The average speeds that can be attained on a road bike (or any bike) are determined by several factors:
- Terrain/roadDistance covered
- Road surface
- Wind
- Equipment, meaning the bike and components
- Distance coveredTerrain
- Age and fitness of the cyclist
- Teamwork
Based on observation and considering all that, on primarily flat, non-windy, terrain, on a decent road bike, my estimates would be:
- Novice cyclist - a short distance (10-15 mi): 10 - 12 mph
- Casual, fit, cyclist - a short/medium distance (~25 mi): 15 - 16 mph
- Average club cyclist/fitness cyclist - a medium distance (+/-40 mi): 16 - 19 mph
- Experienced club cyclist/amateur racer - a medium/long distance (+/-55 mi): 20+ mph
- Pro cyclist - long distances: 25+ mph
Anecdotally... on one of my frequent rides through some farmland, there is a 12 mile, mostly flat stretch. With a tailwind, I'm a pro. A headwind, I'm a novice. With no wind, I'm somewhere in between, depending on the day.
Another anecdote... In my area I frequently use a particular paved bike path on both a road and mtn bike, mainly because it leads to a park full of single-track, and beyond that to nice countryside for road cycling. Anyway, I have 8 years of ride data for that route. Along that path there is a 6 mile, flat open, stretch that was repaved 2 years ago; originally it was very rough, chunky asphalt and upon repaving it was converted to very smooth asphalt. Lo and behold, after the repaving, my average speed on both bikes jumped up ~4 mph.