caution vs danger signal

caution

noun
  • A careful attention to the probable effects of an act, in order that failure or harm may be avoided. 

  • A formal warning given as an alternative to prosecution in minor cases. 

  • Security; guaranty; bail. 

  • A yellow card. 

  • Prudence when faced with, or when expecting to face, danger; care taken in order to avoid risk or harm. 

verb
  • To warn; to alert, advise that caution is warranted. 

  • To give a yellow card 

danger signal

noun
  • An indication that a potentially bad or dangerous situation could develop. 

  • A signal set at danger and showing a red light or other stop indication, which a train driver must not pass until it changes or until manually cleared past the signal by a dispatcher. 

How often have the words caution and danger signal occurred in a corpus of books? (source: Google Ngram Viewer )