While driving across South Dakota today, I came across three points along I-90 that had big red drop gates, which would close the highway if dropped. A mile from these points there are signs that say “Highway closed if lights are flashing.” The gates are placed just after an exit, so that you can take the exit, and just go back the way you came. Another sign, at the exit where the gate is placed, actually says that! It says “If highway is closed, exit here and return to [name of city]” I’ve never seen anything like this on any US Interstate before. Anybody know what there are about? They’re in the middle of nowhere too. Weird.