A ghost bike is a memorial for a cyclist who has been killed in a motor vehicle accident.
These bikes can be seen all over the world, painted white and located at, or near the site of the accident.

As well as celebrating the life of the cyclist, ghost bikes are intended to raise public awareness of bikes on the road.

A ghost bike memorial project was started in St. Louis, Missouri, in October 2003. After observing a motorist strike a bicyclist in a bike lane on Holly Hills Boulevard, Patrick Van Der Tuin placed a white-painted bicycle on the spot with a hand-painted sign reading “Cyclist Struck Here”. Noticing the effect that this had on motorists in the area, Van Der Tuin then enlisted the help of friends to place 15 more “ghost bikes” in prominent spots in the St. Louis area where cyclists had recently been hit by automobiles.They used damaged bikes, in some cases deliberately damaged to create the desired mangled effect.

Wikipedia.