danger signal vs distress signal

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. 

distress signal

noun
  • An indication of distress. 

  • An emergency signal sent by radio (or other means) from a ship or aircraft in distress or danger. 

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