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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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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)? –  Annoyed 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. –  Annoyed Mar 25 at 15:04
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2 Answers

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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From Wiki on milestones in the US - although it's non-cited, that doesn't mean it's not true.

In the U.S. Interstate Highway System, the numbers usually measure the distance to the southern or western state line, while other highways use the county line as the benchmark.[citation needed] Often, the exits are numbered according to the nearest milepost, known as the mile-log system. Some historic and scenic routes – such as along the Blue Ridge Parkway in North Carolina and Virginia and the Overseas Highway of the Florida Keys – use mileposts to mark points of interest.

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One could assume that the numbering west to east on the even numbered interstates is because when you put the map down you write left to right. That's why I specifically asked about South to North because if we follow the map rule the numbering and mileposts should increase west to east and north to south. –  Karlson May 23 '12 at 21:31
    
that's why it also says from the south state line. So if you're close to the south line, the numbers will be low, and in the north high - ie, increasing from south to north. –  Mark Mayo May 23 '12 at 21:48
    
but but but... the OP asked why it is so... –  littleadv May 24 '12 at 7:57
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