Policing levels can have an effect on violent crime and, unfortunately, cities don't have enough police officers. Research by Aaron Chalfin and Justin McCrary of the University of California at Berkeley found that each dollar spent on police is associated with about $1.60 in reduced victimization costs.
According to Chalfin and McCrary, "the precision of our estimates allows us to confirm the common and somewhat surprising finding [in earlier research] that police have more of an influence on violent crime than on property crime."
The researchers compare costs and benefits of policing in various cities, finding, for example, that every dollar spent on policing in Sunnyvale, Ca., yields only 20 cents in benefits in terms of crime reduction. By contrast, every dollar spent on policing in Gary, In., yields $14 in benefits in terms of crime reduction.
The Full Report: http://emlab.berkeley.edu/~jmccrary/chalfin_mccrary2012.pdf