pace car vs safety car

pace car

noun
  • A car that paces the race cars to keep them running at a slower pace due to race conditions, and which the other cars cannot pass. 

  • A car that other cars drive behind before a race starts. 

safety car

noun
  • A vehicle employed to lead the competitors in an auto race for the duration of a few laps, with the intention of restricting their speed while a hazard exists on the circuit. 

How often have the words pace car and safety car occurred in a corpus of books? (source: Google Ngram Viewer )