US Department of Transportation Hazardous Materials Placard
These will only be seen on trucks, trains, or other vehicles that transport hazardous materials. If you can see this placard from safe distance, tell the 911 dispatcher the color of the placard, any symbols on it, and the 4 digit number in the white box (not all placards will have this). Vehicles generally only need to be labeled if they are carrying over 1000 pounds of a single material. If they are carrying a mixed load and each material is less than 1000 pounds, they are not required to have a placard.