Road bike or mtb w/hp slicks, flat pavement, no wind, just cruising along at that certain speed where you're comfortably within your aerobic envelope/comfort zone - not dawdling, not hustling - just feeling good... the pace you could keep up for 3-4 hours...
You look at the speedo. What does it say? How old are you?
I'm 55. When I was younger I used to push it all the time and would feel guilty if I didn't average 17mph plus on spins. I got sense. I cycletour all the time, almost always in seriously hilly terrain, as that's where the scenery is. We average 11-12 mph touring, doing maybe 70m/day. Casually cycling with my friends on Sundays, which we've done for years and years, doing anything from 40-100m, but usually about 60, we seldom average more than 14.5 and usually about 14 mph. That's what we enjoy. The enjoyment of taking it easy is not to be underestimated. I've stayed cycling, all the time. The constant pushers always chuck it in.
I'm 55. When I was younger I used to push it all the time and would feel guilty if I didn't average 17mph plus on spins. I got sense. I cycletour all the time, almost always in seriously hilly terrain, as that's where the scenery is. We average 11-12 mph touring, doing maybe 70m/day. Casually cycling with my friends on Sundays, which we've done for years and years, doing anything from 40-100m, but usually about 60, we seldom average more than 14.5 and usually about 14 mph. That's what we enjoy. The enjoyment of taking it easy is not to be underestimated. I've stayed cycling, all the time. The constant pushers always chuck it in.
To be fair, the question the OP asked is different from the one you seem to be answering. He asked about one specific condition, but you're giving an overall average.
Again, I'll normally ride 18 - 19 mph on the flat, no wind. But my average for a long ride will be significantly less. I never do long rides that are flat with no wind! Besides, if I'm on a long ride, I'm bound to stop many times. And if I'm riding with others, there's a good chance I'll be riding slower, for sociability if nothing else.
I guess I'd say a "long ride" is three hours or longer.
If I'm doing one of those on my own, I'll probably average about 15 mph in somwhat rolling terrain, depending on my mood. But usually I do my long rides with others. And usually they're slower.