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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?

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    Hi @Quuxplusone, welcome to Skeptics Stackexchange. This site is mainly for challenging notable claims. "My [friend/relative/..] said ..." is generally not notable. Do you have a website that makes this claim? Then please post a link to the website. Also, please describe any research you have already done yourself regarding the topic. More info on what a notable claim is you can find here. – redleo85 Feb 8 at 11:52
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    I highly doubt it was a consideration when they were numbering the highways, considering they just started south and went up numerically. I could see them skipping numbers if there was a large enough gap, but if you draw 9 lines across the US and number them 10-90 and space them out evenly, there's probably gonna be approximately X% of the US south of interstate X because that's just how math works. – DenisS Feb 11 at 16:22
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    Furthermore, I want you to compare the routes of I-10 and I-90. There is a lot more land north of I-90 than there is south of I-10. South of I-10 is FL, 1/3 of TX, and small slivers of CA, AZ, NM, LA, AL, and MS. North of I-90 are the entire states of ME, VT, NH, MI, ND, and large portions of WI, MN, SD and MT (not to mention parts of NY, MA, ID, and WA. – DenisS Feb 11 at 16:26
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    And that doesn't even include the problem children of this whole claim, like for example I-84, which actually designates two seperate sections of highway, one going from Pennsylvania to Massachusetts and another going from Oregon to Utah. I-82 is another example, it's a 100 mile segment in OR/WA that runs more north/south, in spite of the fact that even numbered highways are supposed to go east/west. – DenisS Feb 11 at 16:43
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    4.3% is approximately 10% for a given definition of "approximately" (a.k.a. order of magnitude). That's a massive problem with your question (regardless of the extreme notability concerns). Each insterstate that is a multiple of 10 is going to divide the US into approximately 10% because cutting any object into 10 slices is going to leave 10% between the slices. Of course the highways are going to be relatively equidistant from each other because there's no reason to run 10 highways along the southern border and non anywhere else. – DenisS Feb 11 at 18:37
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The numbering system is sequential. It is not a measurement system for land area. The scheme requires that I-80 is farther north than I-10, I-20, and so on. But it does not prescribe that I-80 be a specific distance north of I-10 for land-area-measurement.

Interstates are very roughly evenly spaced because of the places they connect.

The federal information about the interstate highway system is here: https://www.fhwa.dot.gov/programadmin/interstate.cfm

The route planning was supposed to connect major metropolitan centers, which would argue against equal-land-area division as a primary design criteria (emphasis added):

In the Federal-Aid Highway Act of 1944, the Congress acted on these recommendations. The act called for designation of a National System of Interstate Highways, to include up to 40,000 miles "... so located, as to connect by routes, direct as practical, the principal metropolitan areas, cities, and industrial centers, to serve the National Defense, and to connect at suitable points, routes of continental importance in the Dominion of Canada and the Republic of Mexico.

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    TLDR: Given the clear intent to have interstates highways reasonably evenly distributed across the country with north-south and east-west directions, with sequential numbering, it would be surprising if the numbering DID NOT crudely correspond to the land area spanned. – Daniel R Hicks Feb 21 at 3:06
  • @DanielRHicks: Indeed, I should have found a way to include basically that in the answer. – user8356 Feb 21 at 14:31

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