back street vs viaduct

back street

noun
  • A usually small and narrow street or alley, especially one in inferior or poorer parts of a city, away from the centre. 

  • A secret, clandestine or illegal scene. 

viaduct

noun
  • A bridge with several spans that carries road or rail traffic over a valley or other obstacles. 

How often have the words back street and viaduct occurred in a corpus of books? (source: Google Ngram Viewer )