Obviously I use the term very loosely. My 15+ mph days feel "blisteringly fast" to me.

Unfortunately, most of my commute is a rather dull, suburban and commercial environment so I take my mind off the boring surroundings by focusing on the numbers and making sure I'm not slacking off too bad on hills and taking advantage of the long flats and downhills (or thinking about what I'd change about my bike to make it faster if money weren't an option). Other routes I could take don't provide much different surroundings, either .

I've yet to have a day under 13mph, so rather than having a fast day and then slower days, I've been thinking about intentionally limiting my average speed to 14 every day to see how it affects my fatigue level and consistency.