Why are interstates numbered the way they are

The major route numbers generally traverse urban areas on the path of the major traffic stream. Generally, this major traffic stream will be the shortest and most direct line of travel.

Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix. To prevent duplication within a State, a progression of prefixes is used for the three-digit numbers.

For example, if I runs through three cities in a State, circumferential routes around these cities would be numbered as I, I, and I The same system would be used for spur routes into the three cities, with routes being numbered I, I, and I, respectively. This system is not carried across State lines. As a result, several cities in different States along I may each have circumferential beltways numbered as I or spur routes numbered as I Interstate System Dwight D.

Interstate Exit Numbers The States typically use one of two methods of numbering the Interstate interchange exits.

The Consecutive numbering system -- Starting at the most westerly or southerly point on each Interstate route, interchanges are numbered consecutively. Thus the first interchange becomes Interchange 1.

Each succeeding interchange is numbered consecutively as 2, 3, 4, etc. The Milepost numbering system -- All Interstate routes are mileposted beginning at the most westerly or southerly point. The beginning point is milepost '0'. Likewise, the name I was reserved for the farthest south major interstate traversing an east-west route.

Image by the author. The short and inglorious I, shown in red. Public domain image. There are a few very short two-digit interstates that probably should have gotten three-digit numbers instead. 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:.

Three-digit interstate naming system. For example there are seven different Is , and four different Is. There are of course numerous exceptions. In the east, U. In the west, U.

The interstate highway numbering system is similar to the U. Main interstates get one- or two-digit numbers, while loops and spurs get three-digit numbers that reflect their parent roads. To prevent confusion, one important difference distinguishes interstates from U.

While east-west interstates also get even numbers, those numbers get smaller not larger as you travel west.