By Gail Marsh
I know some RVers prefer to travel on state or county highways, while other folks like to take the direct routes that our Interstate Highway System provides. No matter your preference, it’s important to understand highways by the numbers. [FYI: The highway system is technically called the “Dwight D. Eisenhower National System of Interstate and Defense Highways,” and commonly called “The Interstate System.”]
The basics
The first thing to know about the U.S. highway system is that it follows some general rules or guidelines. The second thing you should know is that there may be exceptions to those guidelines. Why? General numbering rules may not apply to newly constructed roads or improved roadways that receive their numbered designation well after other highways have been named.
Even and odd
In the United States, highways with an odd number generally run north and south. Even-numbered highways will help you travel in east or west directions.
Ascending and descending order
Odd-numbered highways increase from west to east. So, the north/south highways in California, for example, have been given a lower numeral than north/south highways in the eastern part of the country. I-5 is a major highway in the west while I-95 is a major highway in the east.
Even-numbered highways increase from south to north. Interstate 10 runs east/west in the southern U.S., while I-90 takes you through the northern tier of states.
Divisible by 5
All major cross-country Interstates are given a number that is divisible by five. That means these thoroughfares will end either in a zero or a five. I-95 was reserved for the most eastern highway number, and I-10 was named for the most southern interstate route. Other roads were numbered in sequential order.
Look at a U.S. highway map and you’ll notice the I-10, I-20, I-40, I-70, I-80, and I-90 east/west routes. These major interstates run the length of the country in ascending order from south to north. The same ascending highway number principle is evident in the north/south roadways, beginning with I-5 in the west and ending with I-95 in the eastern U.S.
There is only one highway I-95 and one I-70 in the U.S. system because these (and other roads ending in zero or five) are major thoroughfares across the country.
Repeats
Major highway numbers may not be repeated, while some minor highway numbers have been reused as long as the highways are a distance apart. For instance, there is a separate I-87 in New York state as well as an I-87 in North Carolina. Because neither highway is considered to be a major one, and the highways are a good distance apart, the highway number is repeated.
Three-digit numbers
Three-digit interstates are generally shorter highways that serve larger cities. These three-digit highways often connect two larger highways as a bypass, spur, or beltway.
The first number in a three-digit bypass highway will usually be an even number if the bypass connects to the major highway in two places. Beltways and spurs that only intersect once with the major highway are usually given an odd first number.
The second two numbers in a three-digit highway designation help drivers know what major highway the spur will connect to. For example, in the St. Louis metropolitan area, I-270 connects with I-70. Highway I-255 connects with I-55.
By the numbers
So, there you have it—a quick overview of U.S. highways by the numbers. If this information is new to you, take a few minutes to look at a map as you review each heading. Knowing the dedicated system for numbering our highways will help you drive more safely and confidently.
RELATED
- Poll: How would you describe the condition of America’s interstate highways?
- How a leaky milk truck changed our highways forever
##RVT1147
I know some RVers prefer to travel on state or county highways… Ascending and descending order Odd-numbered highways increase from west to east. Why are you ignoring US national highways? Odd-numbered highways increase from west to east should read Odd-numbered Interstate system highways… US national highways are the opposite with US 1 running along the east coast and US 101 from Washington to California.
You didn’t mention any exceptions, such as I69, which is mostly a northeast/southwest highway, but partially east/west in Michigan, then kinda north/south through Indiana, then kinda hit and miss on it’s way to Mexico via Texas.
Read the paragraph that says “Repeats”. It addresses that very issue.
I69 is not a repeat. Eventually it will be a complete Interstate highway from Mexico to Canada.
This article is incomplete, you are only mentioning the Interstates, think about all the US highways, like US-131 or US-10 in Michigan. Normally with exceptions they start in the North East and progress West and South, US-2 runs along the Northern boarder. Maybe ChatGT was used to write this.
Mileage posts generally follow same convention. South to North and West to East the mile markers increase. I’ve also driven all those interstates and the worst is I40 in Arizona from the California border to Flagstaff. It should be embarrassed to call itself a highway let alone an Interstate.
Very interesting! I knew some but not all of this.
Thank you, Gail! What an interesting column! I did know the north-south, east-west numbering, but everything else was new information. This should prove very helpful to better understanding which road we take in unfamiliar places. We use an RV GPS, but knowing this information elevates my comfort as a driver. Thanks again, and safe, sunny travels! 🙂
I’ve been aware of the Interstate numbering system for a long time, but I still can’t make sense of the STATE road numbering systems. It’s like they’re all random!
This information generally applies to INTERSTATE HIGHWAYS. The ODD numbers due run south to north with the lowest (I-5) on the west coast and the highest (i-95) on the east coast. The even numbers run east to west with the low numbers in the south and high numbers in the north.
HOWEVER
US numbered highways are the exact opposite. Sort of. Odd numbers still run north to south and even numbers east to west (Kinda – Think US-66). But the low numbers for US highways are in the north and east, with the highest ones in the west and south.
State and local roads have all sorts of ways to tell (if you can) where they go. Know the early presidents, and trees and flowers alphabetical.
Ooops Gail! Seems you missed a couple biggies. Like I-94 and US hiway 2 which crosses the northern tier of the US to Seattle. Across Mn and ND, MT et al. I-94 was built over US Hiway 10. I-94 divided by 5 = 18.88 (according to my calculator).
Well, you are forgiven this grievous error (HA!), as knowing the hiway numbering system is important information for any major travelers – so you done good Gail!
Yes, the author and headline writers should have been clear this was only about the Interstate system. I’ve read more detailed articles about that system than this.
Trip planning idea! Pick a US highway and drive its length. On Interstates this is just boring. We have driven US 20, US 1, US 101, US 90 (think south of I 10). Not all of these have been driven in a consecutive run – US 1 took several decades. You will encounter many small towns and all of them have interesting stops. One of our favorites is US 15 from Rochester NY to Washington DC. It is no longer drivable as I 99 is being built right over it.
Gail, you stated that I-20 runs across the country from coast to coast. This is not correct. I-20 mergers into I-10 in West Texas between Pecos and Van Horn Texas. Check your map.