Ready Reference Files

Highway Numbering System

In 1925, the American Association of State Highway and Transportation Officials set the guidelines for numbering interstates and U.S. highways. Some of these guidelines are:

• Interstate north-south routes have odd numbers, with numbers increasing from west to east.
• Interstate east-west routes have even numbers, with numbers increasing from south to north.
• Interstate highway routes have one- or two- digit numbers.
• North-south interstates ending with a 5 and east-west interstates ending with a 0 are typically major cross-country routes.
• A three-digit interstate always ends with the two-digit number of the main interstate it loops off from, except I-238.
• Three-digit road numbers beginning with an even number are either beltways that go around a city or freeways that go through a city.
• Three-digit road numbers beginning with an odd number branch off the main interstate.
• U.S. highway north-south routes have odd numbers, with numbers increasing from east to west.
• U.S. highway east-west routes have even numbers, with numbers increasing from north to south.
• U.S. highway east-west routes ending in 0 tend to be cross-country routes.
• Three-digit U.S. routes contain the two digits of their parents routes, but there is not an odd and even number system.

See also: American Association of State Highway and Transportation Officials website: http://www.transportation.org/

Source: Summarized from an article in Via, March/April 2000. p.13.

Verified by: DS 6/00

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While the Library has verified the information presented in these files in what it considers to be reliable and authoritative sources, it cannot take responsibility for nor guarantee the accuracy of the information presented.