high road vs slip road

high road

noun
  • A main road or highway. 

  • Used other than figuratively or idiomatically: see high, road. 

  • A course of action which is dignified, honourable, or respectable. 

slip road

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

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