Buildings and Cities
Walkable cities prioritize two feet over four wheels through careful planning and design. They minimize the need to use a car and make the choice to forego driving appealing, which can reduce greenhouse gases emissions. According to the Urban Land Institute, in more compact developments ripe for walking, people drive 20 to 40 percent less.
Walkable trips are not simply those with a manageable distance from point A to point B, perhaps a ten- to fifteen-minute journey on foot. They have walk appeal, thanks to a density of fellow walkers, a mix of land and real estate uses, and key design elements that create compelling environments for people on foot. Infrastructure for walkability can include:
- Density of homes, workplaces, and other spaces.
- Wide, well-lit, tree-lined sidewalks and walkways.
- Safe and direct pedestrian crossings.
- Connectivity with mass transit.
Today, too many urban spaces remain no- or low-walking ones, and demand for walkable places far outstrips supply. That is because walkable cities are easier and more attractive to live in, making for happier, healthier citizens. Health, prosperity, and sustainability go hand in hand.
compact developments…people drive…less: Ewing, R, K. Bartholomew, S. Winkelman, J. Walters, D. Chen, B. McCann, and D. Goldberg. Growing Cooler: The Evidence on Urban Development and Climate Change. Chicago: Urban Land Institute, 2007.
“general theory of walkability”: Speck, Jeff. Walkable City: How Downtown Can Save America, One Step at a Time. New York: North Point Press, 2012.
manageable distance…ten- to fifteen-minute[s]: Hooper, P., M. Knuiman, F. Bull, E. Jones, and B. Giles-Corti. “Are We Developing Walkable Suburbs Through Urban Planning Policy? Identifying the Mix of Design Requirements to Optimise Walking Outcomes from the “Liveable Neighbourhoods” Planning Policy in Perth, Western Australia.” International Journal of Behavioral Nutrition and Physical Activity 12, no. 1 (2015).
cost of…infrastructure: Ewing et al, Growing Cooler.
Walkability…enhances…public transit: DeWeerdt, S. “Mobility: The Urban Downshift.” Nature, 531, no. 7594 (2016): S52-S53.
more people walk…safer: DeWeerdt, “Downshift.”
physical activity…health and well-being: DeWeerdt, “Downshift”; Frank, Lawrence D., Michael J. Greenwald, Steve Winkelman, James Chapman, and Sarah Kavage. “Carbonless Footprints: Promoting Health and Climate Stabilization Through Active Transportation.” Preventive Medicine 50 (2010): S99-S105.
Urbanites…population in 2050: DESA. World Urbanization Prospects: The 2014 Revision. New York: United Nations Department of Economic and Social Affairs, Population Division, 2015.
[problems with] municipal policies: Gravel, Ryan. Where We Want to Live: Reclaiming Infrastructure for a New Generation of Cities. New York: St. Martin’s Press, 2016.
low-income countries…transportation budgets: DeWeerdt, “Downshift.”
Project Drawdown defines walkable cities as: designing and retrofitting urban environments to encourage walking for commuting or transportation. This solution replaces the conventional practice of driving internal combustion engine (ICE) cars in cities.
Walking is the most neglected mode of transportation: the simplest, most sustainable, and cheapest medium of locomotion. In 2014, humanity walked 1,523 billion kilometers, or an average of 200 kilometers (130 miles) per person per year – barely 7 minutes per day. We drive 7 times as much as we walk. Walkability is still associated only with leisure and recreation in the majority of urban projects around the world; however, the notion of walking as a competitor in the area of sustainable urban mobility is increasing.
Walkability has commonly measured according to the “7Ds”: 1) Density of activity, 2) Diversity of land uses, 3) Design of the street network, 4) Destination accessibility by distance or time, 5) Distance to transit, 6) Demand management, and 7) Demographics (which is not part of the built environment, but still important). These D’s influence patterns in travel choices based on the characteristics of users. Because there is insufficient measurement of all variables for large numbers of cities, this analysis focuses on population density as the key indicator for walkability. 
This solution measures the impact of urban dwellers increasingly switching from ICE car use to walking.
Total Addressable Market 
The total addressable market for walkable cities is defined as the total urban mobility, expressed in passenger-kilometers. Several sources are used to project growth of the market from 2014-2050.  Current adoption  is estimated at 5.4 percent of the market, based on data published by the Institute for Transportation and Development Policy (ITDP) and the University of California–Davis (UCD).
Higher population density can lead to increased walking as destinations become easier for residents to get to on foot. It is assumed that the density threshold for walkability is 3,000-4,000 people per square kilometer,  and that high-density cities could lead to residents walking for 6.5-7 percent of urban trips. By contrast, residents in non-dense cities of less than 3,000 people per square kilometer typically walk around 2 percent of all urban passenger-kilometers. With no single definitive metric to measure a walkable city, and due to the high variability of cities globally, projected growth in urban population density is used as a proxy for the increased adoption of walkability in general.  It is important to note that city densities have been declining by up to 2 percent annually (Angel et al, 2011).
Adoption Scenarios 
Impacts of increased adoption of walkable cities from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the total amount of walking remains constant. 
- Plausible Scenario: Adoption is aligned with projections from the 2015 publication of the ITDP/UCD joint report, “A Global High Shift Cycling Scenario,” with 4 percent of urban mobility by walking in 2050.
- Drawdown Scenario: Using the default density threshold and walking mode-share values discussed above, the amount of walking that would happen in 1,737 cities worldwide is projected, representing 57 percent of the global urban population. This estimate is then scaled to 100 percent of the world’s urban population.
- Optimum Scenario: This scenario takes the maximum foreseeable mode share of 8 percent of the total addressable market and projects a linear adoption from current adoption to this mode share in 2050.
Emission reductions are calculated by replacing fuel consumption and indirect emissions associated with the production of cars with walking, which has zero direct emissions. Indirect food emissions (e.g. from food distribution increases caused by eating more) are excluded.
Costs for ICE cars includes fixed operating costs (e.g. insurance), fuel costs, and other variable operating costs like maintenance and depreciation.  While increased walking may result in the need to purchase higher quantities of food because of burning more calories, implementation and operating costs are considered to be zero.
The Plausible Scenario estimates that 2,589 billion passenger-kilometers of urban travel will be done on foot by 2050. This results in 2.9 gigatons of carbon dioxide-equivalent greenhouse gas emissions avoided, saving travelers US$3.3 trillion  in net operating costs of driving a car. The Drawdown Scenario shows a reduction of 8.8 gigatons of emissions by 2050, and the Optimum Scenario avoids 11 billion tons of emissions.
This most basic mode of travel has been neglected for a long time; however, our study shows that it can be a part of a set of global drawdown strategies. It is acknowledged that there are challenges to increasing walking. We ignored implementation costs, as those are likely highly variable and city-dependent. Making cities denser and taking other steps to improve walking, however, takes a lot of time and requires skills that may not be available in many cities. Walking more often requires partly a cultural change in many places; as walking may be seen as the “poor man’s option,” changing attitudes must go hand in hand with changing cityscapes.
At the same time, making walking easier in cities can make other sustainable modes of transport more attractive: cycling, e-biking, and mass transit all stand to gain from improved walkability in cities. Ideally, all of these more efficient modes can be promoted together. There are likely some benefits of combined promotion that we have not included in any of our analyses due to lack of data. In any case, walking ease is a critical part of adoption of the other modes, and cannot be ignored.
 Population density is only one of seven variables, but it has the most available global data. These seven variables are highly interrelated and are challenging to model independently.
 For more on the Total Addressable Market for the Transport Sector, click the Sector Summary: Transport link below.
 Sources include: the International Energy Agency (IEA), the International Council on Clean Transportation (ICCT), the Institute for Transportation and Development Policy (ITDP) and the University of California –Davis (UCD).
 Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
 This simplification hides the fact that no switch happens suddenly at a 3,000 person per square kilometer density. We use this as a guide to examine the general phenomenon noted in the literature. This model provides a useful first approximation of one of the key variables that might encourage more walking, and what might be the impact of more walking.
 This simplified assumption ignores many other interconnected factors that contribute to growth in this solution, that are beyond the scope of this study to address.
 For more on Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Transport Sector-specific scenarios, click the Sector Summary: Transport link.
 This contrasts with the standard Drawdown Reference Scenario, which assumes a fixed percentage at current levels of market adoption, because the standard Reference Scenario is too optimistic given that sources indicate that walking is on the decline (Buehler and Pucher, 2012).
 All costs were harmonized with other transportation solutions, but here ownership is included as a depreciation cost rather than as a first cost as in some other solutions.
 All monetary values are presented in US2014$.
Full models and technical reports coming in late 2017.