The average speeds that can be attained on a road bike are determined by several factors:

 - Terrain/road surface
 - Wind
 - Equipment, meaning the bike and components
 - Distance covered
 - 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.