The semantics aren't exactly meant to be "% of land area", but they do represent a rough coordinate system[1], from (5, 8) in the SW corner of the contiguous US (San Diego) up to (95, 90) in Boston.

Interestingly, US highways follow the opposite system[2], going from (1, 2) in the NE corner of the country (Maine) to (former) (101, 80) in San Diego.

[1] https://en.wikipedia.org/wiki/Interstate_Highway_System#Numb...

[2] https://en.wikipedia.org/wiki/United_States_Numbered_Highway...

Those us routes came first and the interstates are purposely numbered backwards of it to avoid confusion.

Numbering from west to East and south to north and using evens and odds to mean East-west and north-south makes sense.

But that doesn’t imply anything about proportion of country, however the measurement is defined.

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#Numb...