It’s a bright taillight...until you hit the brakes. Then, it flashes four times fast - pauses - three flashes a little slower - pauses - two more real slow, and it’s on full time as a stop light. The entire flash/pause sequence takes 4 seconds and repeats each time you re-apply the brakes. This complex pattern is commonly and intuitively recognized as a deceleration warning. The bulb’s reliable digital electronics are all contained inside the bulbs base. It uses a widely available, easily replaceable ‘G-4’ halogen bulb. (An easy to carry extra is also included.) Fits standard #1157 dual filament brake/taillight bulb sockets. Legal disclaimer: May not be approved for use in all states.