Interstate Route Numbering Explained
Major Interstate routes are designated by one- or two-digit numbers. Routes with odd numbers run north and south, while even numbered run east and west. For north-south routes, the lowest numbers begin in the west, while the lowest numbered east-west routes are in the south.
Decode the interstates: What highway numbers actually mean
The one/two-digit system
There are three factors that go into determining the number for a two-digit interstate:
* East-west roads get even numbers, while north-south roads get odd.
* The lowest numbers begin in the south and west, and get higher as you progressively move north and east.
* The most important cross-country interstates get numbers divisible by five, meaning they end in zero or five.
Thus, for example, the name I-95 was reserved for the farthest east major national cross-country interstate that traverses a north-south route. Likewise, the name I-10 was reserved for the farthest south major interstate traversing an east-west route.
You can see how it works on this map, illustrating only the “zero” and “five” major interstates:
The three-digit system
Three-digit interstates are shorter routes that serve individual metro areas, as opposed to the two-digit intercity routes. They connect to longer two-digit routes, and act as beltways, spurs, or connectors. There are two factors that go into three-digit numbering:
* The latter two digits reflect whatever two-digit interstate the route connects to. For example, I-395 connects to I-95, and I-270 connects to I-70.
* The first digit reflects the purpose of the road. Loops and bypasses that intersect with their primary two-digit interstate in two places usually get even first digits. Spurs and connectors that only intersect once usually get odd first digits.
Odd numbers run north–south with numbers increasing from west to east, while even numbers run east–west with numbers increasing.
