All of the supplement routes for Interstate 95 (I-95) are designated with a three-digit number ending in "95": I-x95.

There are three states that have no auxiliary Interstate Highways: Alaska, Arizona (though there was a planned Interstate 710 in Phoenix that would connect I-10 with the University of Arizona), and New Mexico.

North Dakota has an auxiliary route, but it is unsigned, and Wyoming's does not meet Interstate Highway standards.

Auxiliary Interstates are divided into three types: spur, loop, and bypass routes.

For instance, I-115 contains an odd number in the first digit (1), which indicates that this freeway is a spur.

In some cases, original routes were changed, extended, or abandoned, leaving discrepancies in the system.