Placards should be used:

Question

Warning placards are required to be used any time a vehicle is carrying designated types and amounts of hazardous materials. A placarded vehicle must display placards on all four of its sides.

Accepted Answer

Any time a vehicle is carrying hazardous materials.

Answer

When there is extra space on a truck.

Answer

Only when the amount of hazardous materials on a truck exceeds 200 pounds.