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.
Read more: More bicyclists means fewer accidents, Phila. finds