cloverleaf vs level crossing

cloverleaf

adj
  • In the form of the leaf of a clover plant; cloverleafed. 

verb
  • To turn by following a cloverleaf, or as if by following one. 

noun
  • (with plural cloverleafs or cloverleaves) A cloverleaf interchange. 

  • The leaf of a clover plant 

level crossing

noun
  • An at-grade crossing between a railway line and an ordinary road, with rails and road at the same level. 

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