A warning system housed in a license plate bracket and installed on a vehicle. The system includes a self-contained power source, an accelerometer to measure the deceleration force of the vehicle, and a microprocessor unit (including a microprocessor chip). When the system senses that the vehicle is decelerating at a rate greater than a threshold level, it illuminates warning lights provided on the license plate bracket to thereby alert drivers of neighboring vehicles of the vehicle's rapid deceleration. The warning system is self-contained and self-powered to avoid the need to, and associated drawbacks of, drawing from the vehicle's power source.