Average anything (Speed, Cadence, HR) would only be of use if you did the same thing over and over. Even with uncontrolable enviormental variable like stoplights, traffic, and wind, it you did the same thing over and over long enough in the end a properly plotted graph could yeild data.

Change the course, time of day, etc, or look at a small data set, and the information is useless.

My club does not use "average speed" to classify groups. We use distance, course difficulty, and regrouping to designate. I get asked all the time what the average speed of the A Group is, and the answer is always same - "too fast for you if you have to ask."