As a vehicle approaches from behind... an inatentive driver, perhaps texting or dosing. If you are driving say at 55mph and a vehicle is fast approaching from behind you at a rate of speed that will colide with you, sensed by the rear bumper sensors, when the onboard computer determines the safe braking distance (of vehicle from behind) is getting dangerous your brake light would automatically activate, flash if the vehicle gets closer. Maybe the textor or dosing driver would be alerted by your brake lights and avoid slamming into you from behind.