The 200-mile extension of Interstate 69 south of Fort Wayne, Indiana will soon cause an unusual event for the Indiana Department of Transportation – the changing of all of the exit numbers on the existing stretch of freeway north of Fort Wayne to the Michigan border.  The American Association of State Highway and Transportation Officials (AASHTO) requires that Interstate mile numbering begin at mile 0 at the south end; so what was once Exit 1 will now be Exit 201 (they were glad for the nice round number, I’m sure!).

But mile markers are only a part of the prescriptive numbering system AASHTO uses for the Interstate System.  In 1956, the Interstate System was planned and funded to supplement the U.S. Highways, and a similar numbering system was implemented.  Odd Interstate numbers are assigned to north-south highways, and the numbering begins in the West with Interstate 5.  East-west highways have even numbers, and begin in the south with I-4.  The US Highways were assigned in 1926 to address the need for standardization.  Routes were often named differently from state to state and as interstate travel increased, so did driver confusion. The Interstates are numbered in reverse order from the US Highways so they wouldn’t coincide: US Highways running north-south were assigned odd numbers, beginning with US 1 in the east.  Those running east-west have even numbers, beginning with US 2 in the north.

Interstate highway map of Indiana and Michigan
Interstate highways in Indiana and Michigan – click to enlarge

Interstate Highways with numbers divisible by 5 were intended to be major corridors, stretching further across states than the others.  That holds true for the most part, but, for example, I-94 (which, as it runs through Michigan is one of the longest Interstate stretches through the state) is longer overall than several of the Interstates that end in 5 or 0.  Indiana has more major Interstates than Michigan, including I-94 through Northern Indiana, as well as I-65, I-70, I-80, and I-90.

In the Interstate numbering system, the primary highways have one or two digits. The highest numbers are I-96 and I-99. But many large cities have auxiliary routes that either branch away from a primary route (these are called spurs), or loops away and then back to the same primary route (these are called circumferential highways). So that you can always know whether you will eventually get back onto the primary route, spurs get an odd digit affixed to the front of the two-digit primary route number, and loops get an even digit. For example, I-196 branches away from I-96 in Grand Rapids, and ends in Benton Harbor at I-94, so it is a spur. I-465 loops around Indianapolis from I-65, so it is circumferential, as is I-496 as it loops into Lansing, then intersects again with I-96.

Who needs GPS??