2. Following the September 11 attacks, the use of video surveillance in public places became more common to deter future terrorist attacks. Under the Homeland Security Grant Program, government grants are available for cities to install surveillance camera networks. In 2009, there were an estimated 15,000 CCTV systems in Chicago, many linked to an integrated camera network. New York City's Domain Awareness System has 6,000 video surveillance cameras linked together, there are over 4,000 cameras on the subway system although nearly half of them do not work, and two thirds of large apartment and commercial buildings use video surveillance cameras. In the Washington D. C. area, there are more than 30,000 surveillance cameras in schools, and the Metro has nearly 6,000 cameras in use across the system. The Cheshire figure is regarded as more dependable than a previous study by Michael McCahill and Clive Norris of UrbanEye published in 2002. Based on a small sample in Putney High Street, McCahill and Norris extrapolated the number of surveillance cameras in Greater London to be around 500,000 and the total number of cameras in the UK to be around 4,200,000. According to their estimate the UK has one camera for every 14 people.