I agree, its NOT about an average speed its about traveling at a safe speed for what the terrain dictates
and a lot has to do with what your rriding goals are
back in my younger days when was all about pushing the limits, I drove things at speeds that were extremely fast, that most folks couldn't
but then again my machines were NOT stock, had high end suspensions on them and I was a honest racer, I raced MX bikes and other things for 30+ yrs
so, that being said, if I was running a Sport ATV about trying to push MY own skills and limits, the speeds might be 3 times higher or more than what someone just out of a joy ride might be
but I would also be wearing a lot more safety gear than most guys
heck, I seldom even see folks anymore even wearing helmets period, never mind other safety gear(boots gloves goggles, chest protection, and so on!
when I ride about the farm, I might be going 3 mph for most of what I do, but then on roads about it, maybe 35 mph, on a WORK ATV,
so a LOT comes down to the TYPE of ATV here, and the SKILLS and desires of the rider on it!
so its a VERY hard question to answer not knowing so many things
age of rider, skill of rider, desire of rider, type of terrain YOU ride on, and what YOU feel is safe or not
loaded question here to be honest