Apparently, the signmaker accidentally added an extra "l" & gave a discount if the owner took the sign as-is, thus condemning at least 2 generations of Sioux Falls kids to a lifetime of trying "barrell," "barell" & "barrel" & still never being sure which spelling is right.
Lloyd Eagan first launched a drive-in in St. Cloud, MN. According to Eric Renshaw in the Sioux Falls paper, the Argus Leader, "While the Minnesota location struggled, Eagan was inspired to build another in Sioux Falls, a place he'd never been before. He secured a plot of land south of town on which to build. In 1939, the street lights of Sioux Falls were not installed as far south as 31st and Minnesota, and the light from the Barrell was a welcome sight to thirsty crowds."