Placards should be used:

Question

Placards must be placed on the outside of vehicles carrying hazardous materials. They identify the specific class or classes of materials being carried and warn others of relevant hazards.

Accepted Answer

Any time a vehicle is carrying hazardous materials.

Answer

If there is open space in the truck.

Answer

If the amount of hazardous materials in the truck exceeds 200 pounds.

Answer

If the hazardous materials create an inhalation hazard.