Question: Do those high center brake lights, required on all cars sold in the U.S. since 1986, really reduce rear-end collisions? A randomized comparative experiment with fleets of rental and business cars, done before the lights were required, showed that the third brake light reduced rear-end collisions by as much as 50%. Alas, requiring the third light in all cars led to only a 5% drop. What happened? What is the lurking variable behind the dramatic decrease?