When placards are required
Placards are the diamond-shaped warning labels affixed to four sides of a hazmat-carrying vehicle. They are required whenever a vehicle hauls a hazardous material in a quantity that meets the federal threshold for the specific hazard class — generally one thousand pounds or more aggregate gross weight, with lower thresholds for specifically named "Table 1" materials including explosives, poison gases, and certain water-reactive materials, which require placards in any quantity.
Reading a placard
The number at the bottom of the placard is the United Nations hazard class number (1 through 9). The pictogram at the top represents the primary hazard. The four-digit number that sometimes appears in the middle is the UN identification number for the specific material. As a CDL holder, you should be able to identify the hazard class from the placard at a glance, especially in an emergency response scenario where you may be asked to communicate the hazard to first responders.
Loading and segregation rules
The Hazmat segregation table in the federal CDL Manual is a frequent test target. You should know that explosives and oxidizers cannot be loaded together; that flammables must be separated from oxidizers; and that radioactive materials carry the most stringent loading and placement requirements of any class. Loading practices — the prohibition on smoking near hazmat loads, the requirement to brake-set during loading, the no-cargo-near-the-driver rule — also appear consistently on exams.
For drivers preparing for Placards and Hazmat Loading Rules, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.