The DOT requires displaying placards on each side and end of the freight container, unit load device, transport vehicle, or rail car carrying hazardous materials. Placards must be displayed on all 4 sides of a transport vehicle. Some carriers also place two extra signs on either side of the truck cab for additional visibility.  Each placard on a motor vehicle and rail car must be clearly visible from the direction it faces, except the direction of another transport vehicle or rail car to which it is coupled. Placards must be located clear of appurtenances and devices such as ladders, pipes, doors, and tarpaulins and away from any marking (such as advertising) that could substantially reduce its effectiveness, and in any case, at least 3 inches (76.0 mm.) away from such marking. DOT placards must be located where dirt or water from the wheels is not directed. Any lettering or numbers on the placard (when required) must read horizontally, left to right. DOT placards have to be on the background to contrast the color or must have a dotted line or solid line border.