[nerfhammer]

I think it's triple digits that become a mouthful with the "I-" prefix. Double digits sounds best with the "I-", I think.

Triple digits come from being a spur of a two-digit highway incidentally, not to do with region. I-385 is a spur of I-85, etc.

[InitialLastName]

To ultra-pedant it up, generally only odd prefix digits indicate a spur (only connects to the prefixed highway in one spot), where even numbers indicate a bypass or beltway (which intersect the main highway multiple times).