The center-mounted brake light was made mandatory for all cars sold in the United States starting in the 1987 model year. Studies have shown that the addition of a center brake light reduces the incidence of rear-end collisions by 50%. This is probably due to both the fact that the brake light is more in the driver's line of sight and that at night it is normally off, unlike the other brake lights which are on already since they're the taillights, and just get brighter when the brakes are applied.