49 CFR § 172.516 - Visibility and display of placards.
(a) Each placard on a motor vehicle and each placard on a rail car must be clearly visible from the direction it faces, except from the direction of another transport vehicle or rail car to which the motor vehicle or rail car is coupled. This requirement may be met by the placards displayed on the freight containers or portable tanks loaded on a motor vehicle or rail car.
(b) The required placarding of the front of a motor vehicle may be on the front of a truck-tractor instead of or in addition to the placarding on the front of the cargo body to which a truck-tractor is attached.
(c) Each placard on a transport vehicle, bulk packaging, freight container or aircraft unit load device must—
(1) Be securely attached or affixed thereto or placed in a holder thereon. (See appendix C to this part.);
(2) Be located clear of appurtenances and devices such as ladders, pipes, doors, and tarpaulins;
(3) So far as practicable, be located so that dirt or water is not directed to it from the wheels of the transport vehicle;
(4) Be located 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;
(5) Have the words or identification number (when authorized) printed on it displayed horizontally, reading from left to right;
(6) Be maintained by the carrier in a condition so that the format, legibility, color, and visibility of the placard will not be substantially reduced due to damage, deterioration, or obscurement by dirt or other matter;
(7) Be affixed to a background of contrasting color, or must have a dotted or solid line outer border which contrasts with the background color.
(d) Recommended specifications for a placard holder are set forth in appendix C of this part. Except for a placard holder similar to that contained in appendix C to this part, the means used to attach a placard may not obscure any part of its surface other than the borders.
(e) A placard or placard holder may be hinged provided the required format, color, and legibility of the placard are maintained.