If hazardous materials are being transported, how are other drivers warned?

Question

Diamond-shaped signs called placards are placed on the outside of vehicles carrying hazardous materials to alert other drivers to the presence of the materials.

Accepted Answer

By the posting of placards on the truck

Answer

By the flashing lights on the truck

Answer

By a siren

Answer

By the driver waving an arm out the window

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If hazardous materials are being transported, how are other drivers warned?