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.
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.