Interesting question,
Now that I've been riding a 99 Predator four stroke with a great front fork (Suzuki k10) I find myself going faster than when I was riding an unsuspended 2 stroke. Then it was probably around 20 mph. Now it depends on where I am. I have the good fortune in the summers to be in northeastern Minnesota next to Bearhead State Park. The five or so miles through the forest from my place on Eagles Nest Lake to Bear Head Lake is virtually without traffic at all. No potholes. Nice shoulders. It is a perfectly banked winding up and down race track. I like riding at around 30 to low 30's on that road, pushing it up to around 40mph sometimes. I don't feel unsafe when I do that. The bike runs smooth, I have good control and the road is perfect.
In town or on poorer roads or around other vehicles I go slower. There's no death wish, but I do like hearing the exhaust pipe and feeling the wind in my face... being a boy again, in other words.
In the winter I ride back mountain roads in northern Maryland where the drivers are less courteous, in more of a hurry and the roads are narrower and have almost no shoulders. I'd say around 20 to 25 in that situation... goosing it up to 30 once in awhile so I don't forget what that's like. Ha!
With the dog in her trailer everything is slower, down to 15 or 20. And I always wear a helmet, no matter what speed. A mirror with a good sense of what's behind me is essential. I'm sixty seven years old, but on the bike I'm twelve again. Keep it safe and don't do what I do!
SB