I think Catrin hit the nail on the head. Recently I participated in the Texas Time Trials and there's a boatload of hills in that one. Well, hills as far as we flatlanders are concerned. Anyway, I noticed I was riding the brakes a lot downhill and it was causing me a lot of problems making it up the next hill. Anyway, to make a long story short it was because the chipseal was shaking my bike so bad I was having a hard time seeing. When I got to smooth downhills I was perfectly fine with not braking. Having said that the fastest I've gone is 35.

Totally anecdotal, but there was a lady in our club that broke her collarbone not even going 10 mph, so you can get just as injured going fast or slow.