The blue LED lights up when the door is opened. It alerts following vehicles and motorcycles, helping to prevent accidents. Sticks on the door side on the driver's or passenger's side. Can also be stuck on the door side of the rear seat. Set of 2. Battery used: CR2430 (battery included in set) The blue LED lights up when the door is opened. Two sets are required if you want to attach to four doors (front, rear, left and right).