it all varies with the terrain and the weather!!
like today, my average for 30 miles was 14.9mph. i had about 5 miles of climbing where I sat between 8-11mph(3,000ft climb in 4.5 miles about) I am a slow climber! but then my whole way home i was in the low 20s(going downhill)(on the way out going uphill with a headwind i was around 12-14mph).

Back in FL my average for most rides is closer to 18mph.

So I guess what I am trying to say is, you can not tell much about a rider or the ride by just the average mph, terrain and weather play such an important part(and also is it a solo ride or a group ride,etc.)

oh and one last thing, you can expect, when in a group ride, to ride a few mph above your normal average for the same ride, if you were to ride it solo.