E1 How is the Interstate Highway System Numbered?
There’s a very strict plan for numbering Interstate routes. The two-digit routes are the mainlines, with the even numbered routes going east-west, and the odd routes going north-south. The even routes’ numbers increase as you go north, with the odd Interstate roads increasing to the east. The major routes end in 0 or 5. As for three-digit interstates, those that both start and end at a 2-digit route get an even first digit, while those with a dangling end have an odd first digit. The last two digits are the parent 2di.
