billboard vs warning sign

billboard

noun
  • A very large outdoor sign, generally used for advertising. 

  • A sprite that always faces the screen, no matter which direction it is looked at from. 

  • A piece of thick plank, armed with iron plates, and fixed on the bow or fore-channels of a vessel, for the bill or fluke of the anchor to rest on. 

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 billboard and warning sign occurred in a corpus of books? (source: Google Ngram Viewer )