I have a theory. You will see a lot more car-free and car-light folks in this type of town
* university town with younger population
* has better bus transportation
* has more grid-type streets rather than suburban "crescents"
My reasoning is that the town I live in (Des Moines, Iowa) has an older population, awful bus transportation although the actual layout of the town is pretty bike friendly. 30 miles from here is Ames, a smaller town with a large college, much better bus transportation and a similar street layout. In Ames, you see a lot more folks biking for transportation and quite likely many of them are car-free since studying or working on campus is not that convenient with a car. In fact, buses are free for students and car parking is a major hassle.
What's other factors play into a better "biking for transportation" city?