A new study of five years of crime data in Chicago shows that police cameras prevent crime and save the city money.

The study by the Urban Institute looked at crime numbers in Chicago’s Humboldt Park neighborhood before there were cameras and after, and that data was compared to crime numbers over the same period in a similar neighborhood where cameras were never installed.

Urban Institute director Nancy La Vigne says the cameras were responsible for a 12 percent drop in crime and that for every dollar spent on cameras, the city saves more than four dollars. But she says officers need to monitor the cameras.  “If the cameras aren’t monitored and police aren’t intervening on the spot to interrupt crime in progress and apprehend offenders on the street, the offenders are going to get wise to that pretty quickly,” La Vigne said.

The study also looked at crime rates 200 feet and 1,000 feet away from cameras to see if the crime just moved down the street.  La Vigne says they found no evidence of that “displacement” effect.

Listen to Nancy La Vigne discuss the study: