Healthy mobility: untangling the relationships between the built environment, travel behavior, and environmental health

The built environment forms the basis of physical and social environment where individuals and groups perform all functions of life. However, researchers increasingly understand that the current built environment and transportation systems can create unhealthy and unsustainable condition. Low-density and segregated land uses, and poor urban design features deter walking and bicycling, and increase reliance on fossil fuel and automobile. Further, poor families living in a racially segregated neighborhood often lack healthy food options and face more environmental burdens. These problems vis-à-vis the built environment and health are the quintessential example of wicked problems where the relationships are complex and operate through multiple confounders. Given this challenge, my dissertation, comprised of three empirical essays, expands on our understanding of the relationships between the built environment, transportation, and health by using experimental research design and original data collected on the individual level. ❧ The first essay uses panel data to investigate the before-and-after impact of a new light rail transit line on active travel behavior. The panel design provides an opportunity for stronger causal inference than is possible in the much more common study designs that use cross-sectional data. It also provides an opportunity to examine how an individual’s previous activity behavior influences the role that new light rail transit access plays in promoting active travel behavior. The results show that, when not controlling for subject’s before-opening walking or physical activity, there was no significant relationship between treatment group status and after-opening walking or physical activity levels. However, when controlling for an interaction between baseline walking or physical activity levels and treatment group membership, I found that living within a half-mile of a transit station was associated with an increase in walking and physical activity for the subjects who were more sedentary before the new light rail opened. The results imply that future policy and research should consider the possibility that sedentary populations may be more responsive to new transit investments, and a different market segment approach based on people’s activity levels would be needed to promote healthy travel choices. ❧ The second essay examines the effects of mode switching behavior from car to light rail transit (LRT) on air pollution exposure. Few studies have systematically examined under what circumstances traffic exposure changes when switching from car to LRT. In this study, we conducted a real-time air pollution measurement while one technician was driving a car and another technician was taking an LRT at the same time for 20 weekdays (a total of 80 trips). Simultaneous measurement allowed us to control for daily variations in meteorological condition. Further, we conducted a robustness check to test the effects of other vehicle-specific factors, such as fan strength, vehicle speed, and vehicle age. The results indicate that traffic exposure was significantly altered by ventilation status, and traffic microenvironment and other vehicle-specific factors were not strong enough to outweigh the effect of ventilation. The results suggest that mode shift from automobile to LRT will be particularly beneficial to those owning an older vehicle with a sub-par ventilation system. Moreover, the results imply that early retirement of old vehicle stocks and transit subsidy programs targeted to low income households could help poor families avoid exposure to higher traffic emissions. ❧ In the third essay, I examine how air pollution levels change during a car-free street event, called CicLAvia. Few studies have examined how car-free street events affect localized air pollutant concentrations. Using an instrumented mobile monitoring platform, we measured concentrations of particulate matter less than 10 and 2.5 microns in size, ultrafine particle number, black carbon, and particle-bound polycyclic aromatic compounds in three different urban locations in Los Angeles on Sundays before, during and after CicLAvia Sunday. Meteorological conditions were similar across the Sundays measured, and most pollutants did not show statistically significant changes despite an increase in traffic congestion generated by the event. These results suggest that more careful traffic management are needed to reduce disruptions to normal traffic flow during special events like CicLAvia. Furthermore, the results suggest that continuous technological improvements in gasoline-powered vehicles now allow minimal air quality impacts from changes in traffic congestion. This implies that technology-driven solutions could be effective at managing urban air pollution problems in the long run. ❧ In conclusion, the three essays provide empirical evidence to show that the health effects of the built environment are not only affected by the “place” but also by the "people” that interact with the built environment. These findings led me to conclude that the environmental health impacts are the results of the dynamic interactions between the built environment and human behavior. The findings also suggest that health impact assessment of transportation systems requires a close examination of multiple pathways that link the built environment and health. Uncovering these pathways would require new research framework and more fine-grained data collection methods that are capable of capturing human behavior and environmental patterns that occur at multiple spatial and temporal scales. The purpose of my dissertation is to help develop a new methodology and to inform existing policy and planning to consider a broad range of behavioral and environmental factors related to health outcomes. To this end, I hope this dissertation will help us move forward with a paradigm shift to healthy mobility.

建成环境（built environment）是个体与群体开展所有生活活动的物质与社会环境的基础。然而，学界日益意识到，当前的建成环境与交通系统会催生不健康且不可持续的状态。低密度、分隔式的土地利用模式，以及劣质的城市设计特征，会抑制步行与骑行行为，提升对化石燃料与机动车的依赖。此外，居住在种族隔离社区的贫困家庭往往难以获得健康食品选择，且面临更严峻的环境负担。这类与建成环境及健康相关的问题，正是典型的“棘手难题”：其内在关联复杂，并通过多重混杂因素发挥作用。针对这一挑战，本博士论文包含三篇实证研究，通过实验研究设计与个体层面的原始调研数据，深化了学界对建成环境、交通与健康之间关联的认知。

第一篇研究采用面板数据（panel data），探究新建轻轨交通（Light Rail Transit, LRT）线路对主动出行（active travel）行为的前后影响。相较于更常用的截面数据（cross-sectional data）研究设计，面板设计为更强的因果推断（causal inference）提供了可能，同时也可用于分析个体既往的活动行为，如何影响轻轨可达性在促进主动出行行为中所发挥的作用。研究结果显示，在未控制研究对象开通前的步行或身体活动水平的情况下，实验组身份与开通后的步行或身体活动水平之间并无显著关联。但当控制基线步行/身体活动水平与实验组归属的交互项后，研究发现：对于轻轨开通前更趋于久坐的受试者而言，居住在轨道交通站点半英里范围内与步行及身体活动量的提升显著相关。该结果表明，未来的政策制定与研究应考虑到久坐人群可能对新增交通投资更为敏感，且需基于人群的活动水平采取差异化的市场细分策略，以推广健康的出行选择。

第二篇研究探讨了从机动车转向轻轨交通（LRT）的出行方式切换行为对空气污染暴露（air pollution exposure）的影响。目前鲜有研究系统性分析从机动车转向轻轨时，交通暴露在何种情境下会发生变化。本研究中，我们在20个工作日内同步开展实时空气污染监测：一名技术人员驾驶机动车，另一名技术人员乘坐轻轨，总计完成80次出行。同步监测使得我们能够控制气象条件的日间差异。此外，我们还开展了稳健性检验，以验证风机强度、车速、车辆年限等其他车辆特异性因素的影响。结果显示，交通暴露水平会因通风状态发生显著改变，而交通微环境（traffic microenvironment）及其他车辆特异性因素的影响均不足以抵消通风状态的作用。研究结果表明，从机动车转向轻轨的出行方式切换，对那些使用通风系统欠佳的老旧车辆的人群尤为有益。此外，研究结果还显示，提前淘汰老旧机动车存量，以及面向低收入家庭的交通补贴计划，可帮助贫困家庭避免暴露于更高水平的交通排放中。

第三篇研究聚焦于无车街道活动CicLAvia举办期间的空气污染水平变化。目前鲜有研究探讨无车街道活动对局地空气污染物浓度的影响。本研究使用搭载传感设备的移动监测平台，于CicLAvia活动周日的前、中、后三个时段，在洛杉矶的三处不同城市区域，对粒径小于10微米与2.5微米的颗粒物（Particulate Matter, PM10与PM2.5）、超细粒子数、黑碳（black carbon）以及颗粒结合型多环芳烃化合物（particle-bound polycyclic aromatic compounds）的浓度进行了监测。本次监测的各周日气象条件相近，尽管活动引发了交通拥堵的增加，但大多数污染物并未出现统计学意义上的显著变化。研究结果表明，在举办CicLAvia这类特殊活动时，需采取更精细化的交通管理措施，以降低对正常交通流的干扰。此外，研究结果还显示，当前汽油车的持续技术进步，已使得交通拥堵变化对空气质量的影响降至最低。这意味着，从长期来看，以技术为驱动的解决方案可有效应对城市空气污染问题。

综上，三篇实证研究均表明，建成环境对健康的影响不仅取决于“场所”本身，还取决于与建成环境产生互动的“人群”。基于上述发现，本研究认为，环境健康影响是建成环境与人类行为之间动态互动的结果。研究结果还提示，对交通系统开展健康影响评估时，需深入审视连接建成环境与健康的多重路径。要揭示这些路径，需要构建全新的研究框架，并采用更精细的数据采集方法，以捕捉多时空尺度下的人类行为与环境模式。本博士论文的研究目标，即助力开发新的研究方法，并为现有政策与规划提供参考，使其在制定过程中纳入与健康结局相关的各类行为与环境因素。为此，本研究希望能够推动学界向健康出行（healthy mobility）的范式转型。