How Numbering Interstates and Highways Works

Some of us like travelling and navigating using the highway numbers and, if on the interstate, exit numbers. You can find out how mile markers work by reading the article “What are Mile Markers?”. Now we will explain how the numbers of the interstates and state highways were determined.

Even and Odd Numbered Roads

In all cases, numbers ending in an odd number nearly always run from north to south. In contrast, numbers ending in an even number run east to west. If a highway or interstate performs a second function for that similar interstate, it will start with a one, two, three, etc. For example, Interstate 435 goes around a city that Interstate 35 is on. If it is a single digit like 5, the second to last digit is a zero. Another way to look at it is that if there is a three-digit interstate, the last two digits would be the number of a main interstate nearby.

In some scenarios, there may be multiple interstates with the same exact number in different states. For example, There is an I-470 that runs around Topeka, KS and a I-470 that goes near Independence, MO. It is true that you can find them in different states, but never in the same state.

Interstate Numbering

It is easier to figure out the pattern for interstates. Going west to east, odd numbers go from 5 to 95, mostly ending with five. Going north to south, even numbers, mostly ending in zero, go from the Mexican border to the Canadian border. Of course, not all interstates end in 0 or 5, especially in the eastern US.

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Depending where the last digit is located on the number scale with 5, at least at some point, it will be at the right spot. I say this because interstates and highways like to intersect or cross sometimes. This is something to keep in mind because it may not seem like the pattern works where you are, but the pattern may occur in another state.

National highways

A question that may come to mind is, ”What about the nation’s highways that are not interstates?” This may be trickier to spot. I believe it is the case that with state highways, they are, for the most part, random. For the national highways, however, if you look hard enough, you notice there is a pattern. From east to west, U.S. highways are numbered from 1 by the Atlantic Coast to 101 by the Pacific coast. From north to south, U.S. highways go in order from 2 by the Canadian border to 98 by the Mexican border.

In some states, remember, it may not be perfect because again, highways, especially this kind, like to overlap each other from time to time. They don’t even have to meet. At some point, going in order, earlier digits in the number scale should be more north/south or west/east than the other. That is the trick to spotting the pattern for U.S. highways.

All information shared is to the best of my knowledge. Was it helpful? Let us know by showing support! Follow us on Twitter.