A small child's bicycle, painted white and chained to a guardrail on Martin Luther King Drive, stands in tribute to 6-year-old Riley Boyle.

It is a Ghost Bike, a link to a worldwide movement - physical and virtual - that aims to memorialize the deaths of cyclists, spur a change in motorist behavior, and compel cities to make the streets safer.

Riley was killed while riding his bike on May 13, 2006, when a car lost control and hit the metal gates used to close King Drive to traffic at Sweetbriar Drive. The gate swung and struck the Bryn Mawr kindergartner in full view of family members. He died instantly.

Riley's parents found out about the memorial to their son only recently and have not yet visited the site, but they say Riley would have liked the idea.

"It is certainly only a gift when you have more supporters and friends in the community than you thought you had," said Patricia Boyle, Riley's mother.

The Ghost Bike movement started in 2003 in St. Louis, when Patrick Van Der Tuin put up the first memorial cycle after witnessing an accident.

There are now Ghost Bike memorials in at least 27 states and nine other countries and on the Web site www.ghostbikes.org. There are 41 Ghost Bikes in New York City, one of the more active areas of the movement.

In the United States, an average of 600 cyclists are killed each year in motor vehicle-related accidents, according to statistics from the Centers for Disease Control and Prevention.

"I was actually one car behind seeing someone get hit from behind," said Van Der Tuin, 29, a cyclist and bike mechanic in St. Louis.

After talking with friends about what he saw, Van Der Tuin decided to bring more attention to the issue of sharing the road by putting up a Ghost Bike at the scene of the accident. The cyclist eventually recovered, Van Der Tuin said.

"Everyone thought I was nuts," he said. "It stirred the pot and made everyone talk about the subject they didn't want to talk about."

Soon after, a friend put up the Web site, and the movement spread. Most Ghost Bikes are now placed after fatalities.

"It is pretty incredible how viral it went," said Van Der Tuin, who has received news clips of Ghost Bikes from around the world. "It shows the power of the Internet and the power of media today."

The Web site lists two other Ghost Bike memorials in Philadelphia.

"It seemed like there wasn't enough attention given to bicycle deaths," said Steve Bozzone, who installed Philadelphia's three Ghost Bikes. "When [drivers] see these Ghost Bikes or learn about these deaths they start to understand their cars are potentially murder weapons."

One of the Philadelphia Ghost Bikes honored the memory of George Gonzales, 26, who was riding his bicycle south along Delaware Avenue in Northern Liberties on July 26, 2007, when a Greyhound bus crossed his path. He died at Hahnemann University Hospital shortly after. His Ghost Bike, once chained to a stop sign, has been removed, although Bozzone doesn't know who did it.

The other memorial, to an unknown cyclist who was killed on March 3, 2007, was placed at a bike rack at 15th and Market Streets; it is also gone. Bozzone doesn't know what happened to that one, either.

Ghost Bike memorials are far from permanent - the white bicycles can easily be stolen or removed. But the tributes to the fallen riders live on through the Web site.

Bozzone, 26, who recently moved to Portland, Ore., hopes someone else will take over the task of memorializing cyclists in Philadelphia.

The city has not received high marks from the League of American Bicyclists, an advocacy group that promotes bike safety and education in communities, said Andrew Dyson of Neighborhood Bike Works. He said cycling safety varies by neighborhood and correlates with the number of cyclists on the roads. The more riders, the safer the streets.

"People who are riding their bike to work are really doing a service," Dyson said. He said cyclists do not contribute to environmental problems. "The least people can do is look in the right-side mirror so they don't kill us."

James Boyle said other people had been struck by the gate on Martin Luther King Drive but that his son was the first one killed. It was replaced soon after Riley's death, he said.

The Boyles said their son, the middle child of five, was an outgoing, athletic, sporty guy.

"Nobody in our family learned to ride a bike quicker than he did," James Boyle said. The family rarely rides anymore, he said, because it is too dangerous.

"We live in Bryn Mawr and there is not a place to ride," Patricia Boyle said.