US Highway 566 was one of the two most obscure branches of Route 66 (the other being US Highway 366). As planned by the Joint Board of Interstate Highways in 1925, US 566's entire routing existed only in the state of New Mexico and was thus not a true “interstate” highway. The number and its proposed route were submitted to the American Association of State Highway Officials (AASHO, now AASHTO) and approved in November 1926.
Beginning at Hondo, at its junction with US Highway 366, US 566 ran westward for nearly 120 miles to Socorro. There, it ended at the junction with US Highway 85, the major north-south route through New Mexico (today’s Interstate 25). US 566 appears to have existed for no other purpose other than to provide travelers heading west from Roswell (scene of what may have been a UFO crash) with an easy route to Socorro and US 85.
US 566 didn’t last long, however. In June 1931 AASHO made a number of changes in the New Mexico federal highway plan. Among those changes was the creation of a new highway, US Highway 380, from Albany, Texas to Socorro, New Mexico. This new route absorbed the routing of US 566 and eliminated it altogether. The fifth branch of Route 66 had lasted less than five years and is now known 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)
Sanderson, Dale, "End of Historic US Highway 566", US Highway Ends. January 2002. <http://www.geocities.com/Yosemite%2FFalls%2F3369/HwyEnds/End566/end566.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)