Of U.S. cities with more than a million residents, the one where people bike the most is Philadelphia. In 2012, the U.S. Census estimated Philadelphia’s bicycle commute rate at 2.3 percent [PDF], higher than Chicago (1.6 percent) and New York (1.0 percent).
It's just about always been that way. That comes as a surprise to many people, since Philadelphia doesn't have a lot of bike infrastructure. But there are other street design and urban design factors at work, many due to the fact that Philadelphia is an old city.
For one, the city has a lot of narrow streets. That makes it tougher to add bike lanes, but it also means motorists tend to travel at speeds that don't intimidate people on bikes. On average, people also live closer to their jobs than in most other places, making bike commuting a better option. Stop signs are more prevalent than signals, and where there are traffic lights, the sequencing is short, so people on bikes don't have to wait long at intersections. In the end, most people bike because it is the fastest, most convenient option.
Thanks to Alex Doty, executive director of the Bike Coalition of Greater Philadelphia, and all the other bicyclists I got to speak with. They'll tell you plenty more reasons why biking is good there, and how it could be better.