Marginal And Average

I'm not sure if it was on my local subway or on some other subway system I've been on, but recently I saw some posters touting the benefits of taking public transit by informing riders of the difference between carbon emissions of an average transit trip versus those of an average automobile trip (or average per mile, or whatever, can't remember). While this is a useful to think about if you're evaluating the green value of public transit in general, it seriously understates the relative benefits of me, personally, choosing to take the subway instead of driving to the baseball game. I'm reasonably sure the extra carbon emissions the world gets when I choose to board a subway is basically zero.