 A 1927 law gave it the number and extended it west to the Nevada state line, where it became SR 25 to Panaca.In 1935, the portion from Beryl Junction west to Nevada became an extension of SR-56, which ran east from Beryl Junction to Cedar City.A branch from Enterprise west to Nevada, where SR 75 continued as a shortcut to Panaca, was added to SR-18 in 1941, but split off as SR-120 in 1945.