When is it mandatory to use a HAZMAT placard?

Using a HAZMAT placard is mandatory when transporting a quantity of hazardous material that exceeds regulatory thresholds set by federal and state authorities. This requirement exists to ensure that anyone who might encounter the transport vehicle, including emergency responders and the general public, can easily identify the potential dangers associated with the materials being transported.

The thresholds determine the levels at which the risk posed by the hazardous materials warrants clear labeling to mitigate risks during transport. For instance, certain materials might be considered hazardous at lower quantities, while others might require a more significant amount to reach a level where placarding is necessary.

Understanding these thresholds is crucial for compliance with regulatory requirements and for ensuring public safety during the transportation of hazardous materials. This is why the correct answer emphasizes the need for placarding only when these specific thresholds are exceeded, rather than in all scenarios or for all vehicles.