On June 29, 1956, the National System of Interstate and Defense Highways was created. This system has the following numbering convention. When a state highway department wants to assign a number to an interstate highway, they must get that number approved by AASHTO (American Association of State Highway Transportation Offices), which usually enforces the convention. This convention is:
One and two digit route numbers:
Three digit route numbers:
Archival information regarding submissions of proposed routes for both
Interstate and US Highways can be found in the
AASHTO Route Numbering Archive.
There are a few exceptions to the rules, such as I-238 in California, and I-99 in Pennsylvania. Numbers are assigned to avoid collisions with US signed routes, and divided routes (i.e., 15E) have been eliminated (with the exception of I-35E and I-35W).
Outside the contiguous United States, the following rules hold: (Note that the I- prefix is restricted to the continental U.S.)
In Hawaii, H-1 through H-3 exist and are signed as interstates. All the routes are on the island of Oahu.
In Alaska, A1 through A4 are defined, but are signed as Alaska AK-1 through AK-4. A1 is the Sterling Highway/Seward Highway/Glenn Highway/Tok Cutoff between Homer and Tok. A2 is the Elliott Highway/Steese Highway/Alaska Highway between Manley Hot Springs and the Canadian Border. A3 is the Glenn Highway/Parks Highway between Anchorage and Fairbanks, and A4 is the Richardson Highway between Valdez and Delta Junction.
Signing in California / Signing Standards Numbering of Federal Highway System Routes
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Maintained by: Daniel P. Faigin <firstname.lastname@example.org>.