Our interstates are very confusing. Between the numbers, directions, and designations, it could be hard to decipher what an interstate’s number means. Well, I did the research on all of the interstates in the United States of America to explain each type of interstate.

For this article, I looked up all interstates in the main land of the 48 states. In my search, I excluded Alaska, Hawaii, and the territory of Puerto Rico. This is because their interstates start with either A, H, or PR, whereas the interstates on the mainland all start with I. In addition, in Hawaii, Puerto Rico, and Alaska, there is one interstate that has a number higher than 10 (Interstate H-201 in Hawaii). On mainland, I focused on all interstates, whether they are divided up or connected with each other. (e.g. I-84 ends in New York, but picks up in Utah.)

First, I will focus on all interstates with one or two numbers. I will explain why each interstate is numbered what it is and how it is related to other interstates.

One-Digit Routes:

There are only four interstates in the mainland United States that have one digit: I-2, I-4, I-5, and I-8. The even numbered interstates run east and west, while the odd numbered interstates run north and south. One digit-routes do not follow first digit rules of two-digit interstates (shown below) because they only have one digit. However, the last digit rule is prevalent, as shown above.

Two-Digit Routes:

There are 56 interstate numbers with two digits. I did not count the four interstates that appeared in two separate places in the U.S. twice. For instance, I-88 runs in New York, ends, and then continues in Illinois. I counted I-88 once for my count, as it is technically one number. And, there are rules with each number of every two-digit route.

Second Digit:

The second digit rule is prevalent in almost every interstate in the mainland U.S. It also coincides with the one-digit routes’s rules.

The rule is simple. If the second digit is even (0, 2, 4, 6, 8), then the highway runs east to west. Meanwhile, if the second digit is odd (1, 3, 5, 7, 9), then the highway runs north to south. Obviously, there was some math number rules excluded (1 is a unit, and 0 is, frankly, zero). However, that is the main rule of thumb.

Of course, there are exceptions. I-26 in Tennessee, North Carolina, and South Carolina run east-west, but are labeled north-south due to its second digit being even. I-94 also does this between Chicago and Milwaukee. I-64 is an even more extreme. In the Hampton Roads area in Virginia, I-64 does not have directional labels, as you could be on I-64 East (again, non-existent; only an example), but could be going west.

There are also other parts of this rule. Interstates that end with 5 or 0 are usually the most important (I-95 extends from Maine to Florida on the east coast, while I-90 extends from Massachusetts to Washington state) with the exception of I-45, which runs a mere 285 miles while staying exclusively in Texas. (In comparison, I-90 is 3,102 miles total, while I-95 is 1,936 miles total.) Another exception is I-50 and I-60, which is non-existent in the United States.

First Digit:

The first digit of the interstate number correlates with the second digit. And, it is easy to understand.

If the second digit is even, then freeways are east-west. Therefore, freeways in the southern U.S. with an even second digit will have a lower first digit than one more Northern. To make it simple, for even-numbered highways, for the first digit, they increase from the South up. For example, I-66 is north of I-64, but south of I-68 with no even-numbered interstates in between.

There are exceptions to this. For example, I-74 in North Carolina is south of I-70, while it is north of I-70 in Indiana. Even though I-74 follows the rule at some point, the above exception remains.

This is also the same for odd-numbered two digit interstates. These freeways run north-south, and a freeway with a lower first digit is more west. So, to make it clear, for odd-numbered highways, the first digit increases from the West right. For example, I-27 is west of I-25, but east of I-29 with no odd-numbered interstates in between.

There are also exceptions to this rule. I-95 runs on the East Coast, while I-99 runs from State College, Pennsylvania, down. Obviously, State College is west of the coast, so I-99 remains an exception to this rule. Also, I-71 in Ohio is east of I-75, but in Kentucky, goes on I-75’s west side. However, this still counts as an exception.

Suffixes:

Suffixes for a two digit interstate route should be extinct, as they were abolished in 1980 by AASHTO (American Association of State Highway and Transportation Officials) to prevent confusion. However, there are still some around today.

Many suffixes (I-5, I-15, etc.) were taken away in 1980, but seven interstates with suffixes remain. They are I-35E and I-35W near Dallas; I-35E and I-35W near Minneapolis; and I-69W, I-69E, and I-69C in Victoria, Texas, near Mexico.

Suffixes are pretty simple to understand. If the suffix is E, then that part of the highway is east of a certain area, and if it is W, then the highway is west of a certain area. I could not confirm what C meant if it is a suffix, since it is only used for I-69.

Now, I-35E in Texas goes into Dallas, so that is a minor exception. However, it is east of Fort Worth, so it could be viewed as that the split is around Fort Worth. In Minnesota, I-35W goes through Milwaukee, while I-35E goes through St. Paul. Even though they do go through cities, the interstate splits, and that is all that matters.

The I-69 splits is not that important, because a part of the I-69 portion in Texas has not even built yet. As of now, only I-69E is in service; I-69W and I-69C is still in planning.

Another note about suffixes: I-480N exists in Ohio. However, it is only shown on exit/entrance ramp signs and mile markers. It does not even exist on a map, so do not try to look it up.

Suffixes, even with their low amount, are prevalent to learning about the science of interstate numbers.

Now, to three digit routes, where a lot of what you just learned about two digit routes are thrown out the window.

Three-Digit Routes:

There are many three digit routes, and there are too many to count. And, in many cases, direction rules for the last digit do not apply (I-285 in Georgia is North and East at some point), so I will not even venture into that part. In addition, there could be multiple three-digit routes with the same number, so I will not count all the three-digit routes in the mainland U.S. Instead, I will focus on the three kinds of three-digit routes. These include spurs, bypasses, and beltways.

Spurs:

Spurs are freeways with three digits with the first number being odd. They link a main freeway (one or two digits) or another three-digit route to a main city. Usually, this goes into the downtown of a city. They mostly connect once with its parent freeway, and usually ends at another highway with access control (exits) or a regular city street. An exception is I-390 near Rochester, New York. It starts at I-86, has an interchange with I-90, but ends at I-490. In addition, it does not directly go into Rochester. However, most spurs, like I-110 in L.A. and I-190 in Buffalo, NY do go directly into the city from their parent highways (I-10 and I-90, respectively).

Bypass:

Bypasses go around a city, but make a semi-circle shape. Their first digit is even, and both ends of the interstate end at another interstate. A good example of a bypass is I-440, which forms a bypass around Nashville’s south side.

Beltway:

Beltways are the true deal. Often called loops, they make a full circle around a city. They do not end anywhere; their mileage only resets to zero at a certain point. Their first digit is also even. A great example is I-495, which is conveniently named “The Capital Beltway”. It encircles Washington D.C. while traveling in Maryland and Virginia. Another good example of a beltway is I-275 in Ohio, Kentucky, and Indiana. With its 84 miles of length, it encircles the city of Cincinnati.

There are numerous exceptions to all of these, of course. I-520 in Augusta, Georgia, is a loop, even though its first number suggests it as a spur. A local exception is I-279. After part of it was taken away and given to I-376, I-279 now acts like a spur, going from I-79 into the heart of Pittsburgh. However, because of its number, it should act like a loop.