This is the supplemental brake light that is automatically activated by unexpected stops via a built-in sudden braking sensor. It works independently from your car’s braking system and instead monitors your braking force over 100 times per second. When the device detects a sudden hard stop, it emits an ultrabright pulsing red light, signaling an otherwise distracted driver of your sudden halt. It attaches to a rear window via included adhesive tape and fits nearly any vehicle. Easy to install, it requires no wires or charging cables and is powered by a built-in battery rated to last for up to four years. 91/4" W x 11/2" D x 1" H. (4 oz.)