Question If hazardous materials are being transported, how are other drivers warned? Answers By the posting of placards on the truck By the flashing lights on the truck By a siren By the driver waving an arm out the window Explanation 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. Take a FREE Practice Test Now This questions appears in the following tests: Arkansas HazMat , District Of Columbia HazMat , Nevada HazMat , Oregon HazMat ,