Three years ago, a bike painted entirely white appeared at the corner of Hill Road and Smith Avenue, where Boise cyclist Kevin Pavlis was killed by a car in June 2009.

That “ghost bike” remains, serving as both a memorial to Pavlis and a powerful, albeit unconventional, public message for bike safety.

Three years have passed since Pavlis and two other local cyclists, Jim Chu and Thomas Berrenger, were fatally struck by vehicles in a three-week period in Boise.

The ghost bike is out there 24/7 as a stark reminder that a guy out for a bike ride died there.

“It helps to remind not only drivers but especially cyclists that they need to be aware of their environment,” said John Yarnell, a friend of Pavlis’ who helped make the sign posted at the bike.

“I am not sure every driver understands what a white bike means, but I think every cyclist understands for sure,” Yarnell said.

Such memorials are showing up around the world, with one website listing about 600 in more than 100 U.S. cities and two-dozen countries.

The bikes are as varied as the people they memorialize. The smallest is 2 inches long. Most are a ghostly white, but at least one is bright pink. Some are smashed with sledgehammers to signify wreckage, and in South America they like to hang them off the ground. Those memorialized by the bikes are as young as 6 — a boy killed by a car in Philadelphia.

The ghost bikes also are meant to be reminders of the struggle to share the road. Perhaps nowhere is that struggle more apparent than in New York City, where more than 100 have been erected. The city is also home to a group of cyclists that maintains ghostbikes.org, a site dedicated to catalog the memorials and the closest thing the movement has to a hub.

“I was one of those people who very much thought bikers have the right of way, and these trucks need to yield to me,” said 41-year-old Ryan Kuonen, one of about 10 volunteers who maintains the site. “But now I just slow down and let the truck go because I know how many people have been killed. Sometimes the law may be in your favor, but I tend to rather be safe than sorry.”

Overall, hundreds of cyclists die and thousands are injured in accidents each year in the U.S. The National Highway Traffic Safety Administration reported that 618 cyclists were killed and 52,000 injured in crashes with motor vehicles in 2010, the most recent year for which those statistics are available. The deaths accounted for about 2 percent of motor vehicle-related fatalities. More than 70 percent of cyclist fatalities happened in urban settings, about 30 percent took place at intersections and more than half happened between 4 p.m. and midnight.

Ghostbikes.org is filled with links to stories about the victims and photos of the memorials. The people who create them are usually anonymous. The volunteers typically vet submissions with news stories or obituaries of the dead. The site is the most comprehensive list of the ghost bikes, but its organizers acknowledge that there could be many more that aren’t reported to them.

Kuonen and others say the idea appears to have originated about a decade ago in St. Louis, after Patrick Van Der Tuin witnessed an SUV drift into a bike lane and strike a cyclist in the fall of 2003. The biker lived, but Van Der Tuin was moved to create a reminder of the cyclist’s plight.

“It was one of these subject matters people didn’t want to talk about,” says Van Der Tuin, executive director of BWorks, a nonprofit group that helps low-income children earn bikes, computers and literacy skills. “That was the most frustrating part, that no one wanted to discuss it.”

Van Der Tuin placed a junked bike near the scene of the accident. He painted it white, simply to increase its visibility at night.

More than a dozen of the bikes sprung up in St. Louis in the next few months, he said. The idea slowly spread to other cities.

Cities’ attitudes toward ghost bikes vary.

The Ada County Highway District, which controls most of the roads in the county, does not have a formal policy about such tributes in the public right-of-way.

“We let them be unless the display itself poses a traffic hazard and impedes pedestrians,” said ACHD Communications Director Craig Quintana.

In Austin, Texas, which prides itself on its bike-friendly culture, supporters have even cemented bikes to the ground. Barring complaints, the city has permitted them. The organizers of the website are aware of at least eight there.

In San Diego, city government spokesman Bill Harris said government workers systematically remove them. He said the memorials violate two state codes and local right-of-way regulations.

“You don’t want to put something that blocks a wheelchair access ramp on a sidewalk, for instance,” Harris said. “If they’re distractions — they are ornate and ghostly like — it’s our policy to remove them.”

“I think ghost bikes aren’t only a memorial but art,” said Timur Ender, a supporter in Raleigh, N.C. “Whenever I pass one I have a moment of silence — they’re a reminder of how fragile life can be.”

In Boise, Pavlis’ death motivated his friends to try to foster change. Yarnell, Jason Keeble and several other people founded Look! Save a Life to educate cyclists and drivers about rules of the road.

“I see that white bike as a reminder to be vigilant,” Yarnell said.