Auxiliary Interstate Highways

Auxiliary Interstate Highways are circumferential, radial, or spur highways that principally serve urban areas. These types of Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of a nearby primary Interstate Highway. Spur routes deviate from their parent and do not return, with a few exceptions; these are given an odd first digit. Circumferential and radial loop routes return to Interstate Highways, and are given an even first digit. Due to the large number of these routes, auxiliary route numbers may be repeated in different states along the mainline. Some auxiliary highways do not follow these guidelines, however.

Unlike primary Interstates, three-digit Interstates are signed as either east/west or north/south, depending on the general orientation of the route, without regard to the route number. For some looped Interstate routes, inner/outer are used as a directional labeling system, as opposed to compass directions.