FWIW, signs in Charlotte completely ignore Atlanta, even though that is the next major city on I-85 south, and is a hell of a lot bigger than either Spartanburg or Greenville. Same deal with GDOT mentioning Greenville instead of Charlotte on I-85 north after it has split off I-75.
Although I do suspect that it's probably because of that fierce rivalry going on between the two cities...hey, if VDOT can put Atlanta and Miami on signs, then why won't NCDOT do the same? Well, come to think of it, Miami gets a pass because you have a much longer way to go before you make it there anyway, but leaving off Atlanta is certainly no excuse.
The determination of which control city to name on signage has nothing to do with the biggest or most influential at a further distance.
The only reason Miami, Durham and Atlanta are mentioned in VA Southbound is because:
!) I-95 is arguably the most important highway in the country and at Petersburg it forks with the start of another very important corridor through the Southeast passing through 20-30 million people and it's one of the main industrial corridors
It wouldn't make sense to mention the end of I-85 because that's not a major destination. Atlanta is the biggest region along 85 and somewhat near it's end.
Miami and Atlanta are good indicators of how this fork of two East Coast interstates will send you in different directions.
Greenville Spartanburg are perfectly appropriate control cities whether you're coming from the South or the North.
The gist was that NC is so full of itself that it will avoid using out of state control cities all the way until the borders as if no world exists outside of the state that's worth naming, lol.
My hometown of Louisburg is mentioned on I-95 and I-85, on I-95 it's merely because it's the the end/start of NC561, (NC581 too). On I-85 it's mentioned because it and Franklinton are close on NC56 and technically they are part of the Triangle.