As 1 cites, the proximity of the similarly-deployed I-85 was the determining factor here. However, I for one would have preferred AASHTO have & use a policy that states that if there is an unused number (or a set of them) available for an Interstate corridor, such a number would be applied to the corridor; the only time a duplicate number would be designated would be if none other were available. Since the HPC 13 corridor could be realistically construed as either east-west or north-south, an even number from 46 to 62, subtracting US-route duplication, would be applied. Period.
The duplications of 76, 84, 86, and 88 wouldn't apply as precedent (even 74, although that's a bit of a stretch), because no unused numbers were available for that area of the overall grid.
Since the original "89" north-south suggested designation was more or less pulled out of a hat to satisfy NC's no-nearby-state-route-duplication internal policy, 89's characteristic as a north-south facility was less a deliberate choice than a number chosen for in-state convenience. In this instance, AASHTO overlooked the obvious and "rubber-stamped" the N-S characterization of this corridor although substituting an odd number more to their liking (maybe their vetting process needs a few adjustments -- or maybe the hotel bar at their meeting place shouldn't be opened that early!).
Perhaps it was just naivete on my part, but prior to this instance I had presumed that there actually was an internal policy with AASHTO (and FHWA, for that matter) that discouraged or even prohibited duplicate trunk Interstate designations unless no other number was available; the fact that the duplicate numbers in the '70's and '80's fit that description exactly -- and had persisted since the demise of single-ended suffixed routes some three dozen years ago -- formed the basis for this assumption. Also, due to the decidedly rectangular physical form of the lower 48, I also thought it would be a cold day in hell before I saw a duplicate odd number. I suppose the corresponding designation of I-42 -- previously unused & grid-appropriate -- serves as something of a Pyrrhic victory re my prior assumptions.
