Averages are pointless unless everyone is riding the same route in the same conditions. That being said I average 14-15 mph on the way in to work. For some strange reason I just do not feel motivated to hammer it in to work.
My quickest solo ride occurred yesterday. 69.2 miles at a 19 mph average. I was riding a route with very small hills and sat in my drops 90% of the time. Basically the flattest route I have ever seen around here. I doubt I will replicate this anytime soon.
I generally tend to average in the 16-18 mph range solo.