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

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?

• Does it matter? Its irrelevant whether a choice like this is arbitrary, so long as it's consistent. (Which it is!) Mar 25, 2014 at 2:47
• Inquiring minds wanted to know. :) Mar 25, 2014 at 3:01
• Conversely, why would they increase North to South instead of South to North (obviously the natural choice)? Mar 25, 2014 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. Mar 25, 2014 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. Mar 25, 2014 at 15:04

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.

• 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. Apr 13, 2012 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. Apr 13, 2012 at 22:30
• You're confusing the mileposts and the route numbers. Apr 14, 2012 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). Apr 14, 2012 at 11:10

Comparing milepost numbering to the route numbering of the US Interstate Highway System is wholly unsatisfactory, as the practice of numbering from south to north predates the Interstates. At first I thought it was following the precedent of US 1, which famously has mile marker 0 in Key West. Then I thought maybe the tradition went back to the Boston Post Road, but then I realized that many of these early American roads (the National Road, Lincoln Highway, etc. in addition to the Boston Post Road) had literal milestones that marked the distance remaining to cities of general interest. So, for example, on the National Road traveling westward from Baltimore, one would see the miles counting down to Wheeling, WV; then Columbus, then Dayton, and so on. But eastward, they would count down to these same cities in reverse order, finally ending in Baltimore. So clearly this wasn't it.

Finally, I came across another website that made the point that mathematical graphs count up as one moves to the right on the X axis, and upward on the Y axis. Plotting this along most maps, one would see numbers increase as one moves to the north and east. So it seems that's the answer: Because highway engineers decided to follow the convention set by mathematicians.