There's a lot of talk about gas prices these days and how they are influencing people to commute by bike.
I think that it certainly factors in but my car gets close to 40 mpg and the commute is only 10 miles each way so actually it doesn't add up to a ton of money: $1.50/round trip. Even if my car get 20 mpg it's still only $3/round trip...people spend that on coffee per/day.
What I really hate is driving in traffic and I hate looking for a place to park in downtown Boston or Cambridge or any other congested area. To park in Boston is prohibitive and if the meter runs out it means a guaranteed ticket unless the parking gods are really on your side. Being stuck in traffic and to then have to negotiate downtown streets either circling for 1/2 an hour for a metered spot or just giving in and paying an outrageous amount at a garage or lot is a bigger factor.
It could mean a few things:
#1 That even reduced gas prices or cars with even better mileage will not solve many of the issues that make driving undesirable. Even if cars ran on water it wouldn't solve the problem of too many cars and not enough room for them.
#2 That the reduction in use of fossil fuels and emissions is actually a secondary benefit and not a primary reason for some bike commuters.
So here's my poll, choose the primary reason even if yours may be a combination of many.
edit: funny thing is as I just went to vote on the poll I realized that, for me, environmental concerns sums up a lot of them.