During my recent cross-country drive, I did a lot of thinking about interstates, because if you’ve ever driven Interstate 40 through Oklahoma and Texas, you’ll know there’s not much else to think about, except maybe windmills.

Many years ago I learned some cool facts about interstates. With the prevalence of GPS devices, it’s not likely you’ll ever need them to help you navigate somewhere, but you never know. If nothing else, they make for semi-interesting cocktail-party factoids. (As with everything in America, there are exceptions to just about all of these rules, so don’t yell at me if you have an example of something contrary.)

  • Interstates are numbered from south to north and west to east. That means that lower-number interstates (like Interstates 5 and 8) are found in the southern and western parts of the country. Higher-numbered ones (like 80 and 95) are in the northern or eastern parts of the country.
  • Mileage along the interstate is labeled the same way and starts over in each new state or at the beginning of each new branch. For example, when you enter a new state along its southern or western border, you’ll start with mile marker 0. When you enter at the north or east, you’ll start with the last mile of interstate in that state.
  • You can use mile-markers to determine whether you’re traveling in the right direction. If the numbers are going up, you know you’re headed north or east. If they’re going down, you’re headed south or west.
  • Exit numbers correspond to mile markers. So if you’re at mile 185 and your exit number is 285, you know you have 100 miles to go. (I try to keep the math simple around here.)
  • Interstates that run north-south end in even numbers. Ones that run east-west end in odd numbers. (Any variation here tends to happen in major metropolitan areas.)
  • Interstates that run all the way from the west coast to the east coast end in 0. Interstates that run from the top of the country to the bottom end in 5.
  • Primary interstates are numbered with single or double digits. Off-shoots have three digits and end with the same number as the primary interstate they branch off of. (For example, Interstate 177 branches off of 77. 405 branches off of Interstate 5.)
  • Generally, if the branching-off route begins with an even number, it will reconnect to the primary interstate at some point. (280 branches off of 80 and will likely merge back with it.) This is usually done to create an alternate route around heavily traveled urban areas and can be a good way to avoid congestion.
  • Conversely, branches that begin with an odd number generally do not connect back to the main line. (175 branches off of 75, but won’t meet up with it again.)

Now go find a cocktail party and wow some people. (Have a designated driver for afterwards!)