Auxiliary route (Interstate)

Dwight D. Eisenhower National System of Interstate and Defense Highways
Highway shield for Interstate 295
Interstate Highways in the 48 contiguous states
System information
Formed:	June 29, 1956
Highway names
Interstates:	Interstate X (I-X)
System links
Interstate Highway System Main Auxiliary Suffixed Business Future

The auxiliary Interstate Highways (also called three-digit Interstate Highways) are a supplemental subset of the freeways within the Interstate Highway System of the United States. Similar to the mainline Interstate Highways, these highways also meet all Interstate Highway Standards (with rare exceptions), they receive the same percentage of federal funding (90%), and they comply with other federal standards.

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 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 these as bypasses.

These shorter auxiliary routes branch off of main routes and are numbered based on the number of the parent route, such that all I-x95 routes are supplemental routes for I-95. 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 may repeat in each state along their route, but they will not repeat within a state.

The basic tenets of the auxiliary Interstates are divided into three branches: spur, loop, and bypass routes. Each one signifies a different characteristic of the auxiliary route.

The first digit of the three digits usually determines whether a route is a bypass, spur, or beltway. The last two digits are derived from the main Interstate Highway. For instance, I-515 contains an odd number in the first digit (5), and this indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "15" in I-515 shows that it is a supplement to I-15.