boulevard vs slip road

boulevard

noun
  • A broad, well-paved and landscaped thoroughfare. 

  • A strip of land between a street and sidewalk. 

  • The landscaping on the sides of a boulevard or other thoroughfare. 

slip road

noun
  • A segment of roadway that joins a motorway to ordinary roads (in either direction) 

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