Many years ago I went shooting with a state trooper. He told me a story of how he and his supervisor each had their weapons drawn on a guy 50 feet away. Before he knew it, that knife was sticking in his chest (bulletproof vest on, as I recall).I was a sprinter in HS. Could go from dead stop to 50' in about 1.7 seconds. 10' gives a person about enough time to realize that they just screwed up by letting an assailant too close to them.