A hazardous materials placard is a diamond-shaped sign that must be affixed to any motor vehicle that carries hazardous materials. Ordinarily, it has a four digit number in the middle of the placard and a one digit number at the bottom that shows the hazard class and specific material being carried.
Title 49 of the United States Code of Federal Regulations (49CFR) also known as the Federal Motor Carriers Safety Regulations (FMCSR) requires the use hazardous materials placards when shipping hazardous materials cargo and dangerous goods in the U.S. Many other countries such as Canada and Mexico also have similar regulations that also require the use of these placards.
Apart from the federal regulations, there are state rule relating to hazardous materials placards. For example, Chapter 393 of the New Hampshire laws imposes a $ 25 or $ 15 license fee requirement on any vehicle that carries enough hazardous materials or waste (typically, 1000 pounds) to fall within the federal rules requiring it to display a diamond-shaped hazardous materials "placard." [New Hampshire Motor Transport Asso. v. Flynn, 751 F.2d 43 (1st Cir. 1984)].
The following is an example of a federal regulation on hazardous materials placard:
49 CFR § 174.85. Position in train of placarded cars, transport vehicles, freight containers, and bulk packagings.
(a) Except as provided in paragraphs (b) and (c) of this section, the position in a train of each loaded placarded car, transport vehicle, freight container, and bulk packaging must conform to the provisions of this section.
(b) A car placarded "RADIOACTIVE" must comply with train positioning requirements of paragraph (d) of this section and must be separated from a locomotive, occupied caboose, or carload of undeveloped film by at least one non-placarded car.
(c) A tank car containing the residue of a hazardous material must be separated from a locomotive or occupied caboose by at least one rail car other than a placarded tank car.
This section also provides a table which shows where the placarded cars transporting hazardous materials should be placed.