I mostly ride at a relaxed pace. I'm in no great shape, I sweat a lot and I don't have a shower at work, so I take my time usually (around 10mph). Also, my commute involves a four-way traffic signal at almost every intersection so I'm not going to be able to get to any great speed and maintain it for long (though I know some bike riders in San Francisco do). But there are days I push harder when it feels good.