Traditionally, cities have been viewed as the sum of their locations – the buildings, monuments, squares and parks that spring to mind when we think of ‘New York’, ‘London’ or ‘Paris’.

In The new science of cities (Amazon US| Amazon UK), Michael Batty argues that a more productive approach is to think of cities in terms of flows, connections and relationships – in other words, as a network. Places like Times Square or the Champs Elysée are not big, famous or busy because of their inherent qualities, but rather because they sit at the intersections of movements of people, wealth, information, or power.

An aerial view of the City of London by photographer Jason Hawkes

Flows are not just the connectors between these important locations. Rather, the locations become important because – at least in part – they’re at the intersections.

Urban flows

When we think of urban flows, the hourly and daily movements of traffic or commuters spring to mind, but flows can also be more abstract (information, wealth, power) or longer term (shifting demographics, infrastructure or land uses).

Nathan Yau’s visualisation of RunKeeper data showing running routes around London. View the full set on FlowingData.

Ernst Georg Ravenstein’s currents of migration

These ideas are not new, and metaphors of flow have always abounded in the way we talk and write about the city. For example in the Sherlock Holmes stories Dr Watson, at a loose end after returning from the Afghanistan War, finds himself drawn towards Piccaddilly Circus, that ‘great cesspool into which all the idlers and loungers of the Empire are irresistibly drained’.

There may be more truth in this image than Conan Doyle realised. A famous nineteenth-century map by geographer Ernst Georg Ravenstein showed the ‘currents of migration’ around the British Isles, with people being sucked towards the major cities.

Cities and network analysis

Viewing cities as networks allows us to use the toolbox of network analysis on them, employing concepts such as ‘cores’ and ‘peripheries’, ‘centrality’, and ‘modules’. Batty says that an understanding of how different types of network intersect will be the key that really unlocks our understanding of cities.

Cities, like many other types of network, also seem to be modular, hierarchical, and scale-free – in other words, they show similar patterns at different scales. It’s often said that London is a series of villages, with their own centres and peripheries. but the pattern also repeats when you zoom out and look at the relationships between cities. One can see this in the way that London’s influence really extends across Europe, and in the way that linked series of cities, or ‘megalopolises‘, are growing in places such as the eastern seaboard of the US, Japan’s ‘Taiheiyō Belt‘, or the Pearl River Delta in China.

The new science of cities can be a bit turgid in places, and focusses more on methodology than insight, but it’s a useful primer on a fascinating and fruitful way of thinking about the places where more than half of the world’s population now live.