That’s a good question! I was just assuming that most bicycle trips are for pleasure, not for work.

I searched online, and the best data I could come by are from the National Household Transportation Survey, and this is just US data from 2009.

http://nhts.ornl.gov/det/Extraction3.aspx

According to this data set, Americans traveled 9.0 million miles by bike in 2009, and for 1.9 million of those miles the purpose was “to earn a living”.

So that’s 21% of bike miles traveled for commuting.

More than I thought! But still a relatively small percentage.