"To what extent do police prevent crime?
If police begin chasing immediately after a crime has been done, and chase very fast, criminals may think twice about doing crime to begin with, which puts our well-being at the moral discretion of criminals."
You ask an interesting question, and the hypothetical conclusion you suggest may have merit. The answer to this question depends upon several factors:
How do we define "police"? Are we limiting our discussion to armed, uniformed men employed by the state? What about unarmed people employed by the state? What about armed men hired privately? In a very rural or sparsely-populated area, would we consider random, voluntary "protectors" within the community "police"?
Does your question refer to mere police presence, actual police activity, or both?
Do the police actually "begin chasing immediately after a crime has been done", as you suggest? If so, can we actually measure the result you suggest and can we causally link that result to police (and not some other cause)? If not, what do the police actually do? Would some other mechanism (for instance, everyone and anyone) produce the same or better result?
One corollary consideration, and one I believe is just as important as your question, is (supposing that police actually do prevent crime) do the benefits outweigh the costs? Would it be worthwhile to spend $5 per month to prevent the theft of a $500 stereo? Would it be worthwhile to spend $100 per month? What if it wasn't a stereo, but a life?
To answer your question (lol) one way is to examine the "Kansas City preventive patrol experiment" (@Kansas City preventive patrol experiment). Take a look at it for yourself, research it, and come to your own conclusion.
In an article by the Police Foundation (the authors of the study itself), the author observes:
"Patrol is considered the backbone of police work. Billions of dollars are spent each year in the United States to maintain and operate uniformed and often superbly equipped patrol forces. The assumption underlying such deployment has been that the presence or potential presence of officers patrolling the streets in marked police
cars deters people from committing crime.
But the validity of this assumption had never been scientifically tested. And so, in 1972, with funding and technical assistance from the Police Foundation, the Kansas City Police launched a comprehensive, scientifically rigorous experiment to test the effects of police patrol on crime."
What they found, in a nutshell, was that police presence had essentially zero impact on crime. From the Police Foundation:
"Interestingly, citizens did not notice the difference when the level of patrol was changed. What is more, increasing or decreasing the level of police patrol had no significant effect on resident and commercial burglaries, auto thefts, larcenies involving auto accessories, robberies, or vandalism–crimes traditionally considered to be prevented by random, highly visible police patrol.
The rate at which crimes were reported to the police did not differ in any important or consistent way across the experimental beats. Citizen fear of crime was not affected by different levels of patrol. Nor was citizen satisfaction with police.
"Ride-alongs" by observers during the experiment also revealed that 60 percent of the time spent by a Kansas City patrol officer typically was noncommitted. In other words, officers spent a considerable amount of time waiting to respond to calls for service. And they spent about as much time on non-police related activities as they did on police-related mobile patrol."
One reaction to these findings is found in the advent of what's been termed "proactive policing". The Canadian Department of Justice states, "One American study found that proactive policing resulted in more arrests, detention and filing of reports than reactive policing (Seagrave, 1997)"; that full article can be found here: @6.0 Policing styles: Reactive versus proactive policing. What's left out of the Canadian Department of Justice's otherwise very comprehensive article is whether these arrests, detentions, and reports reduced crime.
It would appear that these activities while "doing something", and costing money in the form of, well, arrests, detentions, and reports, do nothing to make a community any safer or reduce crime. So what does reduce crime?
Time and time again, across study after study, the answer is the same: decentralized, distributed, real-time response to crime is the only truly effective solution. In other words: stopping crime depends upon YOU stepping in and doing it yourself. Call the police, yes: call them to come investigate, verify, and document that you did not criminally assault, batter, or kill the criminal you've already defended yourself from!
The very idea that calling some "special" person to your aid from somewhere far away, necessitating his driving at nearly (or sometimes actually) reckless speeds, forcing traffic off the road, to save you from a threat that you and those in your immediate presence could just as easily defend yourselves from with only a modicum of training is insane. Having "police" everywhere and nowhere within a community, leaving criminals without the means to determine just who will use force to stop him is a real deterrent to crime.
@Detroit police chief: "There's no question" that armed citizens are helping to reduce crime in our city
https://www.quora.com/To-what-extent-do-police-prevent-crime