I ride a steel touring bike, so...

Unloaded - on flat terrain, about 15mph, on hilly terrain, about 12-13mph
Loaded (with an added 10kgs or so) - on flat terrain, about 12-13mph, on hilly terrain, 10-11mph...

Definitely room for improvement!

I get totally intimidated by ppl who say they average 20mph... HOW???