U-turn vs backtracking

U-turn

noun
  • A reversal of policy; an about-face, a backflip. 

  • A turn in a vehicle carried out by driving in a semicircle in order to travel in the opposite direction. 

verb
  • To shift from one opinion or decision to its opposite. 

  • To turn a vehicle in a semicircle so as to face the opposite direction. 

backtracking

noun
  • The usage of a runway as a taxiway, especially at private strips and smaller airports. 

  • The act of building all possible solutions to a problem incrementally, abandoning any candidate solution if it cannot lead to a valid solution. 

  • The act of one who, or that which, backtracks; a retracing of one's steps. 

How often have the words U-turn and backtracking occurred in a corpus of books? (source: Google Ngram Viewer )