[Cooter]

Three-digit Interstate routes have a leading digit with the last two digits being the mainline route that it supplements, so I-x95 routes would have routes such as the ones in Virginia, I-195, I-295, I-395 and I-495. I-664 is an I-x64 route. An odd-number leading digit signifies a spur route off a mainline route (examples are I-195 and I-564). An even-number leading digit signifies a loop around a city (examples are I-295 and I-495), or a branch route through a city (an example is I-264). Three-digit Interstate route numbers can duplicate, but not in the same state.

[nuffsenuff]

I was going to write that out. But I got lazy.