Home » Living in US » How does Interstate Highway system in America work ? Exits, Numbering ?

How does Interstate Highway system in America work ? Exits, Numbering ?

When I first came to America, I had challenges understanding the interstate highway system. I would constantly run into questions like, why some interstates were numbered with 3 digits, where as some of them were numbered with 2 digits?  Odd numbers vs. Even numbers? What is an exit and how does it work?   Some of these things are very interesting and can be helpful sometimes when you do not have a GPS …This article will focus on some of the fundamentals of Interstate system in USA and some tips for you to drive according to rules.

What are Interstate Highways in USA ? How does the highway system work ? Exits ?

As the name signifies, interstate highways are nothing but highways between states connecting major cities in a state with major cities in other state. In fact, it is like a big network interconnecting various cities of all states in USA. They are often called as Freeways or expressways.  These interstate highways do not have any traffic signals or traffic lights. So, you can basically drive freely without any stops….You can get on and off from these high ways using exits.  An exit is nothing but a path, which is a lane, branching out from freeway….Exits enable vehicles to get  off the interstate without stopping your vehicle ….You just have to slow down when taking an exit. Typically exits are often one mile apart, but if you are in a big metro city they are much closer with A, B, C D and other letters assigned to the exit number… Also, the exit numbers starts from 1 as you enter a new state and goes up by 1 for every mile until you cross the state. Key thing to note is, every time you cross the state the exit numbers reset and start from 1 or a large number depending on the direction you are going.  So, do not need to freak out if you miss numbers at times…

How does the interstate highway system numbering work ?

Typically, major highways interconnecting cities are numbered with 2 digits and few of them with single digit. They are typically less than 100.

  • East-West interstate highways are even numbered
  • North-South interstate highways are odd numbered.

Interstate Highways System in USA

If an interstate highway is numbered with 3 digits, then it means it is not a primary highway and it is kind of loop (Very important, not to get lost) or connector in a city either diagonally or around a city. They are called Auxiliary interstate highways. You can see the below image.

Sometimes the interstates are also labeled as interstate business routes; it is just a route parallel to the actual interstate, but goes through city or small town. You do not want to get confused and take these during long drives because it will take your time away as these have traffic lights.

USA Interstate Highway system Rules and Information

All interstates have speed limits varying from 50 mph an hour to 75 mph depending on the state and position of the interstate. You have to drive within the speed limit, otherwise you can be pulled over and get a ticket. Read my experience Why NOT to drive over speed limit for consequences.  Also, in general slower traffic always stays towards right. Left lane is typically passing lane. Depending on state, it can be emergency or quick passing only lane too ( I was pulled over in Illinois for driving in left lane for 10 minutes continuously)  If are going in wrong direction or miss an exit, you should take next exit and then turn around properly, do not try to take short cuts by taking the feeder or small passing lanes between two freeways…they are meant for cops and service vehicles only…be careful.  Also, the interstates have zigzag patches or saw kind of patches on the end of the road, they are meant to alert you by making noise if you loose control by sleeping.   Coming to shoulder, there is a solid yellow or white line dividing the main free away lanes, do not drive in shoulder they are meant for Cops and service vehicles. Some of you may be used to driving in shoulder in home countries, not here in US.

Do you have any other things to share about Freeways in America or Interstate Highway system in USA ?

Image Credits :

http://upload.wikimedia.org/wikipedia/commons/0/0a/FHWA_Auxiliary_Route_Numbering_Diagram.svg and



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  1. What is the mile marker precedance when two interstates share the same segments for a ceratin distance. Such as I-20 and I-59.

  2. Thank you for that information, In the great state of California , there is some additional information regarding exits.

    When approaching interstate exits due to include the weather(Fog) the DOT she wrote she interstate exits, due to implement weather to weather weather parentheses foggy OK the department of transportation do you DOT has placed reflective markers on the ground . So when you are 3/10 of a mile from exit there will be theee markers next to each other on the side of exit. When at 2/10 of Mike there will be 2 markers at exit, when at 1/10 of mile their will be 1 marker.

  3. 1. When you are in a highway, if you need to get to a street, mostly you will have to exit from your right side. If you want to go from one highway to another highway you may have to exit from the left or right.

    2. ‘Lane ending’ If you are in the slowest lane (right most), Some times due to construction, they will signal you to cross over to the the lane on the left. In addition to overhead display with a arrow mark lights, and Orange drums, you can also look on the road for shorter stumpy lane separating white painted bars .

  4. we’re driving from sarnia,canada and going to denver. If i look at directions they ofter make me get off at an exit go through a city just to get you bac on the same highway, how to know not to take those exits and stay on main highways. Ex( are roads with i-and a number will make you go through a city and send me back on highway after. How can i avoid that and stay on highway only unless i want to visit that city. 3 didgits roads vs 2 like i-70 please explain and i would stop waisting so much time… Thank you so much

    • If you stay on the main interstate you should be fine. All interstates are labeled with an I in the beginning of the number. For example, if your on Interstate 80, or I-80, you would stay on there. Three digit Interstates are simply freeways that branch off of the main interstate. The first number should be a number between 1 and 9, and the last two digits should be the Interstate you’re on. So for example when you come into large metropolitan areas driving on I-80, you might see I-280, I-580, I-880, and so forth. Watch for sings because sometimes the main interstate exits and the freeway you were driving on becomes another freeway or another interstate as seen along I-80 in Sacramento at the US50 interchange and in Salt Lake City at the I-80 and I-15 Interchange. Hope this helps!! 🙂


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