I stand corrected. Riding a bike under the influence may be legal. I did a little research and came across this.
“Starting July first, South Dakota bar owners may want to install hitching posts and bike racks out front. That’s when a new law takes effect that insulates bike and horse riders from being arrested for drunken driving. Intoxicated South Dakotans should be able to ride horses or bikes home from bars without fear of being arrested for drunken driving, legislators decided Wednesday.”