Average speed is somewhat subjective. Average for a one-mile sprint? For my 10-12-mile commute? or a Century??
My all-time best flying mile was a tad over 27mph. You Strava-junkies are safe!!!!
My commute -- where I ride quickly for the first 80%, then have a slower 'cool-down' portion so I don't arrive to work all sweaty is more like 17-18mph (including counting the time stopped at traffic lights)
For a Century, my goal is to finish in under seven hours including rest/meal stops for a 14.2 average speed, but a closer to 16mph riding speed.