The radioactive placard on a vehicle indicates that the vehicle carries some amount or regulated radioactive material.
It is a DOT hazmat placard meaning "compressed oxygen."
Many hazard warnings exist: flammable, corrosive, toxic, radioactive, explosive, etc.
The DOT hazard classification system can be used to identify acute hazards.
The Dangerous When Wet placard is a square on point (or diamond) placard with white markings on a blue background. The words "Dangerous When Wet are printed across the middle, with a drawing of a flame centered in the upper corner, and the numeral four (4) in the bottom corner to indicate the Hazard Class.
explosives
The number 4 on the NFPA 704 placard indicates the highest hazard in any of the three categories (health, fire, reactivity).
COLOR
Radon is a colorless radioactive gas considered to be a health hazard.
color of label or placard
The most common radioactive product carried in a tanker is uranium hexafluoride. If I was under a tank of this, I would put four placards on it: radioactive, corrosive, dangerous when wet and inhalation hazard. Technically all you need is the first two, but you could put the other two on and still be okay.
color of the label or placard
A placard identifying hazard class 6 describes the presence of a poisonous gas. For an image, see the related link, below.