I think 12-15 mph is fairly average. Average speed is impacted by many factors including nutrition, hydration climbing, weather/temperature, winds, etc. I generally average 14-15 mph on very rolling and sometimes hilly rides. On the rare occasions when we hit flats, I can ride about 24 mph, but not sure how long I could sustain that because flats are few and far between in Georgia !

I know there are group rides in my area where they say that the average is 17-19mph and I know these rides go over some very hilly terrain, so I am 100% positive that they would drop me if I attempted to ride with them.

I think average speed is dependent on your cycling goals. Example: Sprinters train differently than climbers.