomplete. Second, what documents remain are scattered between various archiving authorities and it is not inexpensive to do an exhaustive document study. Third, documents frequent obscure more than they reveal and sometime outright lie. Documents are quintessentially secondary sources. They are mere description of a physical or legal fact. As you well know from your personal life, the vast majority of your most important activities, with any luck at all, never get recorded on a public document. Those same important acts, though, frequently leave marks on the ground.
Dirt never lies. It may be disturbed or incomplete but, marks on the land are more than anything else, primary resources. So, the TPA seeks the marks and only then does it turn to documents to explain what the mark might mean. That way we think we'll find and understand more than if we start with secondary materials and work back toward the dirt.
I hope this gives you an idea of our methodology and its logical underpinnings.