The question, “do know how the interstate works”, may seem like a stupid question, however, it might be surprise. Highways and interstates have actual intelligent design to them. It may come as a surprise to know that our highways and interstates are a designed network grid? U.S. Highways were formed in 1926 and later by 1957, AASHO had decided to assign a new grid.

It’s All About The Numbers

Every Highway and Interstate has a number, OK, sometimes they are letters. Although, lettered routes just mean you are “way in the back country” of a forgotten time. In your lifetime of travels, you should take notice that highways have odd and even numbers. Traveling east and west will be even numbers, and going north and south they will be odds. Though there is more science involved than just that. The highway routes and interstate numbers can actually help you identify where you are in the U.S.

You may not fully clear on the difference between a route and an interstate. Routes are highways, which would be designations for major routes. These routes usually consist of side roads and regular traffic lights. Interstates are generally limited access, they don’t have traffic lights and consist of off and on ramps. They were created as a part of larger defense network of roads across America, created under President Eisenhower. Routes cut right through towns and cities. Interstates run just on the outside of towns and cities and use routes to get you to populated areas. 

U.S. Highways (Routes)

As from above, routes and interstates are laid out in a grid network fashion. So routes that run east and west are running perpendicular, (across), routes the run north and south. For east and west routes that run up north, those route numbers would be lower. East and west route that run across the south have higher numbers. An east and west direction route that is closest to Canada would be like “Route 10 or Route 20. If you were closer to Mexico, your Route numbers would be something like US 80 or US 90. 

North and south routes follow the same grid pattern. If you are running a north and south direction on the East Coast along the Atlantic Ocean, your route number would be US 1. Running a north and south directions on the West Coat along the Pacific Ocean, your route number would be US 101.

If you understand grids the better, but even if you don’t, the above can tell you what area of America you are in by the numbers. Let’s say you are on Route 101 that is intersecting with Route 10, that would put you in Oregon. Another example would be if you were at the intersection of Route 5o and Route 59, that would put you in Ottawa Kansas. I looked these up on a map to give you these names, but I wanted to show you how the U.S. grid works.

U.S. Interstates

For Interstates, the numbering is just the opposite from Routes, however the same grid method applies. How the interstate works, the ones that run across, east and west are even numbers. The ones that run north and south are odd numbers. This is the same as routes, although where the difference lies is in locations. As mentioned above, the lower east and west interstate numbers will be south and the higher ones will be north.

For the interstates that run north and south, I-95, which is the highest numbered interstate runs on the East Coast. Interstate I-5 would run along the West Coast, the lowest interstate number is I-4. Being an even, it runs east and west and runs across Florida. If you were on I-10, you would be running across the United States east and west through Florida, Alabama, Mississippi, Louisiana and Texas. Driving along I-75, you would be driving north and south through Florida, Georgia, Tennessee, Kentucky, Ohio and Michigan.

Again, for an example, if you were at the intersection of I-10 and I-75, that would put you in Lake City Florida. Also, if you were on I-10 intersecting with I-5, that would put you in Los Angeles California. Another example would be if you were at the intersection of I-5 and I-90, that would place you in Seattle Washington. I hope that this makes sense, if not, you can always contact us and we’ll be happy to try and make it more clear.

How Highways And Interstates Are Numbered

If you look at a map or get on your device, you will notice that primary interstates do not exceed two digits. However, routes on the other hand use up to three digits. Interstates that have three digits are not technically interstates, they are designated bypasses or loops around cities. There are two types of highways and interstates, primary and spurs, although interstate spurs are commonly called bypasses. Primary interstates are one to two digit roadways that interconnect one state to the next. Routes that are primary, run from single to three digits. You will find primary routes run in the 1’s to 10’s range. Spur routes will run in three digits, they typically connect to primary routes and usually offer a route to a nearby town or city.

Your major cities usually always have a loop that runs around it. Houston Texas is a great example of how this works. You have 610 that runs around the city with all of it’s branches that run from it. I-10 runs across Houston, which is an even number for east and west. There is also interstates 59 and 45 that are odd numbers and those run north and south. Up at the northwest corner of 610, there is 290. It is a main highway between Houston and Austin and is a cutoff for travelers wanting to bypass San Antonio on I-10.

Routes or highways, run off the interstates. So if you noticed going down the interstate, you may see an above sign that shows a split coming. It may read “I-47 Franklin go straight and 24 East Marion to the left 1 Mile”. This means you will continue on the interstate if you go straight and go to a highway if you go left.

Mile Markers

Mile markers can tell you what direction you are going. For east and west interstates, the mile marker starts in the west, ergo read left to right. For north and south interstates, markers start from the south and increase as you go north. Interstate I-10 practically runs coast to coast. From Jacksonville Florida to Santa Monica California. It is 2460 miles across. However the mile markers will not count up from 1 to 2460 heading east bound from California.

Mile markers usually change numbering at state borders. So if you knew that Texas was 877.5 miles across on I-10 and you start at 1. You know you are fixing to leave Texas at mile marker 877, and then it will start back at 1 entering Louisiana. The same method applies going north and south as well.

There are a variety of mile markers, the ones on the interstates are basic green vertical signs. They say “Mile” at the top and display a number. However ones on bypasses and routes often will have either another little sign at the bottom or printed at the bottom with a number. Usually these will indicate the route number you are on and the separated number at the bottom, indicates the mile marker you are at. Often routes and bypasses call these markers, “Milepost”. 

Being Familiar With Route And Interstate Signs

Below are some signs that you will find on the interstates and highways. All of the images can be enlarged by clicking on them.

Business Routes And Loops

Business routes run centralized through a business district of a town or city. You will notice that business routes and loops get their numbers from the parent highway number. These routes and loops parallel with main route. Say you have highway/route U.S. 20, there can be a a U.S. 20 Business route. Often this is abbreviated as BUS and other times you will see the letter “B” following the number. Example would be U.S. 20B, “B” would stand for Business.  

Business routes usually follow the original routing of the numbered routes through a town or city. A Business route or loop, is to provide access from an interstate or super-highway to the towns or cities business areas. It bypasses the freeways or heavy public traffic. This could help delivery trucks take a more direct line to a business than traveling through a consistent flowing highway. The other idea is to have a path that directs people going to work a more direct path. It is all part of traffic management. To view a Business Route Guide, click HERE…

Exit Numbers

Exit numbers generally coincide with the mile makers. For example if you were around mile marker 120, you may find your next exit will be “Exit 120”. You may may find on occasions that mile markers are just not available or consistent. Exit numbers can help track you location or your pace. Let’s say you heading west and you know you are needing to get off Exit 247. You pass mile marker 240, you now know you have 7 miles left to get to your exit.

In most cases, exit numbers flow with the mile marker relation. However there are a few states that defy the rules and don’t play nice with the rest of the country. Some states, although very few, want to number exits sequentially. Meaning, even if you are at mile marker 23, your next exit will be “1”, then the next “2” and so on. Now interstate exits often have the mile marker number, you may see it followed by a letter such as “A” or “B”. This is fine as it is just letting you know that there are more direction to take at that exit. However, as a general rule, exit number correspond with mile markers.

Helpful Links:

Instate System

Highway User’s Guide

Bureau of Transportation Statistics

Interstate Routes, Mile Markers & Exit Numbering

Glossary of road transport terms