I've hit about 50 in a downhill, had a moment of thinking "Oh S..." when I saw the size of the expansion joint I was about to hit ( No problem, 40mm tires have lots of volume ) generally, I don't worry much about downhill speeds, if I can see the road. On the flat, such a speed would have worried, about my heart leaving my chest catastrophically.

Seriously, if i was going downhill for long enough, I might start to think about that little piece of glass, that's just about to reach the tube.....

i did have a speed wobble once from a single front pannier, that de-tuned me for awhile.