Your question makes me think.

I've done 50+ mph quite a lot on the tandem, on long descents with a good surface and no road junctions where the sight lines are clear. Speed itself doesn't bother me.

But unpredictability does. Put a road junction, a change of surface, an incoming 'vanishing point' on a curve and I become very conservative indeed - on any 2 wheeled vehicle, powered or not. And 4 wheeled, come to that.

When the conditions were right, I had no apprehension about redlining a number of Ducatis. (Which, on a good day, could approach the national speed limit!)

And after a lot of years and miles, I hurt myself at 2 mph

So, like many previous responses - not the speed, all about the conditions