Sure it's anecdotal, I just heard it from my parents as a kid. And obviously it's not exact, just look at how close I-94/I-90 are around North Dakota and compa

Sure it's anecdotal, I just heard it from my parents as a kid. And obviously it's not exact, just look at how close I-94/I-90 are around North Dakota and compare it to I-90/I-80 or I-80/I-70. And a lot of the "East-West" interstates go diagonally or even North-South in places. But I doubt they made the roads by laying rulers across the map (except maybe in Kansas or Oklahoma), and it does seem like they generally increment from 0-100 in each direction with odd/evenness decided by the primary axis. Wikipedia has a lot to say on the matter, but I got bored reading all the special cases like how they number loops that connect two highways: https://en.wikipedia.org/wiki/Interstate_Highway_System#Numbering_system https://en.wikipedia.org/wiki/Interstate_Highway_System#Numb...