culvert vs safety valve

culvert

noun
  • A channel crossing under a road or railway for the draining of water. 

verb
  • To channel (a stream of water) through a culvert. 

safety valve

noun
  • A relief valve set to open at a pressure below that at which a container (such as a boiler) would burst. 

  • Any mechanism offering relief from pressure or tension, e.g. emotionally. 

  • A United States law, codified at 18 USC §3553(f), authorizing a judge to disregard mandatory minimum sentences for some criminals with few or no prior offenses. 

How often have the words culvert and safety valve occurred in a corpus of books? (source: Google Ngram Viewer )