Countrywide natural experiment reveals impact of built environment on physical activity

TL;DR

This study addresses whether the built environment directly shapes physical activity by exploiting a countrywide natural experiment in which 5,424 relocation events among 1,609 U.S. cities expose individuals to different walkability levels. Using objective, smartphone-based accelerometry, the authors show that moving to more walkable areas increases daily steps and MVPA, with effects persisting for at least three months and across most demographic groups. They demonstrate a monotonic, symmetric relationship between walkability changes and activity, argue against residential self-selection confounding, and quantify population-level impacts via simulations that project large increases in people meeting aerobic guidelines under walkability improvements. The findings provide robust, policy-relevant evidence that urban design can directly enhance health-promoting physical activity and inform public planning to maximize population health benefits.

Abstract

While physical activity is critical to human health, most people do not meet recommended guidelines. More walkable built environments have the potential to increase activity across the population. However, previous studies on the built environment and physical activity have led to mixed findings, possibly due to methodological limitations such as small cohorts, few or single locations, over-reliance on self-reported measures, and cross-sectional designs. Here, we address these limitations by leveraging a large U.S. cohort of smartphone users (N=2,112,288) to evaluate within-person longitudinal behavior changes that occurred over 248,266 days of objectively-measured physical activity across 7,447 relocations among 1,609 U.S. cities. By analyzing the results of this natural experiment, which exposed individuals to differing built environments, we find that increases in walkability are associated with significant increases in physical activity after relocation (and vice versa). These changes hold across subpopulations of different genders, age, and body-mass index (BMI), and are sustained over three months after moving.The added activity observed after moving to a more walkable location is predominantly composed of moderate-to-vigorous physical activity (MVPA), which is linked to an array of associated health benefits across the life course. A simulation experiment demonstrates that substantial walkability improvements (i.e., bringing all US locations to the walkability level of Chicago or Philadelphia) may lead to 10.3% or 33 million more Americans meeting aerobic physical activity guidelines. Evidence against residential self-selection confounding is reported. Our findings provide robust evidence supporting the importance of the built environment in directly improving health-enhancing physical activity, in addition to offering potential guidance for public policy activities in this area.

Figures (17)

• Figure 1: Physical activity levels undergo significant changes following relocation between U.S. cities of different walkability levels.a, During the observation period, 5,424 subjects relocated 7,447 times between 1,609 U.S. cities. Circle area is proportional to the square root of the number of relocations to and from the city. b, Subjects' physical activity levels were tracked through smartphone accelerometry over several months before and after relocation, creating a countrywide study of 7,447 quasi-experiments. c,e Physical activity of subjects moving from less walkable locations to New York City, d,f in comparison to subjects moving in the opposite direction (Methods). Activity levels change significantly immediately after relocation and are symmetric but inverted for subjects moving in the opposite direction. All error bars throughout this paper correspond to bootstrapped 95% confidence intervals.
• Figure 2: Relocations with changes in walkability are associated with corresponding changes in physical activity across most demographics.a, Difference in average daily steps aggregated across all relocations. We find that significantly more walkable locations are associated with increases of about 1100 daily steps, and significantly less walkable locations are associated with similar decreases (for 49-80 point Walk Score increase/decrease). Moving to locations of similar walkability is associated with unchanged physical activity levels. b, Higher walkability is associated with increased daily steps across age, gender, BMI, and baseline activity level groups. Bars show the steps gained per day for each point increase in walkability score (assuming linear model; Methods). Positive values across all bars reveal that, with increasing walkability, more steps are taken by every subgroup, which is significant for all the subgroups except women over age 50 (Student's t-tests, all $P < 0.05$; women over 50 $P = 0.14$).
• Figure 3: Improvements in walkability are associated with increases in moderate-to-vigorous physical activity (MVPA) and with twice as many subjects meeting aerobic physical activity guidelines (49+ point increase).a-c, Changes in physical activity stratified by intensity of physical activity (steps/minute) following relocation to more (a; more than 16 point walkability increase), less (b; more than 16 point walkability decrease) and similarly walkable environments (c; 16 point walkability difference or less). a, We find that walkability-induced additional physical activity (Figure \ref{['fig:panel2']}a) predominantly consists of MVPA, which has been shown to be beneficial for many health outcomes.Lee2012WHO2010b, Moving to less walkable locations is associated with a symmetric loss of MVPA that is equivalent to the increase in more walkable locations (a). c, Further, moving to similarly walkable locations is associated with an unchanged distribution of intensity levels. This suggests that relocation, in and of itself, is not generally associated with increases in physical activity, for instance due to an individual's motivation to increase physical activity. d, Change in MVPA (minutes/week) versus differences in walkability. $\Delta T(I)$ is defined as the change in weekly minutes of activity at intensity level $I$ after relocation, in units of steps per minute. $\Delta T_{MVPA}$ is computed by summing $\Delta T(I)$ for $I \geq 100$ (inset). Large increases in walkability (i.e., 49-80 points) are associated with an increase of about one hour per week in MVPA. e, The increases in time spent in MVPA lead to twice as many subjects meeting national and international aerobic physical activity guidelines of 150 min/week or more in MVPA (before 21.5%, after 42.5%). f, A simulation based on these estimates predicts that if all U.S. cities had the walkability of Chicago or Philadelphia (Walkability score 78), subjects would increase their average activity by 443 more daily steps and 24 more minutes of MVPA per week, and 11.2% or 36 million additional Americans would then meet national physical activity guidelines for MVPA (Methods).
• Figure 4: Subjects' physical activity levels undergo significant changes following relocation to and from specific locations of different walkability. Examples show physical activity levels for subjects moving from/to New York, NY, San Jose, CA, and Albuquerque, NM (differences in walkscore of more than one standard deviation of 15.4 points). Physical activity levels change significantly by about 1,200 - 1,400 daily steps depending on the location. Note the symmetry between moving from (left) and to (right) specific locations.
• Figure 5: Simulating the impact of walkability improvements on United States physical activity levels.a, Estimated fraction of population with 150 minutes or MVPA or more per week following an increase in walkability across all represented U.S. locations. b, Average amount of MVPA added across population following an increase in walkability across all represented U.S. locations. c, Average amount of daily steps added across population following an increase in walkability across all represented U.S. locations.