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Highway Numbering
Highway Numbering in California

Numbering of Eisenhower Interstate Numbered Routes

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:

  • Odd. North/South, lowest numbers in the West.
  • Even. East/West, lowest numbers in the South.
  • Multiples of 5. MAJOR interstates that typically go cross-country.

Three digit route numbers: 

  • First digit odd. Spur into metropolitan area (at time of route adoption).
  • First digit even. Spur around metropolitan area (at time of route adoption).
  • Last two digits. Indicate the route from which the spur or loop diverged.

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.

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