Eisenhower Interstate System naming rules:

All interstate highways start with the letter I and end in a number.

For north-south interstates, the numbers used are odd. For east-west interstates the numbers are even.

Numbers are assigned by region. For east-west interstates the southern routes use the lower numbers (ex. I-4) and the northern ones use higher numbers (ex. I-92).
For the north-south interstates the lower numbers are in the west and increase toward the east.

All transcontinental (coast-to-coast or border-to-border) interstate numbers end in 0 or 5 (ex. I-65 and I-90).

Not all transcontinental interstates will end in 0 or 5 - interstate 69 is planned from mexico to canadia.

Thanks Ed - I completely forgot 3 digit interstates. But you are sorta wrong - it is much simpler. An odd first digit should mean it is a spur - only one end is at another interstate. An even first digit means both ends are at other interstates. This usually happens but sometimes like in california there are too many 3dis and not enough 2dis. Of course the last 2 digits are the parent interstate. kurumi.com has more info.

But that is a very interesting theory!

A hundreds digit of 1 is a local spur into a city. A hundreds digit of 2 is an express through a city. A hundreds digit of 3 is a bypass between highways. A hundreds digit of 4 is an express around a city. A hundreds digit of 5 is a local spur into a city. A hundreds digit of 6 is a wide bypass around a metropolitan region. A hundreds digit of 7 is a wide bypass around a metropolitan region. A hundreds digit of 8 is an express through a metropolitan region.

no offense
Off the top of my head, I write this. If I find official or specific info, I'll amend it.

As a trend, though probably not strictly adhered anymore.

  • Major Interstates have two digits.
  • Even number is east-west. Odd is north-south.
  • A hundreds digit makes a local sister highway.
  • A hundreds digit of 1 is a local spur into a city.
  • A hundreds digit of 2 is an express through a city.
  • A hundreds digit of 3 is a bypass between highways.
  • A hundreds digit of 4 is an express around a city.
  • A hundreds digit of 5 is a local spur into a city.
  • A hundreds digit of 6 is a wide bypass around a metropolitan region.
  • A hundreds digit of 7 is a wide bypass around a metropolitan region.
  • A hundreds digit of 8 is an express through a metropolitan region.
  • On a larger scale, the middle digit gets larger as you go east, or north. That is, I10 is very southern and I90 is very northern; I5 is very western and I85 is very eastern.

    The San Francisco Bay Area has: I80, I280, I380, I480, I580, I680, I880, I980. Most metro areas have at least an I4xx. Dallas and Ft. Worth have a special split: I35E and I35W allow the I35 to visit both cities.

    A correction to Ed Halley, major interstates do not have to have 2 digits. One of Ed Halley's examples I-5, is a major interstate with only one digit.

    The correct explanation is found in the 2nd paragraph of SPUIs writeup.

    From Chapter 6 of "Read Your Road," a publication of the Federal Highway Administration.

    http://safety.fhwa.dot.gov/factsfigs/ryr/chapter6.htm, Accessed July 31, 2000.

    Decoding the Interstate Numbering System

    Knowing what the numbers on Interstate shields have to tell you can be a great aid to navigation. Many motorists think they know how to read the numbers, but they may understand only part of the story. Other motorists are confused by the numbers; still others have never really thought about it.

    When you're traveling in unfamiliar territory, Interstate numbers give you valuable clues to your location and direction, if you know how to read them. Here's the key:

    One- or two-digit even- numbered Interstates are always east-west routes. The numbers increase from south (I-10) to north (I-94).

    One- or two-digit odd-numbered Interstates are always north-south routes. Numbers increase from the West Coast (I-5) to the East Coast (I-95).

    TIP: Since not all Interstates run due east-west or north-south, the closest cardinal direction that applies to most of the road is used. So, even if a stretch of an east-west Interstate shifts to the south for a while, it is still considered an east-west highway.

    Three-Digit Interstates

    Interstate highways with three-digit numbers connect to other major highways.

    If the first of the three digits is an even number, the highway usually connects to another Interstate at both ends - often in a circular beltway or loop.

    If the first of the three digits is an odd number, the highway is usually a "spur" route that connects with an Interstate at only one end, sometimes going into a city center.

    Exceptions to the Interstate Naming Rules

    I-238
    There is no Interstate 38, but there is an Interstate 238. It runs east-west just south of Oakland, California, connecting I-580 in Castro Valley to I-880 in San Leandro.This 2 mile long portion portition of California State Route 238 was upgraded to Interstate highway status in 1983, but there were no available numbers ending in -80 in the San Francisco Bay Area, so the American Association of State Highway and Transportation Officials (AASHTO) granted an exception to the rules.

    Interstate H1, Interstate H2, Interstate H3
    These "Interstate" highways on the Hawaiian island of Oahu run the wrong ways by the mainland numbering rules - H1 & H3 run east-west, and H2 runs north-south.

    Interstate 35 also splits into I-35W and I-35E in the Twin Cities. These are the only two places in the Interstate system where that happens; all others have been decomissioned.

    The Twin Cities Interstates are also interesting in that the ring road is split into two numbers--I-494 and I-694. They share a common origin for the mile markers, near the Minneapolis-St. Paul International Airport.

    Also, Read Your Road has moved to http://safety.fhwa.dot.gov/media/ryr.htm .

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