hard shoulder vs neutral ground

hard shoulder

noun
  • A verge to the side of a highway, which should be used only in case of an emergency (especially on a motorway) 

neutral ground

noun
  • The median strip or central reservation between lanes of traffic in a divided highway. 

  • Used other than figuratively or idiomatically: see neutral, ground. 

How often have the words hard shoulder and neutral ground occurred in a corpus of books? (source: Google Ngram Viewer )