What the Numbers Mean on Interstate Highway Signs

December 2, 2022

Interstate highway signs

Interstate Highways are the most heavily traveled roads in the United States, and a massive number of drivers use them almost every day. But did you ever stop to wonder what the numbers on the signs mean, and why they are numbered the way they are?

In case you didn’t know it, Interstate Highways in the United States that travel east to west are given even numbers, and north to south routes are assigned odd numbers. Odd route numbers increase from west to east, and even routes increase from south to north.

Interstate Highway routes don’t have to cross state lines to be designated as an Interstate Highway. Two such examples are the three roads that are Interstate Highways in Hawaii and Interstate 45 that runs from Dallas to Galveston in Texas. There are many other examples of Interstate Highways in many other states that never cross a state line. Any roads under the Federal-Aid Highway Act that receive federal funds are considered Interstate Highways.

The three-digit Interstate Highways (also called Auxiliary Interstate Highways) around urban areas have their own meaning. The last two numbers indicate the parent route, while the first digit indicates if the road is going directly to a city (which is an odd digit), or looping around the city (an even digit). Of course, there are a few exceptions.

interstate highway numbers

The Interstate Highway System, which is also known as the Dwight D. Eisenhower National System of Interstate and Defense Highways, got its start in the late 1930s. It began as a study of a system of superhighways that would be supported by tolls. The Bureau of Public Roads, which was the predecessor of the Federal Highway Administration, found that a toll system wouldn’t be able to support the road network.

In 1944, the National Inter-Regional Highway Committee proposed a 33,900-mile system of roads with an additional 5,000 miles of urban routes. The Federal-Aid Highway Act of 1944 was enacted by Congress, and in 1947, the first 37,700 miles of road were selected.

Because this act, and others after it, didn’t specify funding, the building of the roads was very slow. It wasn’t until the Federal-Aid Highway Act of 1952 when funding was specifically granted for construction.

interstate highway numbering
How Interstate Highways are Numbered Devinhorn/Wikimedia

President Eisenhower and the Federal-Aid Act of 1956 were the true catalysts for the full construction of the Interstate Highway System. Federal taxes from gas and vehicle user fees were used to pay for the federal portion of construction and are used today in the Highway Trust Fund. One of Eisenhower’s requirements was that the entire program would be self-financed without adding to the federal deficit.

The longest Interstate Highway route, I-90, runs from Seattle, Washington to Boston, Massachusetts, for 3,020 miles, while the shortest, I-87, runs 12.9 miles from Raleigh, North Carolina, to Wendell, North Carolina. The shortest signed three-digit route is I-110 in El Paso, Texas, at 0.92 miles, and Texas has the most mileage at 3,233 miles of Interstate Highways.

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Daniel Ganninger - The writer, editor, and chief lackey of Knowledge Stew, the author of the Knowledge Stew line of trivia books, and editor of Fact World and the Knowledge Stew sister site on Medium, our ad-free subscription sites (you can find out how to join below). I hope you find things here to annoy those around you with your new found knowledge.

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