List of auxiliary Interstate Highways

Auxiliary Interstate Highways (also called three-digit Interstate Highways) are a supplemental subset of highways within the American Interstate Highway System. Auxiliary routes are generally classified as spur routes, which connect to the parent route at one end, bypasses, which connect to the parent route at both ends, or beltways, which form a complete circle intersecting the parent route at two locations. There are 323 auxiliary Interstates in the United States. There are some routes which connect to the parent route at one end, but connect to another route at the other end; some states treat these as spurs while others treat them as bypasses. Similar to the mainline Interstate Highways, these highways also meet all Interstate Highway standards (with rare exceptions), and they receive the same percentage of federal funding (90%).

The shorter auxiliary routes branch off main routes and are numbered based on the number of the parent route. All of the supplement routes for Interstate 95 (I-95) are designated with a three-digit number ending in "95" in the form I-x95. While some exceptions do exist, generally spur routes are numbered with an odd hundreds digit (such as I-395), while bypasses and beltways are numbered with an even hundreds digit (such as I-695). Because longer Interstates may have many such supplemental routes, the numbers can repeat from state to state along their route, but they will not repeat within a state.

There are only three states that do not have any auxiliary Interstate Highways: Alaska, Arizona, and New Mexico. North Dakota has an auxiliary route, but it is unsigned, and Wyoming's does not meet Interstate Highway standards.