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The cameras are getting smarter. New software goes beyond passive recording to alerting law enforcement about suspicious activity in real time. Video analytics enable what is called "activity forecasting." By applying artificial intelligence to video, these services issue alerts of what researchers call "anomalous" behavior—such as when cameras detect people leaving bags behind in public places.

The technology, from companies with names like IPVideo Corp. and ObjectVideo, is still new. It might not have been good enough to have identified the bags left behind by the terrorists in time to disarm the Boston bombs. As these systems improve, however, there will be a growing gap between cities that make full use of surveillance technologies and those that don't.

Surveillance cameras and video analytics can be abused. Authoritarian regimes in countries like China and Iran employ these new tools to monitor peaceful critics and suppress dissent. The U.S. has constitutional limits on unreasonable searches, and legislation will need to be updated as technology progresses on how data on suspects and law-abiding citizens can be used.

But even before the terrorism in Boston, the benefits of applying technology to security altered the trade-off with privacy expectations. On a radio talk show last month, New York Mayor Mike Bloomberg rejected criticism by the New York Civil Liberties Union of the thousands of cameras in New York and the prospect of also using new aerial surveillance drones