Bannered highway

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)

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.

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Wikipedia