One of the more obscure branches of Route 66, US Highway 366 is the only one that was commissioned twice.
When the Joint Board of Interstate Highways published their recommendations in 1925, the report called for five (later six) branches to extend from the parent highway, and US 366 was planned to be the third branch.
US 366’s first incarnation, when it was commissioned in 1926, was completely within the states of New Mexico and Texas. Its northern terminus was at a junction with 66 at Amarillo, Texas. From there, it wound southwesterly down into New Mexico, through the cities of Clovis, Roswell (not yet the famous UFO crash city), and Alamogordo. US 366 then turned due south and re-entered Texas briefly, ending at El Paso. This version of the route, totaling over 470 miles, served as a major regional connector route.
Such was the highway’s routing until the American Association of State Highway Officials (AASHO, now AASHTO) reworked the highways of both states in June 1931. AASHO decided to extend US Highways 60 and 70 through Texas and New Mexico, resulting in a loss of the US 366 designation between Amarillo and Hondo, New Mexico. The remainder was absorbed by an extension of US Highway 54, the alternate westward route from Chicago by way of Wichita, Kansas.
AASHO was not finished with 366, however. Instead of being retired, the number was moved to a short 73-mile stretch of road completely in New Mexico that had been US Highway 470. In this version, US 366 now began in downtown Albuquerque, at another junction with US 66, and ran in a southeasterly direction to the small town of Willard. There it remained until 1938, when an improved highway carrying US 66 was built from Santa Rosa, the New Mexico state capital, to Albuquerque. This new highway absorbed the part of US 366 between Albuquerque and Moriarty; the rest, from Moriarty to Willard, was given a state highway number. The second US 366 thus lasted just over a decade, and is largely known today only to highway historians.
Droz, Robert V., "Sequential List of US Highways", US Highways From US 1 to US 830. July 2003. <http://www.us-highways.com/us1830.htm> (February 2004)
Weingroff, Richard F., "U.S. 666: Beast of a Highway?", Federal Highway Administration Highway History. June 2003. <http://www.fhwa.dot.gov/infrastructure/us666.htm> (February 2004)