That only works if you take the same route every day. If you vary your route from day to day (as I and many others here do for various reasons), it's not so easy to track. In my case, all my bikes have computers and I track each day's rides in a spreadsheet (including whether or not they were commutes). Most people are not this anal, of course, and even if they are, it's still a lot of work create and maintain such a tracking system (hey, I'm bored, and it beats watching TV!). Now that you mention it, though, I'm curious how others keep track of commuting versus total miles. Aside from taking the same route every day, or having a dedicated commuter bike w/ computer, or using a GPS, is there better way?