I wouldn't say average speed is meaningless, even on flat terrain. One of my occasional riding buddies is a little younger and a lot stronger rider. He averages 20 mph on our roller coaster terrain. Whenever I've ridden with him I'm struggling to keep up even when he dials it down a little. Even on flat terrain the wind resistance above 15 mph or so is a major factor. And he doesn't use aero bars, doesn't often even use the drops. He rides the hoods or tops, looks very relaxed. Dammit.
