Interstate Highway Numbers in the United States

Did you ever wonder whether there was a plan behind the numbering of interstate highways in the United States? It makes sense that someone has to handle it to avoid repetition or confusion. So how logical is the process and what are the rules?

The interstate highway network, or more properly the “Dwight D. Eisenhower National System of Interstate and Defense Highways,” extends more than 46,000 miles. It forms the largest collection of high-speed limited access roadway in the world. Obviously the sheer immensity of it requires sophistication and control. Thus the numbering scheme conforms to standards developed by the American Association of State Highway and Transportation Officials (AASHTO).


Underlying Principles

Map of current Interstates. Author SPUI based on National Atlas; in the Public Domain

The process follows a fairly basic set of rules. Understand, however, that exceptions exist. I may explore some of those in future posts.

  • East-west highways feature even numbers that increase from south to north. Interstate 10 runs from Santa Monica, California to Jacksonville, Florida. Meanwhile, Interstate 90 runs from Seattle, Washington to Boston, Massachusetts.
  • North-south highways use odd numbers that increase from west to east. So, Interstate 5 runs from the Canadian border at Blaine, Washington, to the Mexican border at San Ysidro, California. Interstate 95 runs from the Canadian border at Houlton, Maine to Miami, Florida.
  • Numbers divisible by 5, as in the the examples above, represent the most significant long-distance arteries.
  • Numbers not divisible by 5 may still be very important and carry a lot of traffic. However they generally cross shorter distances. These include Interstate 71 between Cleveland, Ohio and Louisville, Kentucky, and Interstate 44 between Wichita Falls, Texas and St. Louis, Missouri.
  • Some interstate highways have three digit designations. AASHTO considers these auxiliary highways. These are radial, spur or circumferential roads emanating from a nearby primary interstate highway. Generally they appear in urban areas. Spurs often start with an odd number while circumferential and radial loops often start with an even number. An example includes the famous Beltway, a circumferential road around Washington, DC. This carries the designation Interstate 495 (concurrent with I-95 on the eastern side of the loop).

Geo-Locating

Now that we know the code we can use it to figure out many basic geographic locations. Going back to the I-495 example, the “4” designates that this is an auxiliary highway springing forth from I-95 and returning to it at a later point. The “95” itself signifies that this as a particularly significant north-south highway in the easternmost section of the United States.

Exceptions to the numbering plan do exist. Actually the exceptions are as much fun as the code itself, so keep that in mind when playing this game.

Comments

2 responses to “Interstate Highway Numbers in the United States”

  1. Bill Harris Avatar
    Bill Harris

    The most infamous counter-example is I-99 in central PA and considered by some to be a political vanity project:

    http://en.wikipedia.org/wiki/Interstate_99

  2. Dan Tilque Avatar
    Dan Tilque

    There’s another exception to the 2-digit rules. I-82 (Oregon and Washington) is completed north of I-84. This was caused by them renumbering I-80N to I-84.

    In addition, there’s half a dozen or so places where two even or two odd numbered interstates cross each other, so making for partial exceptions.

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