There are always more data about cities and urban systems. This is unprecedented in our history and opens the exciting possibility of a new 'Science of Cities', with the aim of understanding and modeling phenomena taking place in the city. Urban morphology and morphogenesis, activity and residence location choice, urban sprawl and the evolution of urban networks, are just a few of the important processes that are discussed for a long time but that we now hope to understand quantitatively. Now is the time to participate to the first steps towards quantitative urbanism. This effort towards understanding an object as complex as a city is necessarily interdisciplinary: we will need to build up on early studies in quantitative geography and spatial economics, on the knowledge of architects, urbanists and urban sociologists, and on the tools of geomatics together with modeling approaches coming from statistical physics.
Many different actors are operating in urban processes and various layers of complexity add up in these systems. Understanding the main and dominant mechanisms governing urban systems was already on the agenda of quantitative geographers 40 years ago, and one way to reach it now is to analyze empirical data, extract stylized facts and to propose models in agreement with these observations. This approach reveals various problems, and in particular, the crucial question of the possibility of modeling cities and the importance of self-organization. In other words, a central issue is the possibility to consider the city as a single emergent phenomenon that takes place on various substrates determined by geography, culture, and history.
Different data, different scales
We can divide the available data about cities according to their temporal scale and some of the recent results pertaining to this new science of cities are the following ones, from time scales of order a day to long times such as decades and centuries.
- At a typical time scale of the order a day, mobility data gathered by mobile phones, gps, or RFID’s inform us about where and when people move in the city, revealing in depth the spatio-temporal structure of activities in a city and statistical patterns of mobility. The availability of mobility data renewed the interest for understanding the laws governing the trips of individuals, such as the gravity law, and questioned its validity, leading to new, more accurate models. Also, these datasets provide a clear picture of the spatial distribution of activities, and of the existence and locations of multiple activity subcenters, allowing us to discuss the possibility of a typology of cities based on their morphology.
- At a larger time scale, of a year, socio-economical surveys provide us relevant informations such as the total yearly gasoline consumption, the total yearly number of miles driven, the relation between density and area, etc. The key of understanding how these different quantities scale with population size is the mobility spatial pattern, and a simple model, inherited from previous studies in spatial economics is able to explain these behaviors. In particular, we can propose theoretical arguments to show that for large cities, the cost of diseconomies such as congestion can overcome the scale economies for infrastructure. As a result, cities based on car traffic are not sustainable for populations beyond a value typically of order a few millions.
- Finally at very long scales such as decades and centuries, remote sensing and the recent digitization of old maps allows us to study the evolution of urbanized areas, transportation and road networks. We can then observe that large subway networks seem to converge to the same structure, characterized by similar values of morphological indicators, revealing the existence of dominant mechanisms independent from cultural and historical considerations. We can also observe the large-scale evolution of road networks, allowing us to characterize quantitatively the natural, ‘‘organic’’ evolution of an urban system. For some cities and some systems of cities, the influence of urban planning was crucial and this type of analysis permits to observe quantitatively the effect of such external, top-down processes.
This list of examples illustrates the extent of our agenda for a new science of cities. At stake is the possibility of a quantitative urbanism with practical implications for the planning of future cities.
"Morphogenesis of spatial networks" Springer 2018. Available from the 31st january.
Book on cities
"The Structure and Dynamics of Cities" Cambridge University Press (Dec 2016).
Available on the CUP website and on Amazon
News & Recent studies
New paper in PNAS
Our paper "From global scaling to the dynamics of individual cities" is published in the current issue of PNAS (the revised version will soon appear on the arxiv).
Paper published the Journal of the Royal Society Interface
Our paper 'Tracking random walks' is now published in the JRS Interface.
Review paper on mobility on the arXiv
Our review paper 'Human mobility: models and applications' is now on the arxiv.
New paper on the arXiv
Our paper 'The coalescing colony model: mean-field, scaling, and geometry' is now on the arxiv. Now published in Phys Rev E
New paper on the arXiv
Our paper 'Structural invariants in street networks: modeling and practical implications' is now on the arxiv.
New paper in Scientific Reports
Telmo Menezes and Camille Roth published their paper 'Natural scales in geographical patterns'. Available here.