There is a lot of confusion about the Tueller drill and what we can learn from it. Over the years the drill has turned into the '21 foot rule' and some police policy has been driven by incorrect interpretations of it.

Simply put, most people can cover about 14 feet of distance per second. A police officer can typically draw and fire one shot in 1.5 seconds. This means that a threat with contact distance weapon 21 feet away can close the distance, and strike the officer at the same time as the shot breaks assuming that the attacker and the police officer (the defender) are reacting to the same stimulus (usually a buzzer or a whistle.)

There are a lot of flaws with what is modeled by this simple example. The error is that the attacker and the defender do not react to the same stimulus. The attacker initiates whenever he feels like it, and the defender has to recognize the hostile action and react.

Another problem with the example is that it is a tie. The defender gets one shot off as the attacker's knife cuts him. Handguns do not reliably stop threats with a single shot, and the attacker has lot of inertia built up from his run. Even with a fatal gunshot wound there is a good chance that the attacker can still cut the defender more than once.

Variations on the drill have the defender start with his hand on the gun, the gun already drawn, or the defender is allowed to move. These variations change the time/distance equation to get a tie at different distances.

The Tueller drill is an object lesson in "how far, how fast." An assailant with even a contact distance weapon is a credible threat at distances greater than 21 feet if you are planning on standing flat footed and shooting it out.

How to beat the 21' rule in real life:

• Getting started as early as possible is the key to winning.
• Get your feet moving before the threat is confirmed.
• Get some obstacles between you and the threat to slow his movement.
• Engage early with verbal commands to remove some of the ambiguity as to the attackers intent.
• Get a hand on a weapon.

References
The Tueller Drill