The Interstate Highway Numbering System

Interstate Highways are the major highways in the US. The system was based on the Autobahn which is the highway system in Germany. Dwight Eisenhower discovered these highways when he was in World War II and wanted to implement a similar system here in the United States. The highways were started in the 1950s. Though all the highways on the original plan have been built by today, new highways are still being built. There are a number of highways that are being worked on currently and more that are planned.


Interstate highways use a numbering system to help people know where they are going. Each number symbolizes the highway’s location in the United States. From the number you can determine what direction the road goes and the part of the United States you are in (the north, south, east, or west). The highway numbering system forms a grid. From any given highway, one can determine the location.

The simplest part of the system, routes that end in odd numbers, go north/south and similarly routes that end in even numbers go east/west. So for example Interstate 81 goes north/south. It starts in Tennessee and ends at the Canadian border. On the other hand, Interstate 90, starts in the state of Washington, and ends in Boston, Massachusetts. So I think the basic directional part of the system is clear. Now there are two other important facets to keep in mind.

Not all interstate highways are created equal. Some go longer distances than others. Some connect many major cities; others only connect one or two cities. The major highways, the ones that go from coast to coast or from border to border end in 0s and 5s. Interstate 30 and 45 are the exceptions, and there are no Interstates 50 or 60. A few even highways like Interstate 20 and Interstate 70 are long but don’t quite make it to the coast. In the case of Interstates 50 and 60, no even numbers are used between 46 and 62 as their routing would be too close to a U.S. highway (another type of highway with a similar numbering system) with the same number. The original rule was that it was forbidden for a state to have a U.S. highway and an interstate highway with the same number. Routes in the middle of the United States easily could have the same numbers if these even numbers were used.

The third rule for interstate highway numbering was implemented to keep highways with the same numbers away from each other. Odd interstate highways are numbered from west to east. In the west you will find the lower number highways. For example, interstate 5 is found in California, Oregon and Washington. Interstate 15 is in Montana, Idaho, and Nevada etc. You can see the pattern. Numbers get larger as you go east. The highest numbers are in the east such as Interstate 95 which connects New England, with Pennsylvania, Maryland and the eastern coastal southern states. Highways in the 50’s are in the middle of the country.

For even number interstates, the low numbers are in the south and they increase as you go north. Interstate 10 is in Florida, while Interstate 96 is in Michigan. To avoid number duplication Interstates 46 through 62 were not used, putting Interstates 44 and 64 in the middle of the United States from north to south instead. In case you’re wondering though, no odd number interstate highways are forbidden for being prone to cause duplicates. The major U.S. highway counterparts for going north and south actually end in 1 and not 5 making it harder for the same number U.S. highway and interstate to go through the same state. Due to lack of population in certain areas not all odd number highways are used.

Before Interstate 99 came to fruition all odd number interstates were used from 75-97.


There are still two more types of highways to talk about. Three digit interstates can be divided into two categories. If the first digit is even, then the route is a beltway and goes around the city. Interstate 695 goes around Baltimore and intersects Interstate 95 two places. If the first digit is odd, than it’s a spur route and goes into a city. Interstate 370 goes into Baltimore from Interstate 70. These routes must connect to the highway that shares the last two digits. Interstate 526 for example is a spur route and takes the driver from Interstate 26 into Charleston, South Carolina.

Originally the system was strictly adhered to, but as highways kept getting built some peculiarities have been introduced, though the vast majority of the system still follows the rules. There are several notable exceptions. Interstate 238 which is completely in California, does not meet Interstate 38(it doesn’t even exist!) This highway is actually a section of California State route 238 that was upgraded to interstate standards. In Pennsylvania, Interstate 99 is the bane of many highway enthusiasts’ existence as it is located between interstates 79 and 81. The number isn’t even close to fitting the system. The number of the highway was written into law back in the 1990’s. The portion of Interstate 74 that has been recently built in North Carolina is in the same state as U.S. 74(a rule breaker) and a section actually uses the same roadway!


There are also unsigned interstate highways. These routes are short and are small pieces of another route already numbered. Did you know that Alaska, Hawaii, and the territory of Puerto Rico also have interstate highways? Hawaii has three highways prefixed with the letter H on their main island. In Alaska and Puerto Rico the highways are not signed, but they are written “on paper.”


Now that you understand the numbering system of interstate highways, you understand how they can help direct you to get from place to place. You can see how you can determine your general location, and what direction you are traveling by looking at the number on the sign.

You may wonder where all of my knowledge of the system came from, though I acquired it over the years. My interest stems from “Road Sign Games” a game and information guide that was bought for me from a book order when I was a child. The information booklet I still have, and it talks about how the interstate highway system works on an introductory level. Though it would be years until I actually got to look at the system as a whole (first a picture of a map for the Pennsylvania, Maryland, West Virginia region in a school textbook, and then eventually road atlases themselves) the little booklet got me interested in the numbering system. I always had an interest in both numbers and roads, and this was an area that tied both together.


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