As the number of bicyclists on Philadelphia streets has risen, cyclists and city officials have seen a counterintuitive result: The number of bike crashes and deaths has declined.
This "safety in numbers" phenomenon has been documented elsewhere, and safety experts believe it is because motorists become more alert to cyclists when there are more of them.
Since 2002, the number of cyclists on many Center City streets has more than doubled, according to tallies at key intersections, and the percentage of bike commuters has also doubled. In 2002, there were six bicyclists killed in accidents with motor vehicles; last year, there were two such deaths.
Traffic crashes involving bikes in Philadelphia have fallen from a high of 1,040 in 1998 to 553 in 2010.
"Where cars expect to find bicyclists and pedestrians, drivers are more cognizant of cyclists and pedestrians," said Alex Doty, executive director of the Bicycle Coalition of Greater Philadelphia. He cited a study in Portland, Ore., that found a doubling of the number of bicycles reduced the crash risk by one-third.
"I know I get better treatment now than I did 10 years ago, or even five years ago," Doty said. "Drivers have a better idea what to do. Though there is still quite a bit of room for improvement."
The correlation was reported in 2003 by the medical journal Injury Prevention, when it published what it called an "unexpected result" of a safety study: The likelihood of a cyclist or pedestrian being hit by a car "varies inversely with the amount of walking or bicycling."
The journal's study concluded that "policies that increase the numbers of people walking and bicycling appear to be an effective route to improving the safety of people walking and bicycling."
In Philadelphia, the Nutter administration has created dozens of bike lanes and bike routes, trying to carve out more space for cyclists in a city not known for its bicycle bonhomie.
The safety in numbers phenomenon "is really playing out" in the city, said Stephen Buckley, director of policy and planning in the mayor's Office of Transportation and Utilities. The city has about 220 miles of bike lanes, he said, and the administration hopes to increase that to about 300 miles.
The city's goal is to boost the percentage of commuters who travel by bike from the current 2 percent to 5 percent by 2020 and to reduce injuries and fatalities by 50 percent.
If more biking means safer biking, safer biking is likely to produce more biking.
"The biggest impediment to people riding is they're afraid," said Patrick Cunnane, president and chief executive of Advanced Sports International, a bicycle design and distribution company headquartered in Northeast Philadelphia.
Cunnane, whose bike sales have doubled since 2004 to 300,000 worldwide and 170,000 in the United States last year, said biking is booming in Philadelphia. His company now employs 62 people here and nearly 100 elsewhere in the United States and abroad.
The biggest growth in bike sales, he said, has been in "urban bikes," including inexpensive single-gear bikes favored by many young city cyclists.
And there has been a shift from mountain bikes to road bikes, he said.
Cunnane, who bikes to work from his Jenkintown home, said the city's efforts to make biking easier and safer have paid off.
"The Spruce and Pine Street closures have really opened up the city dramatically," he said of the replacing of one car lane with a bike lane on each of the two Center City streets. "And when I commute to Northeast Philly, as soon as I get into Philadelphia, I have a bike lane the whole way."
Julie Cristol, a nurse-midwife who has commuted by bike from her West Philadelphia home since 1988, said drivers are much more aware of cyclists now, especially in Center City.
"About the worst thing that can happen to you when you're riding a bike is to have someone open their car door. Now, I notice that people, especially if they're parked next to a bike lane, are more likely to look before they open their door," she said.
That doesn't happen often in the suburbs, said Cristol, who now bikes daily to Bryn Mawr. Outside the city, with many fewer bikers on the roads, motorists seem much more oblivious to cyclists, she said.
John Randolph, a retired architect and builder who has been biking on the streets of Philadelphia since his days at the University of Pennsylvania in the 1960s, said bike riding here has gotten appreciably safer.
The increase in cyclists "has changed the consciousness of motorists," he said. "Bicyclists are paid much more attention to now. They're not just an unusual occurrence any more."
"Obviously, the bike lanes make it safer . . . biking east and west through Center City is much more pleasant and safe than it used to be."
Doty, of the bicycle coalition, predicted that the increase in bikers and in biking safety may attract a new breed of riders to Philadelphia streets.
"We can attract more women, more of the 8-year-olds and 80-year-olds, and that's the tipping point that makes biking fully accepted.
"And that's when Philadelphia can turn into something like the Amsterdam of the U.S."