mobility scooter vs roller skate

mobility scooter

noun
  • An electric-powered scooter, similar in use to a wheelchair, that is specially designed for disabled and/or elderly people. 

roller skate

noun
  • A boot having small wheels or casters attached to its sole; used for roller skating. 

verb
  • To skate using roller skates. 

How often have the words mobility scooter and roller skate occurred in a corpus of books? (source: Google Ngram Viewer )