Every city has its odd details, features that don't make sense in the urban context. They may be great or small. New Yorkers, think of Staten Island; Parisians, the Montparnasse Tower. They can be smaller, too, something that could escape your notice. This picture is an example. The sign is on the Mississippi flood wall in a barren area north of the Arch. Hardly anyone works around there and about the only residents are in a nearby tent city built by the homeless. So who would need to evacuate, and for what? I mean, if the river is threatening flood you could take a leisurely walk up the hill to the city center. Is someone anticipating The Rapture?