danger signal vs exhortation

danger signal

noun
  • 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. 

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

exhortation

noun
  • Language intended to incite and encourage 

  • The act or practice of exhorting; the act of inciting to laudable deeds; incitement to that which is good or commendable. 

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