It's long been "known" among police that anybody with a knife or edged weapon within 21 feet is a lethal threat. This so-called "rule" has long been a big pet-peeve of mine.
This anybody-within-21-feet-is-a-threat mentality does result in a lot of crazy people getting shot. And don't get me wrong, I have no problem with police shooting and killing people coming at them with knives. But the idea that anybody within 21 feet could be carrying an edged-weapon and is thus potentially a lethal threat? Get real. Even if the "21-foot-rule" were true, what are you supposed to do with this knowledge? You're a cop. Of course you're going to be dealing with people at a normal talking distance of a few feet.
The first problem with the 21-foot "rule" is that it assumes the officer doesn't perceive a threat. The scenario starts with a holstered weapon. Well if you don't perceive a threat that exists, that's a separate problem. But it doesn't mean you're justified in keeping everybody at a 21-foot distance. The second obvious problem with the "rule" is that it assumes that the man with the blade is a trained skilled stealth ninja (or at least an academy instructor-san much better than you, young academy grasshopper trainee, at hand-to-hand combat).
The relevant question is how close should you let a man with a knife get to you when you are in the drawn and ready position. Based on nothing but my gut experience (I'm sure somebody has better-formed answer), I would probably start shooting at about 6 feet. Maybe 10 feet if they're advancing in more a threatening manner. But of course it all depends on the situation: what kind of person? What kind of knife? How is the person holding the blade? (Blade facing back, arm-down, fist clenched means the guy may know how to use it.)
For small knives that don't make particularly good weapons (like a dinner knife or something without a bolster/finger guard), I'd be more than willing to take my chances defending myself and whacking the guy with my trusty 29-inch wooden straight baton (a far better offensive and defensive weapon than the now much more common expandable metal asp). And I'd be more willing to try and disarm the guy if I could come up from behind while the person with the blade is distracted by the other six officers on scene who are drawn down on him).
[And of course a man with a knife is exactly what the Taser is designed for even if it emasculatingly and shamefully used far more often for routine non-threatening non-compliance situations. Also, you should not mace a guy holding a knife, because then you have an angry blind guy with a knife.]
Here's the thing: most people police face with knifes are not well trained in "edged-weapon combat." They are A) crazy or B) cutting up their loved one. Sometimes both. But police rarely if ever face a trained evil ninja out to assassinate a police officer caught unaware (honestly, there are far easier ways to assassinate a police officer, if you so choose). So basically you have this whole police paranoia based on a situation that never happens.
I checked Officer Down and, since 2000, could find just four officers on patrol killed by an assailant with a bladed weapon: one domestic, one EP (aka: EDP or mental case), and two fatal fights after a foot pursuit. As you might guess, not one of these assailants was an a trained stealth ninja.
Best I can ascertain, only one officer in the past 14 years (Sault Ste. Marie Detective John Weir) could have been saved, maybe, I don't know, by keeping greater distance and being quicker to shoot. The other officers, rest in piece, died doing the job they had to do.
So why has the 21-foot rule persisted for decades despite little basis in fact or police reality? I don't know. I'd love to hear what you think. Could it be just another example of the conservative warrior mentality so pervasive (and usually counterproductive) in policing? Think of this: the instructor teaching hand-to-hand combat in the academy is the most aggressive threat-perceiving police officer out there perhaps (just hypothetical, er, based on my experience) having been pulled off the street and into the academy where he can't shoot another sue-the-city person (all of them technically justified, but still...).
So you get a perpetuating cycle where the paranoid cop too-quick to elevate a threat-level ends up teaching and scaring the next generation of police officers to adopt his code-red us-versus-them ideological world-view in which one must assume the worst about even seemingly non-threatening citizens.
Here's a good recent piece by Ron Martinelli which more scientifically analyzes and partially debunks the 21-foot rule, and inspired this post. If you're still with me, it's also worth clicking-through to the first link on this page.
Update: Based on a useful comment to this post, I also should have included blunt weapons and not just knives. Doing so brings the total number of police officers killed in the past 15 years with any relevance to the 21-foot-rule up to three police officers. It's also worth mentioning that I'm not looking at officers just injured. But we don't have those figures. And one can assume some relationship between fatal and non-fatal injuries.
So let's put the 21-foot-rule in perspective. In this same time period since 2000, as many officers (3) have been killed by a moving trains.
Six officers have been killed by animals: one by cow, one by spider, and one by bee; the other three from horses (interestingly, none by dog, the only animal often at the receiving end of a 21-foot mindset).
If one were truly interested in saving police lives rather than simply building police paranoia and mistrust of the public, we should look at the 515 law enforcement officers killed traffic fatalities. How many of these would have been prevented by officers wearing seat belts? And yet the same officer who won't wear his seatbelt because he claims it gets caught on his equipment (which, speaking from experience, is bullshit) will be quick to spout the absurdity that his life is endangered by anybody within 21 feet, in optimal conditions.
Also, it's come to my attention that the 21-foot-rule has now been upped to 30 feet.