Final answer:
Placards can be removed from a vehicle carrying dangerous goods when the materials are offloaded, or if the quantity falls below the regulated threshold which requires placarding.
Step-by-step explanation:
The placards on a vehicle carrying dangerous goods can be removed when conditions change such that placards are no longer required. This usually means that after the hazardous materials have been completely offloaded and no longer present in the vehicle, the placards can be taken down. Additionally, it may also apply if the amount of hazardous material falls below the threshold that necessitates placarding. It is crucial for the safety of the public and the environment that these regulations are strictly followed.