(Keep in mind, I'm talking about rural areas more so than urban areas)
In East Tennessee, we have some highways (US and SR's) that are of near-Interstate quality with at-grade crossings, with barely any stoplights, if any. These are typically 2 lanes in each direction with a wide right shoulder and either a middle-turning lane or a grass median, with left-turn deceleration ramps.
I've been marking these routes, whether US or a Tennessee Route, as a Major highway. I've also, for aesthetics, have been trying to not downgrade these roads in construction zones or in areas where TDOT has not upgraded the road to this quality.
US-11 makes me scratch my head, personally, for example. It's considered major in all the TDOT long-range maps, and considered more major on the Official Transportation Map for 2012, but most of it is worse than some State Routes crossing it - but then again, it's parallel to an Interstate - so why upgrade it to a near Interstate-quality road?
Here is what US-11 looks like (for most of it outside of Cleveland and Athens, TN):
But, here is another US Hwy, 411 (parallel route quite a ways from I-75 and US-11):
(This is typical construction, from what I've seen, for the more major US and some Primary State Routes)
Similarly, this is SR-30, a east-west route:
This is the typical construction for most Primary State Routes (In this case, Highway 72 is one of a decent number of east-west Primary State Routes):
Last, but not least, Tennessee has many Secondary State Routes. This is Highway 322 - a literal east-west route:
After granting all this information, here is what I'm pressed to do (I've primarily have followed US=Major, SR=Minor, but I think that doesn't work all the time):
Mark roads like US-411 and SR-30 as "Major Highway" (even for stretches not completely up to par, but are under construction or will be in the near future), for aesthetic and functional reasons.
Mark roads like US-11, SR-72, and SR-322 as "Minor Highway": Basically, the quality of SR-72 is better than US-11 and SR-322 as a whole, and I'd like to mark it as such, but SR-322 and US-11 are still major routes - and are also on Transportation Maps. Marking it as a Primary Street would also conflict with local roads that are exactly that - local.