Reaction?
The 21-foot rule is a calculation of time, distance and reaction, which was worked out several decades ago. In fact, I was taught the 21-foot rule when in the police academy back in 1985, along with several other street survival courses I attended. The rule simply states if an assailant is 21 feet or closer to you, and your gun is holstered, you won’t have time to draw your gun and shoot, before the assailant charges and stabs you.
Reaction time is calculated. The average person’s reaction time is ½ a second. It takes the brain this long to recognize a threat, process it, and then send a message for you to react. This reaction time was tested by having drivers operate on a special driving track and being told they would have an obstacle “pop” out and do the necessary avoidance reaction. The drivers were ready, hyper vigilant even, as the test was conducted. But the testers were clever, having a second obstacle in place. The time for the second obstacle was what was used to come up with the 0.5 second rule, as the people tested were not expecting a second obstacle, hence a more natural reaction. As you can imagine, the second reaction was slower than the first, for there was no warning.