This question is about the numbering scheme of U.S. interstate highways. Wikipedia says:
In the numbering scheme for the primary routes, east–west highways are assigned even numbers and north–south highways are assigned odd numbers. Odd route numbers increase from west to east, and even-numbered routes increase from south to north (to avoid confusion with the U.S. Highways, which increase from east to west and north to south). [...] Primary north–south Interstates increase in number from I-5 [...] to I‑95 [...] Major west–east arterial Interstates increase in number from I-10 [...] to I-90 [...] with two exceptions. There are no I-50 and I-60, as routes with those numbers would likely pass through states that currently have U.S. Highways with the same numbers, which is generally disallowed under highway administration guidelines.
A relative of mine reports that in school they were taught that approximately 10 percent of the U.S.'s land area is south of I-10; 40 percent is south of I-40; 90 percent is south of I-90; 5 percent is west of I-5; 55 percent is west of I-55; 95 percent is west of I-95; and so on.
For the listed routes, to what extent is this literally true? (Does it become more true if you include only the continental U.S. and exclude Alaska?)
To what extent was this land-area thing actually a consideration when they were picking the numbers and/or routes for the interstates, beyond the considerations given in the Wikipedia quote above?