Free Safety Signs

What do the hazard signs on the back of semi's/tractor trailers mean?

I know the NFPA signs but what do the white signs that are also shaped like the NFPA signs the ones i have seen have a number and a word usually just hazardous. Any help is much needed.

Public Comments

1. it means there's a hazard......
2. They are called placards and they generally mean that the truck is hauling more than 1000 lbs of a hazardous substance. However certain hazardous substances require placards when any amount is hauled. Such as class A explosives. The placard is there mainly to notify officials as to what type of hazardous cargo the truck is carrying in the event of an accident or other emergency.