I once hit 50.4 mph on the steep A53 down past the Roaches (I remember because it is a 50 mph zone). I had similar thoughts about "what happens if..." and decided I shouldn't go that fast again. It wasn't so much about the polystyrene helmet, it was something I'd heard about motorcyclists crashing. I forget the numbers but it was something like, unless you're wearing the right kit, you lose Xcms of skin/flesh for every metre you slide.

Better cyclists must be much more inured to speed than I ever was. These days 30mph feels fast, and on the tandem I have a built-in safety system that tells me to slow down if we're going anywhere near fast.