Always a can of worms with this question. Some people will brag, some will lie, some will tell the truth, some will try to say that their route is more difficult than other peoples.

My average "average" is really about 12mph or so.

When you factor in all of the things like riding with the kid and wife, riding through a city with stoplights, using the bike to go shopping at the grocery, etc it all adds up.

I'm one of those who don't care what people think about my average. Reality is I can "average" a lot faster in the right environment with not a lot more effort. If I go on group rides with the cycling club, my average could jump up 8mph for that ride. I generally laugh at the ones who feel the need to state their average in only the best situations. If they can always average 20+ mph, they aren't doing enough of cycling in other enviroments.