placard vs warning sign

placard

noun
  • A sheet of paper or cardboard with a written or printed announcement on one side for display in a public place. 

  • A kind of stomacher, often adorned with jewels, worn in the fifteenth century and later. 

  • An extra plate on the lower part of the breastplate or backplate of armour. 

  • The woodwork and frame of the door of a closet etc. 

verb
  • To announce with placards. 

  • To affix a placard to. 

warning sign

noun
  • A sign that indicates a danger, for example to vehicles on a road or of hazardous chemicals. 

  • An indication that someone or something is in danger or at risk 

How often have the words placard and warning sign occurred in a corpus of books? (source: Google Ngram Viewer )