Isaac Newton.
(In his second law.) Don't misunderstand me: I think 3% descents are dangerous too. But the increased speed an 18% descent gives is more dangerous than a crash at lower speed.
Also, being an "awesome descender" doesn't make you safe - in some cases it means you just crash at higher speeds. Vincenzo Nibali is possibly the best (road) descender in the world at the moment (eg 2013 Giro exploits, his attack on the 2012 Milan-San Remo Poggio descent) and yet he crashed in the World Championship road race, and it was only luck that saved him from serious injury.