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California Highways

Numbering Conventions:
Eisenhower Interstate Numbered Routes

 
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Highway Numbering Conventions [Sign Changing] [Interstate Shield] [Federal Shield] [Old State Shield]
[Green State Shield]
[Exit Numbers] [Postmile Bridge Sign] [Business Route Shield] [Forest Shield]
Introduction to
Highway
Numbering
Signing in
California/
Signing

Standards
Eisenhower
Interstate
Numbered
Routes
Federal
Highway

System
California
State
Routes
Exit
Numbers
Post
Miles
Business
Routes
Forest
Routes

[Interstate Shield]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.

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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Numbering: Federal Highway System

©1996-2004 Daniel P. Faigin.
Maintained by: Daniel P. Faigin <webmaster@cahighways.org>.