Special route

In road transportation in the United States, a special route is a prefixed and/or suffixed numbered road that forms a loop or spur of a more dominant route of the same route number and system. The dominant route is generally referred to as the "parent" or "mainline", while special routes are also unofficially or neologistically known as child routes or auxiliary routes.

Special routes are included in many highway systems in the United States; most are found in the Interstate Highway System, U.S. highway system, and various state highway systems. There are numerous types of special routes, each possessing generally defined characteristics and having defined relationships with the parent route. Types of special routes: Business Route, Scenic Route, Truck Route, Spur Route, Alternate Route, Bypass Route, Connector Route, Toll Route, Temporary Route, Loop Route, Permanently signed detour route, and Divided Route (see List of divided U.S. Routes)

Nomenclature

In the field, the special route is typically distinguished from the parent route with the use of auxiliary words or suffix letters placed on the route shield or on an adjacent sign, known as a "banner" or "plate" or according to the Manual on Uniform Traffic Control Devices, a "route sign auxiliary sign". A common roadfan synonym for special route is "bannered highway" or "bannered route", terms coined from the presence of these companion signs. The term is not all-encompassing however, as not all special routes have these banners.

List of auxiliary Interstate Highways

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, 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.