This purpose of this report is to determine the effect of rear turn signal color on the likelihood of being involved in a rear-end crash. Federal Motor Vehicle Safety Standard No. 108 allows rear turn signals to be either red or amber in color. Previous work on this subject includes laboratory experiments and analyses of crash data that suggest amber rear turn signals are beneficial. The present study was designed around the concept of "switch pairs" - make-models of passenger vehicles were identified that had switched rear turn signal color, and crash involvement rates were computed before and after the switch.