I'm done with the research and will probably begin drafting the routes this week. Based on the comments, for now I will plan on treating usatn as a single system, rather than separate primary/secondary. Looking at the routes in GMSV, it's obvious that signage is inconsistent, with primary and secondary signs occasionally being used for the same segment of a route.
How to deal with the unsigned routes isn't as clear, and my solution will probably not please everyone.
- A totally unsigned route that is signed as another route (e.g. TN26, which is signed as US70 between Lebanon and Sparta) will be ignored, even though its number appears on mile markers.
- A totally unsigned route that is not signed as any other route (e.g. TN300, a short freeway in Memphis) will be ignored. I looked for mile markers on TN300, to see if they mentioned the number, and found none. It's less than 2 miles long.
- A partly signed route that is not signed as another route (e.g. TN 8 within Chattanooga) will be included, even if signage seems old or only occurs at one end.
- A route that is signed by itself, but not signed when following another route, will be split into separate routes except when:
- "TO" signage is included at at least one location where the route ends
- the segments of the route exist a short distance from each other, such as being within the same county, and don't follow different routes in between
- the unsigned segment is a short segment of an otherwise long, consistently signed route
An extreme example of a split route deserving separate segments is TN66, which has three separate 0 mile markers over a very short (but convoluted) distance. But that is extreme, and I think there's only one other example of a three-part route (TN77). Most of the routes will only have one segment, and about thirty will have two. Overall there should be around 425 routes, including the unnumbered part of the James White Pkwy, which ought to be in usasf.