An Abrupt Braking Indicator (ABI) that complements a vehicle's existing rear brake light by determining the difference between regular brake pedal usage and dangerous abrupt stops or abrupt-braking that often causes accidents. It is when, and only when, an abrupt stop or hard brake occurs at the differential gravitational value of 0.5 G or more that the present invention emits highly noticeable flashing lights that warn following drivers of the sudden hazard ahead.