only true measure is riding with someone, everyone on this thread would struggle to average 15mph+ on my commute.
Hang on… how do you know, have you ridden with everyone here?
I know because I regularly get top 10s on the strava segments (many with 3000+ riders) on my commute and I very rarely average over 15mph. Traffic, road surface, junctions, students, traffic lights etc etc.. all kill fast speeds.