I had to do a little research on the numbering of interstates for my book "Diary of a Future Politician"
I created a fictitious town called Riverbend USA, which had an interstate. I designated the highway as I-XX. My editor suggested I give it an unassigned number. But even the unassigned number would give a clue as to where Riverbend could be or could not be located. I decided to keep the mystory and stayed with I-XX.