As anarchist Allen Thornton observes, “Police aren’t in the protection business; they’re in the revenge business.” Forget about Batman driving around interrupting crimes in progress. Police patrol does not prevent crime or catch criminals. When police patrol was discontinued secretly and selectively in Kansas City neighborhoods, the crime rate stayed the same. Other research likewise finds that detective work, crime labs, etc. have no effect on the crime rate. But when neighbors get together to watch over each other and warn off would-be criminals, criminals try another neighborhood which is protected only by the police. The criminals know that they are in little danger there.

— Anarchy 101 by Bob Black

Here’s an article about the Kansas City experiment on Wikipedia.