("Auxiliary route" redirects here. For auxiliary Interstate Highways, see List of auxiliary Interstate Highways.) 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.