In many American cities, you can bet someone's watching when you walk the streets.
In the name of crime prevention and the war on terror, police departments around the country have set up surveillance cameras to keep an eye on troublemakers.
The New York Police Department announced Monday that it was creating a web of surveillance in lower Manhattan that will eventually include 3,000 public and private security cameras to track terrorists. By the end of this year, 116 license plate readers will monitor cars moving through the area, which is the city's financial district.
A spokesman for the department told ABCNEWS.com that footage collected by the NYPD's cameras would be kept for a 30-day period before it is discarded or recorded over. "It would be used to intercept a threat coming our way but not to collect data indiscriminately on individuals," NYPD spokesman Paul Browne told The New York Times.
The new initiative adds to the extensive network of cameras that already watch New Yorkers. Nearly 4,200 public and private surveillance cameras are currently located in downtown Manhattan, according to the New York Civil Liberties Union.
Other cities, including Chicago and Los Angeles, have expanded their use of public surveillance cameras and tout the effectiveness of the technology. Some of these cameras, which can cost up to $60,000 each, have night-vision capabilities and can be remotely controlled to pan, tilt, zoom and rotate.
Over the last few years, the Chicago Police Department has set up more than 500 cameras throughout the city. And the CPD claims that the web of surveillance has been an important crime-fighting tool, resulting in more than 1,200 arrests since February 2006.
"Our preliminary research shows that they are effective, especially left in places for over 180 days," said Jonathan Lewin, the CPD's commander of information services. "Once it's in, it's hard to move because the community loves it. If they don't see the camera there one day, we get calls."
The cameras have provided valuable forensic evidence in crime and terror investigations, such as the recent blundered car bombings and the July 7, 2005, terror attacks in London where British officials were able to track the movements of the perpetrators and make arrests.
Yet they seem to have a mixed record when it comes to preventing crime before it happens.
In England, the use of cameras exploded after the 1993 Bulger case in which two 11-year-old boys were videotaped kidnapping 2-year-old Jamie Bulger at a shopping center in Liverpool. Though the cameras did not prevent Bulger's murder, the evidence they provided did help convict the killers.
As a result, the British government dramatically expanded the use of cameras — in 1991, there were only 10, now there are more than 4 million, including more than 200,000 in London alone. By some estimates, Britons are filmed more than 300 times a day.
But have they been effective at cutting crime?
According to a British Home Office review of dozens of studies analyzing the cameras' value at reducing crime, half showed a negative or negligible effect and the other half showed a negligible decrease of 4 percent at most. Researchers found that crime in Glasgow, Scotland, actually increased by 9 percent after cameras were installed there.
In the United States, one of the most prominent examples was Tampa's use of facial recognition technology in 2001. But the city's police department dropped the technology two years later when it failed to result in a single arrest. The use of video surveillance was considered by the Oakland, Calif., police chief, but he ultimately found that "there is no conclusive way to establish that the presence of video surveillance resulted in the prevention or reduction of crime."
"They are good forensic tools — after something happens, they'll tell you what happened," said Jim Harper, the director of information policy studies at the Cato Institute. "And in the rare case where a terrorism case fails, they can be useful to help track down the perpetrators. But they do not provide protection against attacks, and that's a key distinction."
"We are not safer from terrorism with security cameras in our cities," said Harper. "Particularly terrorists who are willing to die, security cameras do not control their behavior. They would not stop them from planning to pull off an attack."
And Harper is concerned about the impact of widespread surveillance cameras on the privacy of Americans. "Cameras don't just watch criminals, but they watch everybody," he explained. "Someone visits their psychiatrist every Monday at two in the afternoon, traveling through public spaces. Where they're going is known to nobody, but a network of cameras could pull that out of obscurity. That info is known to government officials."
Another problem with the technology is that it sometimes shifts crime from one place to another. "If you're the owner of a 7-11 with a surveillance camera and someone robs the store next door to you, then that's a win for you but not for the police," said Bruce Schneier, a security consultant and the author of "Beyond Fear: Thinking Sensibly About Security in an Uncertain World."
Both Harper and Schneier believe that traditional law enforcement techniques are far more effective at preventing terrorism. "Generating human intelligence, targeted surveillance and breaking up terrorist cells — those have been proven to work," said Harper.
Most police chiefs regard the cameras as just one tool for cutting crime. "It's a technology bump for policing and justice," said John Firman, the director of research at the International Association of Chiefs of Police, who said that the issue will be discussed in a roundtable discussion at the IACP's annual conference in October. "We know cameras enhance that capacity but saying for sure that they reduced crime by 20 percent, that's another thing. Anecdotally, we know that they have had an impact."