ghost bike
n. An old bicycle painted white and locked to a fence or post near the scene of an accident that killed a cyclist.
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Bob Shenton rolled up next to a blood-stained roadway with a bouquet of white flowers for a woman he did not know.

"I’m tired of ghost bikes and I’m tired of cyclists getting killed in this city," said the veteran cyclist.
—Natalie Alcoba, “Cyclist dies after being hit by a truck at Toronto intersection,” National Post, November 07, 2011
Around the globe, all-white bikes are being chained to street signs near the sites of fatal accidents involving cyclists.

They are called ghost bikes — somber reminders of tragedy and intended as a quiet statement in support of a cyclist's right to safe travel, proponents say.
—Kevin Clerici, “Ghost bikes aim to honor fallen cyclists, raise awareness,” Ventura County Star, May 23, 2011
2004 (earliest)
They appeared late Monday around Allegheny County, 14 bicycles painted a ghostly white and chained to poles with a cryptic sign: "Cyclist Struck Here."

The roadside memorials are the work of Ghost Bike Pittsburgh, a loosely organized group of bicycle enthusiasts seeking to improve the lot of bicyclists in a sometimes hostile world.

Its goal with the ghost bike project is simple: "to help Pittsburgh become a city where cyclists and motorists can coexist, to the benefit of both groups."
—Jonathan D. Silver, “'Ghost bikes' are grim reminders,” Pittsburgh Post-Gazette, May 27, 2004