Go here and look for the benchmarking study.
It answers this question as well as any other bicycling related question you can possibly think of. Chapter 3 is especially interesting. Since it has all the data related to bicycling.
Funniest thing I found there:
While Portland leads the nation in the percentage of trips to work by bike, they are also cheap skates beyond belief . In Portland 3.6% of people commutes to wrok by bike; 0.6% does that in Atlanta.
The bicycling retail sales per capita in Portland is $70; that number in Atlanta is $216......

People in Atlanta that do ride their bike, spend 16 times as much doing so than people in Portland....

Duppie