If I work hard and have some luck at the lights I can average over 16 (barely) on a commute. Most days it's between 14 and 15 but in winter it slows down considerably. Frankly, I don't even want to know how much slower it is in the winter so I don't usually mount a cyclometer on the winter bike.

A fast group ride will push 18 now and then but that's a grueling pace for me. The route we take has got some hills that are killers, but 2/3 of it is pretty flat. A few sections have frequent stops.

On a closed course sprint triathlon I can average 20 mph if it's fairly flat. If I trained for the extra distance I could probably do an Olympic distance at a slightly faster pace.