Interstate Highway Numbers in the United States

On June 2, 2008 · 1 Comments

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, but 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 is the largest collection of high-speed limited access roadway in the world and the sheer immensity of it requires sophistication and control. The numbering scheme conforms to standards developed by the American Association of State Highway and Transportation Officials (AASHTO).

United States Interstate Highway System
Source of image: (released to the public domain)

The process follows a fairly basic set of rules (recognizing there are some exceptions that I may explore in future posts):

  • East-west highways have even numbers that increase from south to north. Interstate 10 runs from Santa Monica, California to Jacksonville, Florida, and Interstate 90 runs from Seattle, Washington to Boston, Massachusetts.
  • North-south highways have odd numbers that increase from west to east. Interstate 5 runs from the Canadian border at Blaine, Washington, to the Mexican border at San Ysidro, California, and Interstate 95 runs from the Canadian border at Houlton, Maine to Miami, Florida.
  • Numbers that are divisible by 5, as in the case of the examples provided above, are used for the most significant long-distance arteries.
  • Numbers that are not divisible by 5 signify roads that are still very important and may carry a lot of traffic, but generally for 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. These are considered auxiliary highways. These are radial, spur or circumferential roads emanating from a nearby primary interstate highway and are found mainly in urban areas. Spurs generally start with an odd number while circumferential and radial loops generally start with an even number. An example of this would be the famous Beltway, a circumferential road around Washington, DC, which has been designated Interstate 495 (concurrent with I-95 on the eastern side of the loop).

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 and the exceptions are as much fun as the code itself, so keep that in mind when playing this game.

On June 2, 2008 · 1 Comments

One Response to “Interstate Highway Numbers in the United States”

  1. Bill Harris says:

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

Comments are closed.

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