CoolGov uncovered this US Highway Administration document that explains the numbering scheme behind the US interstate highway system.
* Major interstates routes have a one or two digit number associated with them. North-south routes have odd numbers (I-5) while east-west roads have even numbers (I-10).
* Connecting interstate routes or beltway loops around urban areas have 3 digit numbers (the 101).
* To prevent duplication within a state, a progression of prefixes is used for the three-digit numbers. For example, if I-80 runs through three cities in a state, circumferential routes around these cities would be numbered as I-280, I-480, and I-680.
* There’s no set standard on exit numbering, but states generally use one of two systems:
1. Milepost numbering. The southern or western-most point on a given interstate begins the odometer at 0. If an exit is 6.5 miles from that point, it’s exit #6 and so forth.2. Consecutive numbering. Again, starting at the western or southern-most point, each exit is given a number, starting with 1. When they have to shoehorn more exits in, they become #6A, #6B, etc…
(via Cool Gov)