Watched a youtube video about helmet aerodynamics and they claimed the average speed of all rides uploaded to Strava was around 15.5mph for men and about 13.5 for women. Add or subtract a couple of tenths since I am going off memory but that is the ball park. Take out the hilly or mountainous rides, take out the nasty head wind rides and I would think the calm day average flat road speeds would be about 17-19mph for men and 15-17 for women.