YOU may have noticed scenes like these around the city. These photographs depict “ghost bikes,” memorials to bicyclists who have died in collisions with cars and trucks on New York streets.

Image Linden Boulevard, East New York Shamar Porter, 10, Aug. 5, 2006. Credit... Fred R. Conrad/The New York Times

Ghost bikes are typically old bicycles painted white and placed where bicyclists have died. Plaques identify the dead. The memorials are the work of Time’s Up!, an advocacy group, and Visual Resistance, a group of Brooklyn artists and activists. Inspired by similar campaigns in St. Louis and Pittsburgh, the two organizations have erected more than 20 such memorials.