Interstates
a.k.a. The Eisenhower Interstate System numbering plan

Interstates are numbered with the geographic system.

East-West Routes

East-west routes are one- or two-digit even numbers, which increase from south to north. The extremes are I-4 in Florida and I-96 in Michigan.

North-South Routes

North-south routes are one- or two-digit odd numbers, which increase from west to east. The extremes are I-5 on the west coast and I-97 in Maryland. However, there is an I-99, but it doesn't count. (A certain Congressman *cough*clear throat*Bud Shuster* brought home some serious bacon. He got an interstate in his district and got to number it himself!)

Three-Digit Interstates

Ones that start with an even number are either beltways around a city or are freeways that go through the city. Examples: The I-494/694 Twin Cities Beltway; I-480 through downtown Omaha

Ones that start with an odd number are spurs off of the main interstate. Examples: I-394 in Minneapolis; I-380 in Iowa

A three-digit interstate always has the two-digit part of the main interstate it's a spur/loop of.

A three-digit interstate number cannot be repeated in the same state.

When deciding which odd or even digit to use, it appears that some states have a concrete policy; some don't. For example, in Colorado the first through-city routes were I-225 and I-270, and then I-470 was planned. However, if you look at Minnesota, I-494 and I-694 were used for the beltway, and then I-394 was the first spur. The spur off of I-35 is I-535. Some states seem to do it geographically, which the first digit being used increasing as you go across the state.

Major vs. Minor Routes

You can also tell if a one- or two-digit interstate is a more "main" interstate than another by its last digit. By "main", I mean interstates which are cross-country routes, vs. just being a regional route.

On north-south interstates, the major ones end with a 5.

On east-west interstates, major ones end with a 0. The interstates stick to their system amazingly well when you consider that they criss-cross all over each other as they they go all over the country. Of course there are violations, and to take a look at those go to Ron Newman's Guide to Violations.

Letter Suffix Routes: There used to be a whole slew of Interstate routes that had a letter suffix. This was usually done by taking the two digits of a parent route, and using N, S, E, or W to denote the secondary route as branching off in a northerly, southerly, easterly, or westerly direction, respectivily. For example, I-76 between Denver and I-80 used to be I-80S. As I said, there used to be a whole bunch of these, but AASHTO now seriously frowns on these, so almost all of them have been renumbered something else. The ones that do remain are I-35E/I-35W in both Dallas/Fort Worth and the Twin Cities.

Letter Prefix Routes: These are used exclusivly in the Non-Continental US. Interstate highways in Hawaii are I-H1, H2, and H3. Alaska has Interstate highways, but these are purely legislative, and are not signed. They are I-A1, A2, A3, and A4. Puerto Rico would be I-PRIx if it had Interstates. We've had some pretty spirited discussions on misc.transport.road about what an oxymoron it is having an Interstate highway in Hawaii, but as it was pointed out, being an Interstate means more than going across state lines: There's the need for freeways to certain standards, plus the 90% federal funding that comes with Interstates.

For more on interstates, go to an online list of interstate highways. There is also Kurumi's list of three digit Interstates. Another good place to check out is AASHTO's Interstate Highway Sign Policy, from Richard C. Moeur's site.

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