Walkable Cities, a matter of geometry

Seoul, 2019. Ernesto Valero Thomas
Seoul, 2019. Ernesto Valero Thomas

Every time I come across the topic of sustainable urban mobility, walkability is treated as a key factor to improve our experience in cities. However, my feeling is that more often than not, walkability as a mode of transport is associated only with leisure and recreation. A marketing strategy, a sugar coat concept in thousands of urban projects around the world. Our reality is that for the last seventy years humans drive 7 times as much as they walk in the majority of our cities.

The good news is that the act of walking is gaining momentum as a serious competitor in the travel choices of cities. Based on its capacity to reduce tons of avoided carbon dioxide emissions and to decrease costs associated with car ownership in millions of USD per year, this mode of transport ranked 54 in the list of the most significant 100 solutions to reduce global warming on a global scale (see the book Drawdown, 2017). I proudly served as the main researcher of this solution in the book, and among the things that I learned is that walkable cities can thrive with the help of two geometric constants: Diameter of Cities, and Population Density. Here is a brief explanation of both, and its relationship with walkable cities.

Seoul, 2019. Ernesto Valero Thomas
Seoul, 2019. Ernesto Valero Thomas


The evolution of human mobility originates with our feet. The historic configuration of cities has been dictated by the human capacity to walk. Think of the Inca and Aztec civilizations. They sustained large cities and regions for centuries on foot. In France, mechanical mobility equaled walking only during the 1920s (Ausubel, Marchetti, and Meyer, 1998). Pedestrian cities usually have a diameter no greater than 5 km. This is the average hourly speed of a healthy pedestrian. Cesare Marchetti comments that when cars were introduced that traveled 6-7 times faster than a pedestrian, cities increased their connected area 6-7 times in linear terms. (Marchetti, 1994) Walking 5km/hour for 1 hour provides a radius of 2.5 km and an area of 20km2. These are the distances that define a village globally. Even today the area that can be crossed in one hour with dominant modes of transport functionally defines a city. You can test this pattern in your own city, or even in your neighborhood.


Agglomeration of humans in a given area. That is population density and it can be measured. Cities have the potential of being walkable if they promote districts with a population density that starts from 3,000 to 4,000 persons per km2. Long term data from thousand of cities with different varieties of incomes and geographic contexts suggest that once we have reached this threshold (measured by residential and employment density) automobile dependence is significantly reduced and other forms of mobility are incorporated. These densities are rarely observed in suburbs located in the United States and they are limited in Western Europe. However, they are often reached in Asia, Africa, and Latin America For instance, in cities such as Mexico City, Bogota, Mumbai, Lagos, and Sao Paulo, like most other developing-world metropolises, population density levels are higher than this minimum. Residents tend to drive less in dense, compact cities than in sparse, sprawling urban areas. Guerra (2014) has identified six variables as the reason behind this mobility pattern in dense and compact urban environments: shorter trip distances, more congestion, higher car insurance bills, higher gasoline prices, less parking, better public transit, and a more pleasant walking environment. 


Diameter and Density. These geometric properties of cities together make a basic unit of analysis for walkable cities. The amount of evidence regarding these two constants is enormous (see the bibliography below to have a general idea of key works regarding walkability). Architects and Urban Designers did not have this solid evidence 15 years ago. We do now. There is no excuse available today to neglect the geometry of cities when we propose walkable cities.

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