Highway Numbers

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Highway Numbers

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Cynthia Shi, Editor/Journalist

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You may have wondered how the United States interstate highways are numbered, such as Highway 101. Perhaps it could be a random number? Well, these numbers do play a significant part in recognizing where these highways go, especially when road maps were not as accurate as they are today. It is officially known as the Dwight D. Eisenhower System of Interstate and Defense Highways, and the government authorized spending and construction in 1952. The most basic logic of these numbers are:

  1. Routes with odd numbers run north-south across the United States
    1. The lower numbers are closer to the south
    2. For example, Highway 5 runs north-south along the western coast
  2. Routes with even numbers run east-west across the United States
    1. The lower numbers are closer to the west
    2. For Example, Highway 10 runs east-west in the southern parts of the United States.


Apply this logic to some of the following major transcontinental routes, and you will notice how they match up to the rules:

East-west Transcontinental Routes:

Interstate Number From To
I-10 Los Angeles, CA Jacksonville, FL
I-80 San Francisco, CA Teaneck, NJ
I-90 Seattle, WA Boston, MA


North-South Transcontinental Routes:

Interstate Number From To
I-5 San Diego, CA Blaine, WA
I-15 San Diego, CA Sweetgrass, MT
I-35 Laredo, TX Duluth, MN
I-55 New Orleans, LA Chicago, IL
I-65 Mobile, AL Gary, IN
I-75 Miami, FL Sault Ste Marie, MI
I-95 Miami, FL Houlton, ME


Major transcontinental routes intersecting with major urban area beltways (highways encircling an urban area around the city) receive three-digit numbers. They are given an even number prefix and the number of the main route. For example, if I-80 passes through three cities, routes around theses cities may be known as I-280, I-480, and I-680. When these routes rejoin, they form I-80 once again. This system is not carried across state lines, so two states may have different beltways both known as I-280.

Another important aspect of this system are interchanges, or exits. Their numbering involves two methods that the state chooses:

  1. The consecutive numbering system starts off at the most western or southern point on each interstate route, and the interchanges are numbered consecutively. This means the first interchange would be #1.
  2. The milepost system numbers the interchanges according to the number of miles there are, once again starting off at the most western or southern point. Therefore, an interchange that occurs between mileposts 5 and 6 would be named interchange #5.

Now that you know how the highways of the United States are numbered, you can this information to help you navigate where you are going. Of course, that doesn’t mean you shouldn’t use a map. Always refer to a map before you rely simply on the highway numbers. However, you can use this knowledge to find the right route on a map to plan a trip. If nothing else, you can always remember this as a fun fact!





U.S. DOT, Federal Highway Administration [through the two sites above]