Discussing with a friend about how is it that the average speed of the amateur cyclist that I see riding around has move from 30 km/h to around 38 km/h. 35 became the old 28 km/h cruising speed. And I say this taking away the big groups that were always faster but small groups of 2 or 3 guys or single riders. I ride 99% of my time alone. And I see this in the flats roads of central Netherlands, when the weight of the bike isn't a big factor.
We came up with 3 reasons (apart from structural training that deserves a complete post on itself and I won't cover here).