Interstates | Highway Numbering System

Last weekend we drove through Kansas City and Oklahoma City during parts of a visit to see our son. He is a pilot in training at Vance Air Force Base, Enid, OK. Most of the driving was done on the Dwight D. Eisenhower System of Interstate and Defense Highways. That link provides a large amount of information on the system begun on June 29, 1956. On that date, President Eisenhower signed the Federal-Aid Highway Act of 1956. This post is about one aspect of the interstate system, namely how the roads are numbered.

Numbering in General

East-west roads are even numbered. North-south roads are odd numbered. The even-odd scheme applies to the general route of the roads and allows for some deviation in places. The one and two digit numbers start small in the south and west. They progress larger for the roads toward the north and east. Here is an example of a few chosen roads.


Numbering Near Big Cities

Large urban areas pose some different challenges. I constructed a map for a fictitious location called Big City. Interstate highway I-60 passes through as an east-west route. There are two suburbs a few miles to the northeast and northwest. Notice there are two types of connections to I-60. There are loops and spurs.

A spur connects to the main artery at one end such as the roads 160 and 360. The first digit of a spur is an odd number. The next two denote the parent highway. In this case it is 60. A loop connects to the main artery at both ends such as 260 and 460. The first digit of a loop is an even number. The next two denote the parent highway.

Big City

As with all systems and schemes, there are some exceptions and oddities. The Federal Highway System has them, too. I invite you to read the link in Wikipedia for some comments about them. Enjoy your next road trip.


19 thoughts on “Interstates | Highway Numbering System

  1. Steve Gingold

    I’ve traveled a few of the parts that make up the Eisenhower system. We also often use 95 when going to Maine or Cape Cod. 495 makes a nice skirting alternate to the part of 95 that goes from Providence, RI through Boston and then on up to Maine which saves a fair amount of time. In Maine, 95 does a curvy path into the center of the state and 295 straightens the trip out passing by Portland and up to LLBean before rejoining 95.
    Here in MA, we have Rt. 2 crossing the state and almost every town has a 2A.

  2. Steve Schwartzman

    The Long Island Expressway, designated Interstate 495, is not a loop and ceases to be an expressway when it crosses Manhattan on its way to connecting with I-95 in New Jersey. People used to joke (and may still) about the Long Island Expressway being the longest parking lot in the world.

    1. Steve Gingold

      Oh yeah!!!! I “parked” on the LIE a few times. I can’t imagine being one of those lucky folks who “drives” it on a daily basis. I’d end up running screaming and naked up and down the lanes after a while. Hey, I think I just figured out how to thin out the congestion on the LIE. LOL

  3. Jim Wheeler

    I always thought I had a logical mind but for many years I had a blind spot where the Interstate Highway system is concerned. I thought (not that I really thought about it) that exit numbers were arbitrarily assigned and otherwise without meaning. Wrong. Interstate exits are numbered by how many miles they are from the highway’s beginning within a given state. Thus, the number of an exit is a measure of its approximate distance from a state line. The principal exit from I-44 here in Joplin is exit 8, about 8 miles east of the Oklahoma border. When exits occur more frequently than a one-mile spacing, they are given letter designations, as for example, exits 8A and 8B. (One goes north, the other south.) The sequencing begins anew in each state the highway crosses.

    1. shoreacres

      One of the best Waffle House experiences I ever had resulted from taking 8B on my way to Kansas City. I was taking my mom to her sister’s, and we decided to eat there because it was close to the motel. An old guy tried to pick up Mom. She never stopped talking about it.

      The other thing I love about 8B is that it puts you on Range Line Road, the old Route 66. I’m really fond of Route 66.

      1. Jim Wheeler

        Hey, shoreacres, small world! I know exactly where that Waffle House is. It’s still there. Love the story about your mom. Small, intimate, rustic actually, not because of design, which is cookie-cutter of course, but because of the local culture. We’ve eaten there. Reminds me of Mel’s Diner. Talk about slingin’ hash! 😆 😆

        Joplin has been trying for decades to shift emphasis from Range Line Road to Main Street and because of the power of 8B, and they’re still trying! And speaking of old Route 66, we live a block away from it in north Joplin. Nostalgic for sure, and ironic. Mollie and i passed that way between Navy duty stations in 1961 and now we live here!

  4. Steve Gingold

    Oh, I forgot to mention….I love the exit numbering system in Maine. They are numbered by the miles, not sequentially. So we know how far we have yet to travlel. Excellent.

  5. shoreacres

    This is really interesting. I was looking at your diagram, and realized that, as far as I can remember, I’ve never driven a north/south interstate except where it’s necessary in and around cities. I’ve driven I-10 from Texas to Florida and California, and I-80 from Illinois to California. I’ve been on 40 in Kansas, where two counties claim to have the first piece of interstate poured.

    My folks built a new house on the south edge of town in the mid-50s. I remember sitting on the front step, listening to the trucks rolling on I-80, only a couple of miles away. I dreamed about getting on those roads and just going. And, I have — always with some good traveling music.

    1. Jim in IA Post author

      I’m glad you liked this post. It seemed like a silly minor thing. But, many people are touched by the interstates in many ways. We all have some things to share.

      We live a mile north of I-80 now. Most days we can hear some truck sounds. Some days are louder than others. Depends on the weather.

      That music has me jumping. Thanks for it. 🙂

  6. Mrs. P

    Hubby was a truck driver in another life and he explained this to me when we started our road trips. It really does help navigate through the roadways and depending on the time of day we’ll either blast through town or take the loops. Generally we drive at night to avoid daytime congestion so often we just drive straight through town…and traveling music is a must!


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