The Collapse of the Network City

Cities are networks. The roads are links and the buildings are nodes. Some buildings are more important than others. Some for their cultural value, giving us a sense of identity. Some for the type of work that is being done in the building. In network terminology, these buildings are hubs.

Expanding networks that contain hubs, like cities are, are called scale-free networks. Research by Albert-Lazlo Barabasi and others has proven that this type of network is extremely resilient to failure. If any node is taken out, other nodes will take over. For example, if a building is declared uninhabitable, the residents will move out and relocate to other buildings.

However, scale-free networks may not be vulnerable to failure, they are very much so to attack. A network will disintegrate completely when a number of hubs are simultaneously removed.

What this means in the real world is illustrated by a 2005 study by the Bank of England and others. They had a look at the vulnerability of the City of London, London’s financial district. The City is arguably Europe’s most important financial center and houses a number of financial hubs. The study showed that half of the sites that the British banking industry depends on were within ten kilometers (six miles) of the Bank of England. Many of the banks’ computer backup sites were within the same area. A single catastrophe, like a pandemic or a dirty bomb, could therefore easily cripple the banking system. This in turn would have consequences for an area much larger than the City of London, or the United Kingdom for that matter.

Banks in The City of London: View Larger Map

But even when the City itself remains in operation, there are other factors that can influence its operability. For example, when a number of public transport hubs are being affected for a longer time. At the 2005 London bombings the disruption was just periodical, but had the public transit system been closed for a longer period, people would not have been able to go to their work. As a consequence The City would shut down anyway. In terms of network science, this is called cascading failure, or progressive collapse.

In an interconnected world, the hubs are the most important, but also the most vulnerable parts of the system. Therefore it would be interesting to see how we could spread the risk of failure.

Could home-working be an option?

All work on NightlyBuilt.org is licensed under a Creative Commons Attribution-Share Alike 3.0 License. This means you are free to reproduce everything as long as you make it available to the public as well, and refer to the original author and site: Maurits Ruis (CC) 2009 – www.nightlybuilt.org .