Final answer:
Vehicles carrying hazardous materials are required by United States Department of Transportation regulations to have four placards, one on each side of the vehicle, displaying the hazard class of the material.
Step-by-step explanation:
When it comes to vehicles carrying hazardous materials, the United States Department of Transportation has specific regulations that mandate the use of placards. These are usually diamond-shaped signs that are placed on all four sides of a transport vehicle to indicate the presence of hazardous materials. The number of placards required is generally four: one for the front, one for the rear, and one for each side of the vehicle. Each placard must display the specific hazard class of the material being transported. These regulations are crucial for the safety of the public, the environment, and emergency responders in the event of an accident or spill.