What is a crucial factor when deciding to use placards?

The hazard class of the material is a crucial factor when deciding to use placards because placards are specifically designed to communicate the type of hazard that a substance poses during transportation. The classification of hazardous materials is governed by regulations that assign specific hazard classes based on the properties and risks associated with the materials. Different classes indicate different types of hazards, such as flammability, toxicity, or corrosiveness, which must be clearly communicated to ensure safety for emergency responders, transport personnel, and the public. This visual warning system allows for immediate recognition and appropriate response actions in the event of an accident or spill.

Transportation methods, vehicle size, or the identity of the transporter may all play a role in the broader context of hazardous material transport, but they do not fundamentally dictate the necessity or type of placarding. The primary purpose of placards is to inform of hazards, which directly correlates to the hazard class of the materials involved.