State Route 78 (Arizona–New Mexico) State Route 78 (Arizona–New Mexico) Arizona State Route 78 (SR 78) and New Mexico State Road 78 (NM 78) are a pair of adjoining state highways located in eastern Arizona and western New Mexico linking U.S. Route 191 (US 191) and Arizona State Route 75 near Greenlee County Airport to US 180 northwest of Cliff, New Mexico. It is also known in its Arizona stretch as Mule Creek Road. The western terminus of SR 78 is located at a junction with US 191 and SR 75 at Three Way, Arizona near Greenlee County Airport. The highway heads northeast from this
Arizona State Route 79 agency, as of 2011 Arizona State Route 79 was the fastest road in the nation, with the fastest 5% of the vehicles using it clocked at an average speed of 88 miles per hour. SR 79 is the section of what was together U.S. Route 80 and U.S. Route 89. US 80 was removed from Arizona in 1989, and US 89 was removed from southern Arizona in 1992. The original SR 79 was designated in 1924 as part of the original state highways from Prescott to Jerome. By 1926, it extended northeast to Flagstaff. In 1941, this was redesignated as