When the Interstate Highway system was built, it was a huge undertaking. To avoid confusion with the traditional US route system, the first thing they needed to do was invent a way to keep route numbers from being confused. With the US Route highway system, numbers start in the east and north ending with the higher route numbers to the west and south. With the Interstate Highway System, they simply reversed it so that there would not be a conflict between older US routes and Interstate Highways. So, low numbered Interstate Highways are to the west and south with higher numbers in the north and east. In both cases, odd numbers travel north and south and even numbers travel east and west. There are some notable exceptions to this system but for the most part they have stuck to it.
With the Interstate Highway system, major corridors have two digit designations. Three digit designations are reserved for beltways and spurs in and around major cities. The second and third digit of a three digit belt or spur highway designates the two digit major Interstate highway that it connects to. For instance here in Columbus we have I-70 traveling through the city so our beltway is I-270. We also have I-670 that facilitates access between the airport and downtown. Note that both end in “70” because they connect to I-70. This system allows for duplicate Interstate highway numbers when in a distant city there is another belt or spur that connects to the same major corridor. It’s not a problem having three digit duplicates since they will always be geographically separated. However, three digit routes with the same designation are not allowed within the same state. When a three digit Interstate highway goes around a city as in a beltway it will start with an even number as in I-270. If it is a spur that departs the major corridor and ends somewhere apart from the system it will start with an odd number. These are the rules but you will find exceptions.
Mile markers on the Interstate highway system start at the state line to the south and west. Exit numbers are derived from these mile markers. So if you enter a state at mile marker one and the first exit is three miles into the state, it will be exit three even though it is the first exit. This is a brilliant way to allow for the addition of future exits. When there is more than one exit between two mile makers, they will add a letter to the exit number such as “exit 3A and 3B.”
Knowing how these exits are numbered allows you to calculate mileage and driving time quite easily. If you are traveling to exit 101 and you are at exit one, then you’ll have 100 miles to go. It’s easy to calculate time at this point. If you are traveling an average of 60 miles per hour or a mile a minute, it will take you 100 minutes, or an hour and 40 minutes to arrive at exit 100. I’ve used this estimation system often and it is amazingly close to what actually happens. Possibly you are debating whether to stop at a rest area or wait until the next one. Just look at the map and see what exit numbers surround that rest area and then look at the mile marker you are at and you’ll be able to calculate how long it will take to get to the next rest area.
Sometimes the math can get a little complicated if you enter a state at the high side of the mile marker count. Then just use subtraction instead of addition to calculate.
There are some instances where exit numbers are just sequential instead of mile marker based and this makes it more difficult to calculate driving time.
The longest tunnel and highest point on the Interstate highway system is at the Eisenhower/Johnson Memorial Tunnel in Colorado on I-70 cutting through the Rockies. The height above sea level at this point is 11,013 feet.
The longest Interstate highway stretches 3,085 miles. Designated as I-90, it travels from Boston to Seattle.