Chances are that you have seen an unchained white bicycle near an intersection or on the side of the highway. However, you probably drove by unaware that it was a ‘ghost bike,’ which is a tribute to a cyclist who was hit or killed by a vehicle. An article in the Arizona Daily Star reports that there are over 600 ghost bikes listed in more than 100 U.S. cities and two-dozen countries. Although they are typically painted white, the bikes are as diverse as the people they symbolize. Some bicycles are smashed with sledgehammers to indicate wreckage, while others are painted bright pink.

New York City alone has more than 100 ghost bikes. New York City is also where the organizers of Ghostbikes.org currently share stories with their audience about bicycle collision victims. Ghostbikers.org reaches such a large audience that when Timur Ender had learned that a fellow cyclist, and a father of three, was killed by a semi-truck near a busy North Carolina highway, he decided to make a ghost bike to place near the scene as a memorial.

Sadly, accidents such as these are common in the U.S. In fact, The National Highway Traffic Safety Administrationreported that in 2010, 618 cyclists were killed and 52,000 were injured in crashes with motor vehicles. It is also reported that more than 70 percent of cyclist fatalities occurred in urban settings with about 30 percent taking place at intersections.

Ryan Kuonen, a volunteer from Ghostbikes.org, hosts days with fellow volunteers that are dedicated to creating ghost bike memorials out of donated bicycles. “It has its rewards, but it’s emotionally taxing,” said Kuonen of his experience working with ghost bikes. In 2008, ghostbikes.org created 106 ghost bikes for victims and their families.

There are many cities which are extremely supportive of ghost bikes, but also many who are against them. Bill Harris, a city government spokesman, said that the city removes ghost bikes because the memorials violate two state codes and local right of way regulations.

Although ghost bikes are not supported in each city, it is a ghostly reminder for drivers and cyclists to share the road safely. Therefore, the next time you pass by an intersection with a white bicycle, remind yourself of its significance and the severe repercussions that your actions behind the wheel could cause.