By 1981, due to the construction of Interstate 15 and extension of other U.S. Highway designations, US 191 had been truncated to a route between West Yellowstone and Malta, Montana. Utah and Wyoming lobbied AASHTO to create a single highway that would connect several National Parks in their states to facilitate tourism. The initial proposal was to extend U.S. Route 163 north into Wyoming. However it was later decided to add Arizona to this proposal for a new highway and instead extend US 191 south. Most of the southern extension recycled existing U.S. and state highways. However, some new sections were constructed. When the 1981 extension was finished, US 191 reached Interstate 40 in Arizona.