EYEWITNESS NEWS (WBRE/WYOU)— While the naming convention for the U.S. Interstate System may seem a little random, there’s actually logic behind most interstate names.

The idea of the Dwight D. Eisenhower National System of Interstate and Defense Highways, popularly known as the “Interstate System,” was brought into existence back in the late 1930s, and in 1947, the first section of the interstate system was put down.

Since then, the American Association of the State Highway and Transportation has introduced a naming convention for major interstates. These were their rules:

  • Major Interstates running NORTH and SOUTH should be named using one or two-digit ODD numbers (5,15, 25, 35, 45, 65, 75, 85, 95).
  • Major interstates running EAST and WEST should be named using one or two-digit EVEN numbers (10, 20, 30, 70, 80, 90).
Courtesy of Justin Glowacki

Those rules only apply to one or two-digit, major interstates.

When you see an interstate with three digits, something like I-285, that indicates it’s an interstate that splits off from the main interstate. So I-285, for example, would be an offshoot from I-85.

Whether or not the offshoot interstate returns to the main interstate also plays a role in the name.

If the first digit of the interstate is even, that means the offshoot returns back to the main interstate. If the first digit is odd, it does not return to the main interstate.

However, states do not communicate with one another when it comes to this naming system. This leads to interstates sharing the same name in multiple states.

Another flaw in the naming system is human error. Some interstates have been built and incorrectly named. This is where mistakes like I-99 in south and central Pennsylvania come from.

These mistakes unfortunately can’t be fixed by states, it has to be corrected on a national level.

The Federal Highway Administration said they don’t plan to change the name of incorrectly named interstates.

An official from the Federal Highway Administration, in part, said this:

An occasional inconsistency is inevitable in a complicated, evolving network. They cause little difficulty for the traveling public. Most motorists are not aware of the numbering pattern; when driving in areas with which they are unfamiliar, motorists choose routes based on maps, signs, or directions received along the way.

The Federal Highway Administration’s website