If you've spent any time in a big city over the past several years, you may have come across a "ghost bike," an old bicycle painted white and often adorned with plastic flowers and a hand-painted sign.

Usually placed by the families of cyclists who were victims of fatal traffic crashes, the bikes are intended to serve as memorials -- and as reminders to other riders of the inherent danger in biking along with bigger and faster motorized traffic.

The practice began in St. Louis in 2003 and has since spread to other cities in the U.S. and abroad. The Chicago Tribune recently mapped the locations of nearly 30 past and present ghost bikes in that city.

One of the bikes in Chicago was placed in memory of Jacqueline Marie Michon, a 25-year-old personal trainer who was riding her bike on an August night in 2011. At the downtown intersection of Wabash Avenue and Wacker Place, she lost her balance and fell under a dump truck that was stopped for a red light. The light changed and the truck rolled forward, crushing the cyclist.

Now, more than six years later, drivers, bikers and pedestrians who pass by the intersection are reminded of what happened that night and encouraged to take caution when sharing the road.