What Do The Numbers On Interstate Signs Actually Mean?

In order to understand why these signs are numbered the way they are, we'll need to have an understanding of how the Interstate Highway System was developed. During World War II, President Eisenhower served as the Supreme Commander of Allied Forces. During this time, he observed how Germany's Autobahn network of highways allowed for fast transportation throughout the country. Considering that America had just come out of a devastating war and was experiencing the looming threat of atomic warfare thanks to Cold War tensions with Russia, Eisenhower determined that quick transport may be necessary, especially if evacuations were to become necessary. As development began in 1956, so too did the numbering of these highways.

The first step to understanding interstate roadsigns is to see how many digits are on the sign. One or two-digit interstates stretch over multiple regions, while three-digit interstates are limited to a single city and are known as auxiliary interstate highways. Ultimately, these connect to the one and two-digit highways. Once you understand the difference between interstates with differing numbers of digits, it's time to understand the greater numbering process.

While it may seem confusing with the vast number of roads, overpasses, and underpasses it consists of, the interstate highway is actually fairly easy to navigate once you get the numbering nailed down. In a highly logical fashion, east-west roads and north-south roads are differentiated — east-west roads get even numbers while north-south interstates are odd-numbered. Furthermore, the lowest-numbered roads are located in the southwest, with the numbers getting higher as they branch out. Finally, the most important interstates, typically cross-country ones, are given numbers divisible by five, meaning that they end in either a zero or five. Now that you have some understanding of the numbering system, let's take a look at how each interstate actually connects.

Auxiliary interstate highways contain three digits, the last two of which tell you the interstate that it connects to. For example, I-395 connects to I-95. Interestingly, the first digit indicates the purpose of the interstate. While two-digit interstates are normally given an even first digit, spurs and connectors usually end up with odd first digits. Still, this isn't a hard and fast rule, with some exceptions existing as well. Considering that the United States is so large, you're probably wondering whether some of the interstates actually share a name. As a matter of fact, there are some interstates with the same name — the rule, however, is that these interstates cannot be located in the same state as this would cause too much confusion. The biggest exception to the interstate numbering can be seen in Alaska, Hawaii, and Puerto Rico. Because these states and territory don't connect with the U.S. mainland, each one of them has its own interstate highways, albeit with simpler numbering. For example, Hawaii's first interstate is H1, while Puerto Rico's is PR1.