Actually if you combine I-17 and I-19 in Arizona, the combined road would still not cross State Lines. According to the numbering system, a better option would be I-110,I-310,I-510,I-710, or I-910, since they branch off from I-10. Although I-17 could also be a spur of I-40. And the section of I-17 that is south of downtown Phoenix could be considered a loop giving it a I-210, 410, etc, designation.
But I think both highways were numbered as such for strictly political reasons. By using I-17 and I-19 Arizona has 5 major Interstate Highways so we are a progressive developed State.
