With the introduction of Interstate 40 (I-40), US 66 began declining considerably, with some of the towns along the highway becoming ghost towns. Following the completion of I-40, US 66 was completely decommissioned by the Arizona Department of Transportation (ADOT) in 1984, then retired nationwide the following year. Significant portions of the old highway remain, such as State Route 66 (SR 66) between Kingman and the Yavapai–Coconino county line east of Peach Springs. Since 1987, other sections have been designated as Historic Route 66, which is both an Arizona Historic Road and a National Scenic Byway.