A brief examination is made of the likely mechanism of action for the reduction in rear-end collisions arising from the installation of a central high mounted rear brake light. It is argued that the extra light would provide an alerting stimulus to increase the likelihood that the following driver or rider respond at all, rather than to decrease their reaction time. The cost effectiveness of the introduction of such an additional brake light is estimated in an attempt to show that the light is a cost-effective countermeasure to collisions into the rear of cars and car derivatives.