Why did you upgrade this road to primary? The way I see it, neither Drakes Branch is not significant enough on its own to warrant a connection to the network of primary highways by OSM standards.

What I mean by "town center" is the center of commerce, or "downtown", not the geometric center. For some cities and towns, such as Anacortes, this location is not at the geometric center of the town.

I am not using any external data/statistics. I am identifying minor collectors with aerial imagery (to check road quality) and by considering what the most logical route through an area would be, then marking that road as tertiary.