Explore the logic behind U.S. interstate highway numbers.

The numbers on U.S. interstate highway signs may seem random, but they can actually be an important navigation tool. This short video shows how it works.

For starters, the interstate system is divided into majors, mediums, and minors. East-west majors are two-digit numbers ending in zero, with the more northerly routes having higher numbers. This is why the northernmost interstate (between Boston and Seattle) is I-90 and the southernmost (between Jacksonville, Florida and Santa Monica, California) is I-10.

Interstate highways, however, are not always sequentially numbered; I-50 and I-60 do not exist because these numbers were already in use on highways that were part of the pre-Interstate system (which also gave birth to Route 66).

North-south interstates also have two-digit numbers, but this time ending in a "5" to indicate direction, and the most westerly routes have lower numbers. This is why the interstate between Maine and Florida is now I-95. The route between California and Washington is called I-5, a single digit number, but still appropriate for this most westerly route.

Minor interstate routes branch off from major routes and become three-digit numbers. The last two digits match the major from which it branches. If the first digit is even, it means that the minor route will eventually reconnect with its major route. For example, I-670 branches off I-70 in Kansas City, Missouri, and reconnects in Kansas City, Kansas.

If the first digit of the minor route number is odd, the highway does not reconnect with the interstate from which it originated. Therefore, these highways are sometimes referred to as spurs, but they may connect to another interstate highway. Since this system allows only a limited number of numbers (1-9 plus the parent highway's number) to be selected, there are many duplicate minors. For example, both Maryland and Massachusetts have I-495.

Finally, interstate mediums use two-digit numbers, with even numbers indicating east-west routes and odd numbers indicating north-south routes. However, some numbers are duplicated. However, some of the numbers are duplicated. This is permissible as long as the routes concerned are far enough apart to avoid confusion.

Thus, with a few exceptions, it is possible to figure out which direction you are traveling by looking at the interstate numbers and which small interstates connect to which major highways. Keep this in mind on your next road trip.