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That only works if you have enough volunteers. Most people work and don't want to come home to do voluntary security patrols. There will also be people who will take advantage in gang controlled areas...how do you think the folks that live in those area's are going to feel when the gangs run out the volunteers. Just way to many different scenario's that would play out to make such a endeavor impossible.

Voluntary does not mean done by volunteers... It means done without using coercion and the threat of force.