Having a police force larger than many standing armies is a huge boon for making cities safer. Stop and frisk probably doesn't have much to do with it.
I don't know about that. Russia and Singapore have roughly the same number of police (~750) per 100,000 inh. One is way safer than the other. (The US is middling with 233 per 100k.)
Also, as a city, Caracas comes to mind. A city with heavy police presence but intolerable crime statistics.
