In reply to Dave Reeve:
I think the average speed of people on bicycles has gone up, mainly because of the demographic now using them... it used to be kids and older people who grew up with bikes as transport mainly, with a lower proportion of keen/club cyclists. These days loads of relatively fit 30-50year olds have taken it up and raised the average speed.
Cycle club membership has dropped over last 20yrs meaning maybe only the most committed are left? Lots more keen cyclists now ride with mates and informal clubs, which is perhaps where the old second tier cyclists now reside perhaps?
The road bike I had 20years ago as a kid (second hand so would probably be 35 yrs old now) was a decent raleigh and could go at a fair lick, but remembering the tyres etc, I would think I would drop my average now by 2-3mph compared to my current mid range road bike.
Mountain bikes on the other hand I would think the progress in tech would mean I could halve my times over technical terrain as a 16 yr old on my current bike and it still feel slow