Aram Boghosian for The Boston Globe

A white “ghost bike” has been placed along Morrissey Boulevard in Dorchester near where a bicyclist was killed.

Aram Boghosian for The Boston Globe

The bicyclist, Doan Bui, was struck and killed Sept. 14 by an alleged drunken driver.

David L. Ryan/Globe Staff

On West Broadway, mourners have left a “ghost bike,” flowers, Irish flags, and handwritten notes for Tanya Connolly.

David L. Ryan/Globe Staff

Connolly, a 37-year-old Irish native, was killed after being struck by a tractor-trailer in South Boston last week.

David L. Ryan/Globe Staff

"Ghost bikes," usually older models that have seen better days, are painted wheel-to-wheel in white, creating a stark, almost haunting image.

Allen Breed/Associated Press

A "ghost bike" stood along a highway in Raleigh, N.C., in late July after a fatal accident there.

Allen Breed/Associated Press

The ritual, which seems to have begun in St. Louis in 2003, has since spread to cities around the world.