I go shoulder to shoulder at 35+ on the flats in sprints, so nothing in the 30s really makes me take notice unless it's wet and/or a dangerous road.
I've hit 62 in a race, and over 50 many times on my own, but that's about it. If I didn't look at the speedometer, I wouldn't really be able to tell much past 50. At that point it all seems pretty quick.
With or without a helmet doesn't really matter at 40+. You hit something head on and you're pretty much done. It's not something you can think about, really.