An armed attacker can cover 21 feet in less than three seconds. This simple fact is one of the first things I learned at the Sheriffs Academy. We were taught that a man with a knife at 21 feet was a lethal threat. They showed us videos and drilled it into our heads. This video shows that better than most.
And yes, that was fast. Remember boys and girls, the police have no legal responsibility to protect you from harm. Be aware of your surroundings, don't frequent dangerous places. Seek self defense training, only you can be responsible for your safety and that of your loved ones.
As our favorite petite librarian says, "Carry a gun, it's a lighter burden than regret."
Oh, and I almost forgot the link chain:
Caleb covers it.
Joe Huffman covers it.
Robb Allen covers it.