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Not sure if this will be on topic but I'd like to know anyway:

Why do mileposts on US Interstates increase South to North instead of North to South?

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3  
Does it matter? Its irrelevant whether a choice like this is arbitrary, so long as it's consistent. (Which it is!) –  LessPop_MoreFizz Mar 25 at 2:47
    
Inquiring minds wanted to know. :) –  Karlson Mar 25 at 3:01
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Conversely, why would they increase North to South instead of South to North (obviously the natural choice)? –  Relaxed Mar 25 at 5:48
    
@Annoyed South to North is a natural choice? Normally I would expect them to increase in the way your would write. Since it would be looking at the map and writing left to write and top to bottom. –  Karlson Mar 25 at 14:25
    
@Karlson I hoped the italics on “obviously” would make the irony clear enough. My point was that there isn't anything particular about either directions. But, this small quibble aside, I guess there should at least be some historical explanation. –  Relaxed Mar 25 at 15:04

1 Answer 1

up vote 7 down vote accepted

I assume the numbering of mileposts is to match the numbering of the U.S. Interstates themselves -- west to east interstates, which have even-numbers, are numbered from south to north (i.e. I-10 runs through southern states, and I-90 runs through northern states). Likewise, south to north interstates, which have odd-numbers, are numbered from west to east -- I-5 runs along the west coast, and I-95 runs along the east coast.

Note that this is the opposite of the US Highway system -- north to south routes grow larger from east to west. US Highway 1 is on the east coast, and the old US Highway 99 (which is now no longer a US Highway) ran along the west coast. Likewise, US Highway 14 runs through several northern states, and US Highway 82 runs across several southern states.

I believe the reason the Interstates were numbered opposite of the US Highways was to avoid confusion, i.e. when talking about route 5 along the west coast, you know the person is talking about an Interstate, not a US Highway.

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I don't think we can compare to the US Numbered Highway system simply because US 22 runs East West and US 202 runs North South. –  Karlson Apr 13 '12 at 18:52
    
    
@Karlson, the article you referenced says: The two-digit U.S. Routes follow a simple grid, in which odd-numbered routes run generally north to south and even-numbered routes run generally east to west. Three-digit numbers are assigned to spurs of two-digit routes. Not all spurs travel in the same direction as their "parents". My answer was about the major US Highways routes -- the two-digit ones, which do follow the Interstate numbering plan as far as odd/even. I didn't get into the three-dogot spurs for either system. –  tcrosley Apr 13 '12 at 22:30
    
You're confusing the mileposts and the route numbers. –  littleadv Apr 14 '12 at 6:08
    
@littleadv, I'm not confusing the two, I'm saying that both were consistently numbered in the same direction (south to north, and west to east). –  tcrosley Apr 14 '12 at 11:10

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