I spend a decent amount of time reading Streetsblog LA, which is an interesting site in part because many of the people who regularly contribute to it are bicycle advocates. If you've been reading this, you've probably noticed that I don't talk about bikes nearly as much as I talk about walking and transit.
I hardly ride my bike anymore. This isn't because I'm against bikes as a form of transportation. Bikes are faster than walking, clean, inexpensive, and fun. If everybody rode bikes instead of driving cars we'd have less pollution, less habitat lost to urbanization, more money to spend or save, better health, and fewer road deaths. There are very specific reasons why I don't ride much anymore, some of which might surprise you:
1) My apartment is too small to comfortably store a bike (starving student, remember), and I'm paranoid about loosing my deposit for a wall-mounted bike rack.
2) I don't feel safe riding a bike on the arterial streets of my neighborhood. There are no bike lanes in my area that I know of and the cars drive quickly.
3) I've set myself up in a place where I can easily get around walking and using transit.
For me, something actually trumps safety, but for most people, safety is probably the biggest concern. I've learned on Streetsblog that it's actually safer to ride in the street since cars have trouble seeing you on the sidewalk and they aren't expecting fast-moving things to go through crosswalks. Still, psychologically, the street is often a pants-wettingly scary place to ride a bike.
We need to take space away from cars for lots more bike lanes and we need street design that slows down traffic to make things safer for bikes and pedestrians . . . and I need a better apartment :)