There's a local ride that's 30-40 miles, but at a moderate 16 mph-on-the-flats pace. About once a year, a new rider shows up that can't go faster than 12 or 13 mph on a flat road. The ride leaders don't abandon the rider, but might have someone lead them back early or shortcut a loop. Most new riders can handle the pace, and at these slower speeds, the group isn't tightly spaced, either. Probably the longer distance keeps completely new riders from coming out.
I would think that most of your new riders handle the ride with no problems. Even a strong warning notice wouldn't always keep a problem rider from trying the ride.
There's a few local rides that advertise "15-18" average speeds (with hills included). That's a big range. Many riders that could handle 15 would get dropped at 18. A rider needs more than 50% higher power to average 18 mph compared to 15 mph.
Last edited by rm -rf; 05-17-12 at 05:27 PM.