List of auxiliary Interstate Highways
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. 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. Terminology and guidelines. 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. Numerous exceptions to the standard numbering guidelines exist in many places. This can be for  a number of reasons. In some cases, original routes were changed, extended, or abandoned, leaving discrepancies in the system. In other cases, it may not be possible to use the proper number because the limited set of available numbers has been exhausted, causing a "non-standard" number to be used. Spur route. A spur route's number usually has an odd number for its first digit. It is usually one of the following: Examples include: Sometimes, a three-digit Interstate Highway branches off from another three-digit Interstate Highway. These spurs do not connect directly with their parent highways, but  are associated with them via the three-digit highways they do intersect with. Examples include: Bypass. A bypass route may traverse around a city, or may run through it with the mainline bypassing. In a typical 3-digit Interstate Highway, bypasses usually have both its two termini junctioned with another Interstate highway. Bypass routes are preceded by an even number in the first digit. Examples include: In the case of an auxiliary Interstate highway which has both ends at Interstates but not the same Interstate, some states treat these as bypasses while others treat these as spurs — see "Spur route