A deceleration-activated brake light system for a vehicle featuring a speed sensor, a brake sensor, a throttle sensor, and an engine brake switch; and a microcontroller operatively connected to a standard relay switch functioning to activate a standard brake light system of the vehicle, the microcontroller is configured to receive input signals from the sensors when the sensors detect deceleration of the vehicle more than a predetermined percent, coasting of the vehicle more than a predetermined amount of time, application of the brake pedal; or engagement of the auxiliary brake whereupon the microcontroller generates an output command to the relay to cause activation of the standard brake light system of the vehicle.