The third rear brake light increases the visibility of a car when on the roadway, thus reducing the possibility of rear-end collisions. It works because the third light is placed so it is directly in a driver's line of sight.
The inventor was Dr. John Voevodsky, a psychologist who was studying why people got distracted when driving. He created a small, inexpensive brake light and convinced a San Francisco cab company to help with an experiment. The cab company equipped 343 of their vehicles with the device, amounting to almost two-thirds of its fleet. The rest of the cabs were left with the standard two-light system.
Drivers were assigned to the cars at random. The experiment lasted 10 months. At the end, the group with the third light had 60.6 per cent fewer rear-end accidents. Injuries from those accidents also decreased 61.1 per cent from the norm. Repair costs saw a similar decrease.
Dr. Voevodsky's 1974 experiment proved that the third light successfully captured the attention of drivers. By 1986, all new cars built and/or sold in the United States were required to have that third light. Depending on the model, the light is usually on the top or bottom of the back window. It's also sometimes found on the trunk lid.