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In the search to reduce the threat of more terrorist attacks on London there has been increasing focus on preventative security on the Underground.

Technology exists that is adequate to detect bombs and suspicious behaviour but few are willing to claim that technology alone could prevent future terrorist attacks. There are numerous hurdles in the form of cost, legislation and the practicalities of making a new system work.

One of the key challenges is speed. Between 2m and 3m people use the London Underground each day, and Transport for London insists that anything that slows down the movement of commuters is out of the question.

“Any system you deploy would have to have zero effect on the movement of passengers,” says TfL. “We’re all about moving 3m passengers a day - that’s our business.”

It is hard to imagine how London would function if users of its main transport system had to queue for 15 to 30 minutes before each journey. And aside from the inconvenience factor, long queues for airport-style security screening could create an easy target for terrorists.

“You create a mass of people more tightly packed in than in most places,” says Philip Baum, an aviation security specialist. “So there would be relatively no value in doing that, it would cost a fortune and you wouldn’t even have a good result out of it.”

One of the technologies that has received the most attention is millimetre scanners, machines which can x-ray individuals’ clothes as they walk past, showing whether they are concealing anything metallic.

At first glance it looks ideal: the machines can work swiftly and are fairly unintrusive. They are also more effective than the “archway” scanners used in airports, according to Qinetiq, a company spun off from the Ministry of Defence that makes the scanners.

Neil Fisher, Qinetiq’s director of security solutions, says the company has developed a system that can screen 400 people per minute as they walk past.

“Millimetre wave has a role to play in that it can screen what are anomalies in the mass of people in the rush hour,” says Neil Fisher, director of security solutions at Qinetiq.

However, there are numerous barriers to deploying this on a mass scale: an estimated cost of up to £2m per station, privacy concerns as the screens effectively show people naked, and the problem of having enough staff to monitor the screens. While Qinetiq has tested the scanners, its readiness for a mass transit system such as London’s Underground is also disputed.

In 2000, the European Union backed a three-year project into actual and perceived security on mass transit systems. Participants in the project, known as Prismatica, included the London, Roman and Parisian underground train operators and several academics and private companies.

The project examined several technological and non-technological approaches to security and reported on them in 2003. It concluded that facial recognition technology was not adequate for fast-moving systems, and acoustical surveillance, which it said recorded too many “false positives”, or unnecessary alerts.

A technology that received a more positive review in the Prismatica report is automated monitoring of closed-circuit television cameras (CCTV). This involves software that can scan CCTV footage as it is filmed and detect instances of unusual behaviour – such as loitering or leaving a package.

However Kingston University’s Sergio Velastin, an expert in visual surveillance systems who oversaw the Prismatica recommendations and has written widely on transport security technology, remains cautious.

Dr Velastin, who is also the chief executive of a company that develops CCTV monitoring systems, says the technology works.

“But then there’s a big difference between that and saying can we come up with an industrial system that can be rolled out,” he cautions.

Intelligent camera systems can positively identify the type of movement they are trained on about 80 to 90 per cent of the time, but they can have a false positive rate of 4 per cent.

Dr Velastin says makers of CCTV monitoring technology are aiming to make the monitoring cost less than the cost of buying, installing and networking each camera – which can be about £20,000 each.

Then there is the problem of responding to system alarms: no matter how advanced the security monitoring, every alert that is generated must be checked out by a human being and, if necessary, acted upon.

TfL, which operates the London Underground, has trialled both millimetre wave scanners and CCTV monitoring software.

But it says there is simply no technological approach that could be deployed on its network of 275 stations.

“That isn’t to say that a solution won’t be developed tomorrow… if it comes up we will consider it, trial it and see if it would work for the entire network,” TfL said. “The biggest difficulty of all mass transit systems across the world is balancing the movement of vast numbers of people with security.”

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