…What is the appropriate size of cities?
By Oscar Quintanilla
While academics and practitioners are arguing for more urban density, agglomerations and bigger cities, I question whether their apparently simple argument is fundamentally right. Many argue that urban density is more sustainable than its alternative, and while I agree that there are great benefits from having compact and dense cities, this option, by itself, cannot be sustainable either. Looking at rapidly urbanizing developing countries, where sustained rural – urban and urban – urban migration is creating cities of unimagined sizes, I wonder at what point urban agglomeration does more harm than good. What should the size of a city be? What affects this?
As we saw in two recent case studies, cities are faced with tough decisions and tradeoffs, and while density is theoretically a good thing, many factors determine the point where cities should stop growing. Like in the Mexico City case, where the delivery of water was becoming more challenging and costly, or in Bogota, where the city was deciding between a bus rapid transit (BRT) and a Metro system, questions about the size of the city and the appropriate density should be considered carefully.
While many argued that a Metro system would provide the capacity that Bogota needs for its future, should Colombia support a continuously growing Bogota? The Metro might cost as much as 150 million dollars per kilometer, a lot more than the expected 22 million for an additional kilometer of BRT. While the BRT system seems to be reaching its capacity, a Metro system is very expensive and would require an operational subsidy that the BRT doesn’t require. Should Colombia support the infrastructure for an even bigger Bogota? Or should it aim to support growth and infrastructure in mid sized cities?
The Mexico City question is even more pressing. Water scarcity, droughts, and a fast depletion of its ground water supply are challenging the city, the region and the national economy. While the first thought is to invest in the infrastructure needed to support the 8.84 million people that live in Mexico City proper, at what cost for the local, regional and national and governments? Is Mexico better off with a constantly growing metropolis or with more balanced urban concentrations?
The case for density is appealing. More people together in less space is environmentally responsible, allows cities to support major infrastructure and supports the idea based economy of the 21st century. Yet cities are not all made of the same material, the same people, have the same supporting hinterland, or have the geographic characteristics to support it.
Density is subject to the law of increasing costs, and its benefits are also subject to diminishing returns. Find were marginal benefits equal marginal costs and you will find the optimal size for the city. Many factors could affect this, like productivity levels, if the economy is service or manufacture based, or the supporting capacity of the hinterland and the terrain. Countries also need to consider their place in the global economy, and the value that a bigger and denser city provides to their national development strategies.