California Highways
www.cahighways.org

California Highways

Numbering Conventions:
Eisenhower Interstate Numbered Routes

 
powered by FreeFind
California Highways Home Page
State Highway Routes
Numbered County Highways
State Highway Types
Interstate Types and History
Highway Numbering Conventions
State Highway Renumberings
State Highway Chronology
 Maps Trails and Roads Related WWW Links Site Change Log Sources and Credits
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.

<-
Signing in California/Signing Standards		 ->
Numbering: Federal Highway System

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