footbridge vs sidewalk

footbridge

noun
  • A bridge over a road, railway, river, etc for pedestrians. 

sidewalk

noun
  • (usually) a paved footpath located at the side of a road, for the use of pedestrians 

  • any paved footpath, even if not located at the side of a road 

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