The correct explanation is found in the 2nd paragraph of
SPUIs writeup.
From Chapter 6 of "Read Your Road," a publication of the Federal Highway Administration.
http://safety.fhwa.dot.gov/factsfigs/ryr/chapter6.htm, Accessed July 31, 2000.
Decoding the Interstate Numbering System
Knowing what the numbers on
Interstate shields have to tell you can be a great aid to navigation. Many motorists think they know how to read the numbers, but they may understand only part of the story. Other motorists are confused by the numbers; still others have never really thought about it.
When you're traveling in unfamiliar territory, Interstate numbers give you valuable clues to your location and direction, if you know how to read them. Here's the key:
One- or two-digit even- numbered Interstates are always east-west routes. The numbers increase from south (I-10) to north (I-94).
One- or two-digit odd-numbered Interstates are always north-south routes. Numbers increase from the West Coast (I-5) to the East Coast (I-95).
TIP: Since not all Interstates run due east-west or north-south, the closest cardinal direction that applies to most of the road is used. So, even if a stretch of an east-west Interstate shifts to the south for a while, it is still considered an east-west highway.
Three-Digit Interstates
Interstate highways with three-digit numbers connect to other major highways.
If the first of the three digits is an even number, the highway usually connects to another Interstate at both ends - often in a circular beltway or loop.
If the first of the three digits is an odd number, the highway is usually a "spur" route that connects with an Interstate at only one end, sometimes going into a city center.