On the cost-effectiveness test, though, La Vigne says the cameras were a solid success. For every $1 of costs, they yielded $4 of societal benefits (reduced crime, savings in courts and corrections, less suffering for victims), despite their failure in West Garfield Park.
In Baltimore, where cameras are concentrated in downtown and monitored actively 24 hours a day (as distinct from the more passive approach in Chicago), La Vigne found the impact on violent crime was even greater -- and the benefits exceeded the costs by 50 percent. (In Washington, which deployed only a small number of cameras, they found no effect.)
All this may confirm those who see this technology as an unmixed good. But La Vigne herself worries that too much will be made of these results.
"I'm sure there are diminishing marginal returns," she says, meaning that each extra camera achieves less than the one before. "I'd expect very little impact on low-crime areas." If we have cameras on every corner, many of them will be the functional equivalent of potted plants.
Even if cameras have benefits, they narrow the scope of personal privacy, which should not be sacrificed without a compelling reason. In a crime-infested neighborhood, the loss may clearly be modest compared to the dangers of violent perforation. In more tranquil locales, the burden of proof should be on the supporters.
When cameras are used, common-sense restrictions should apply. The American Civil Liberties Union of Illinois recommends that police show probable cause that someone has committed a crime before they use facial-recognition software or conduct nonstop video tracking of an individual. Another proposal is to delete images after seven days unless there is reason to think they document a crime.
The ultimate question is not whether cameras work. It stands to reason that they can work when used wisely -- just as a hammer works for certain tasks. But not everything is a nail.