Walkable Cities, a matter of geometry

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.

Diameter

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.

Density

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. 

Conclusion

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.

• ARUP (2016), Cities Alive: Towards a Walking World http://publications.arup.com/publications/c/cities_alive_towards_a_walking_world
• Organization for Economic Co-operation and Development (2011), Pedestrian Safety, Urban Space and Health
• Marchetti (1994), Anthropological Invariants in Travel Behavior
• Newman and Kenworthy (2006) Urban Design to Reduce Automobile Dependence: http://www.naturaledgeproject.net/documents/newmankenworthyurbandesign.pdf
• Ausubel, Marchetti, and Meyer,(1998) Toward Green Mobility: The evolution of Transport: http://phe.rockefeller.edu/green_mobility/
• McKenzie (2014) Mode Less Traveled: Bicycling and Walking to Work in the United States 2008-2012: https://www.census.gov/prod/2014pubs/acs-25.pdf
• Rode and Floater (2014), Accessibility in Cities: Transport and Urban Form. NCE Cities Paper 03. LSE Cities. London School of Economics and Political Science
• Pickrell, D. 1999. “Transportation and Land Use.” In Transportation Economics and Policy Handbook, edited by J. Gomez-Ibanez, Brookings Institution Press.
• Climate Change 2007: Working Group III: Mitigation of Climate Change https://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch5s5-5-1-1.html
• Ewing and Cervero (2010), Travel and the Built Environment: A Meta-Analysis: http://www.cedeus.cl/wp-content/uploads/2013/09/EwingCervero.pdf
• Cervero and Kockelman (1997), Travel Demand and the 3Ds: Density, Diversity, and Design: https://www.researchgate.net/publication/223845521_Travel_Demand_and_the_3Ds_Density_Diversity_and_Design
• Frank and Pivo, (1994), Impacts of Mixed Use and Density on Utilization of Three Modes of Travel: Single-occupant Vehicle, Transit, and Walking: http://www.reconnectingamerica.org/assets/Uploads/Frank-and-Pivo.pdf
• Bertaud (2004), The Spatial Organization of Cities: Deliberate Outcome or Unforeseen Consequence?  http://escholarship.org/uc/item/5vb4w9wb
• Guerra (2014), The Built Environment and Car Use in Mexico City: Is the Relationship Changing over Time? https://www.researchgate.net/publication/270689348_The_Built_Environment_and_Car_Use_in_Mexico_City
• Reid Ewing and Hamidi (2015) Compactness versus Sprawl: A Review of Recent Evidence from the United States, Journal of Planning Literature Vol. 30(4) 413-432
• Ewing and Handy (2009) Measuring the Unmeasurable: Urban Design Qualities Related to Walkability, Journal of Urban Design, 14:1, 65-84,
• Envision Tomorrow (2017) http://envisiontomorrow.org/#welcome
• Harding et al., (2012) Modeling the Effect of Land Use on Activity Spaces
• Angel et al. (2011) The dimensions of global urban expansion: Estimates and projections for all countries, 2000–2050 http://www.sciencedirect.com/science/article/pii/S0305900611000109
• Annual Urban Population at Mid-Year by Major Area, Region and Country, 1950-2050 United Nations, Department of Economic and Social Affairs, Population Division (2014). World Urbanization Prospects: The 2014 Revision.
• Demographia (April 2016), World Urban Areas (Built-Up Urban Areas or Urban Agglomerations) 12th Annual Edition http://www.demographia.com/db-worldua.pdf
• WASH Advocacy Initiative, 2011 http://www.washadvocates.org/about/
• Lo, R. (2009). Walkability: What is it?. Journal of Urbanism, 2(2), 145-166
• The Institute for Transportation and Development Policy, in cooperation with the University of California, Davis (ITDP, 2015) ITDP UC Davis high shift scenario model (email communication August 2015)
• The international Council on Clean Transportation (ICCT, 2012) ICCT, 2012, “Global Transportation Roadmap Model”, Available at http://www.theicct.org/global-transportation-roadmap-model
• Zegras and Hannan (2011) The Dynamics of Automobile Ownership Under Rapid Growth: The Santiago de Chile Case http://web.mit.edu/czegras/www/Zegras%20and%20Hannan_Revised2.pdf