The Interstate Code Secret You Should Know Before Your Next Road Trip

If you're driving across the U.S., the names and numbers of the interstates might seem completely random. In reality, there's a pattern the highways follow. The U.S. Department of Transportation Federal Highway Administration explains that odd-numbered interstate highways travel north and south, with the lowest numbers in the west and the highest numbers in the east. Meanwhile, even-numbered routes travel east and west, with the lowest-numbered routes in the south and the highest in the north.

The system was recently brought to light by Instagram user @kuyyannunu in a viral Instagram Reel where the content creator shows a map of the many horizontal and vertical interstates covering the U.S., along with their corresponding numbers. While some commenters noted that they already knew this tidbit of highway knowledge, many were just as mind-blown — which might explain why the Reel has earned over 650,000 likes to date. However, the U.S. interstate and highway numbering systems don't end there. There are other codes drivers may want to know before hitting the road.

More than just being a fun fact, the highway numbering system can keep you from getting lost. For instance, if your GPS goes out but you know you want to head east, watching out for north-south interstates with a higher number (I-85 after I-75, for example) might help you stay on track.

You may be wondering where three-digit highways fit into the scheme. According to the Federal Highway Administration, smaller roadways that connect to an interstate are given a three-digit code. Generally, the last two digits of the highway match the larger interstate, and if the highway connects to the interstate more than once (think beltways circling a city), the first digit will be an even number. In California, this includes I-405, which connects to I-5 twice. If a roadway branches off of an interstate but doesn't reconnect to it, it's given an odd-numbered prefix instead, such as I-105, which connects to I-5 once.

While the two-digit interstate system highlighted is generally foolproof, the rules for three-digit highways can sometimes differ, depending on the state. Also, note that you may come across two different interstate highways in two different states that share the same name. With these points aside, the numbering system is easy to learn — and it might save you trouble during your next road trip.