Seems pretty simple so far right? Well we then get to the exciting issue of the 3 digit Interstates. Most Interstates have children when they reach major cities. If the baby Interstate goes through the city or all the way around the city, then it will start with an even number. If it stops somewhere in the city, then it gets an odd number. For example, in Pittsburgh I-279 loops from I-79 into the city and then reconnects with it further north. In contrast, I-376 leaves I-76 to go into Pittsburgh and ends downtown. In principle this shouldn't be that big of a deal, but in practice it is frequently debatable as to whether the first number should be even or odd. Some highly questionable choices have been made, which annoys a lot of us road geeks (and don't even get us started on I-238).
Anyway you are now either ready to plunge into the road listings or you now know that you made a wrong turn and should just return to the International House of ZZYZX.