What the Numbers Mean on Interstate Highway Signs

Interstate Highways are the most heavily traveled roads in the United States, and a massive number of drivers use them almost every day. But did you ever stop to wonder what the numbers on the signs mean, and why they are numbered the way they are?

In case you didn’t know it, Interstate Highways in the United States that travel east to west are given even numbers, and north to south routes are assigned odd numbers. Odd route numbers increase from west to east, and even routes increase from south to north.

Interstate Highway routes don’t have to cross state lines to be designated as an Interstate Highway. Two such examples are the three roads that are Interstate Highways in Hawaii and Interstate 45 that runs from Dallas to Galveston in Texas. There are many other examples of Interstate Highways in many other states that never cross a state line. Any roads under the Federal-Aid Highway Act that receive federal funds are considered Interstate Highways.

The three-digit Interstate Highways (also called Auxiliary Interstate Highways) around urban areas have their own meaning. The last two numbers indicate the parent route, while the first digit indicates if the road is going directly to a city (which is an odd digit), or looping around the city (an even digit). Of course, there are a few exceptions.