Human mobility is a crucial and burgeoning research field in urban planning, transportation, public health, and related fields, representing the movement of individuals and groups through both space and time. It has many social, health, economic, and environmental impacts. A plethora of studies have been conducted to understand the determinants of human mobility patterns. Existing evidence confirms that socioeconomic status and the built environment can either hinder or facilitate mobility behaviors. However, studies to date have primarily focused on investigating mobility behavior within specific modes at the individual level or mobility density within certain places (i.e., collective human mobility). These measurement approaches have inherent limitations such as limited geographic scales and reliance on small survey data. Meanwhile, there is a dearth of systematic investigation on human mobility patterns at the place level (i.e., area level) focusing on incoming and outgoing directions. Moreover, the COVID-19 pandemic has further highlighted the importance of studying human mobility. Despite many studies have revealed disparities in mobility changes based on socioeconomic status during the pandemic, limited research has been conducted to explore the impacts of the built environment on human mobility in this context. Effects of the built environment are essential for urban planning initiatives aimed at fighting future crisis and promoting urban resilience.

This study utilizes data from SafeGraph datasets to examine human mobility patterns within all cities (i.e., urban areas; combined statistical areas) in the United States. To address the research gaps, this thesis extends the scope of measurement from a dual perspective by incorporating both incoming and outgoing human mobility patterns at the census block group (CBG) level (i.e., place level; area level). The CBG-level characteristics of incoming mobility behavior (i.e., inflow, inbound mobility) are identified using three indicators: stop counts density, median dwell time, and median distance from home of visitors. In addition, the CBG-level characteristics of outgoing mobility behavior (i.e., outflow, outbound mobility) are assessed using three indicators: non-home device counts density, median non-home dwell time, and median distance traveled from home of residents. By examining these six indicators, this study introduces a comprehensive understanding of place-level human mobility patterns. To further explore the correlated factors, social distancing index, socioeconomic status, and built environment at the CBG level are measured as independent variables.

This thesis has formulated four research objectives. (1) First, this study aims to systematically assess human mobility patterns at the place level from a dual perspective, examining both incoming and outgoing mobility patterns during normal period and the COVID-19 pandemic at finer-grained geographic scales. (2) Second, this research will investigate the relationships and effect sizes between the place-level human mobility patterns and the determinant factors, particularly in the built environment, during the normal period (i.e., before pandemic or pre-pandemic period). (3) Third, this study aims to ascertain the impacts and effect sizes of the place-level determinants on changes in place-level human mobility during the COVID-19 pandemic, measured by a year-over-year (YOY) difference. (4) Fourth, a synthesized discussion will be elaborated focusing on the place-level factors, especially the effects of the built environment on human mobility patterns and urban mobility resilience.

Regarding the findings and implications, during the COVID-19 pandemic in 2020, significant changes were observed in human mobility patterns compared to the normal periods in 2019, including incoming and outgoing behavior patterns. Specifically, both stop counts density and non-home device counts density, representing CBG-level mobility density, decreased during the pandemic. There was an increase trend in CBG-level dwell time for incoming behavior, while non-home dwell time for outgoing behavior decreased. In addition, CBG-level incoming distance tended to be shorter than usual, whereas outgoing distance increased and fluctuated. Notably, there were exceptions to these trends, such as CBGs not reducing mobility density during the initial two months of 2020 before the national lockdown.

This study found significant relationships and effects of socioeconomic and built environment factors on human mobility patterns during the normal period and changes during the COVID-19 pandemic (March to December, 2020). CBG-level incoming and outgoing behavior, including mobility density, dwell time, and distance, were partially explained by CBG-level socioeconomic and built environment factors. The magnitude of change during the pandemic also correlated with CBG-level attributes. Built environment generally demonstrated greater explanatory power than socioeconomic variables, highlighting their vital role in shaping human mobility. Major findings and implications for the built environment are listed as follows:

(1) Density: developed open space predicts relatively higher mobility density of outgoing behavior, longer incoming dwell time, and shorter mobility distance. In the COVID-19, places with more open space show less decline in mobility density, less change in dwell time and distance, especially for incoming behavior. Developed areas should incorporate adequate open spaces. These areas accommodate diverse human activities, foster vibrant neighborhoods, improve accessibility, and provide green and public open spaces. Furthermore, these areas experience less disruption in incoming mobility patterns during the COVID-19, including density, dwell time, and distance of visitors.

More green space in areas correlates with lower mobility density and longer distance during normal periods. Mobility distance is significantly affected in these areas, while mobility density is expected to be less affected during COVID-19. Green spaces are crucial urban landscape elements for promoting healthy cities. It is important to integrate green spaces more effectively into the development of urban areas to promote place-level mobility resilience.

Places with a relatively higher population density are associated with higher mobility density and shorter mobility distance. Higher population density areas lead to a decline in mobility density and more changes in dwell time of incoming behavior, but less change in area-level incoming distance. Areas with higher population density encourage greater mobility behaviors during normal periods and are generally more accessible. However, extremely high population density will lead to pronounced decline in mobility density and require people to stay at home during the pandemic.

Higher retail, service, and entertainment places attract more incoming behaviors, while retail, office, and service areas predict fewer outgoing behaviors in normal periods. Incoming dwell time is shorter in these four types of areas. Incoming distance is longer in denser office, service, and entertainment places, but this effect is small in retail areas. During COVID-19, incoming trips declined more in retail, service, and entertainment places. However, outgoing trips declined slightly less in retail, office, and service places. There were slightly smaller changes in dwell time patterns in these four types of areas. Incoming distance to office, service, or entertainment areas slightly decreased, while the decline was less pronounced for retail areas. However, outgoing distance in these areas may have slightly increased to access destinations. Hence, it should better to maintain relatively denser individual functional density and make a complement to promote resilience for collective human mobility patterns.

(2) Diversity: diverse areas (i.e., employment diversity, job-worker balance) tend to have more dynamic mobility patterns, including increased incoming trips, shorter incoming and outgoing dwell time, and longer incoming distance but shorter outgoing distance. Diversity has an impact on decline in area-level incoming density and distance during the pandemic. Nonetheless, residents in such areas are less affected by the pandemic in terms of density, dwell time, and distance of outgoing behavior. Therefore, diversity in employment, especially in combination with housing, is favorable. Moreover, it is better to maintain higher jobs per household in neighborhoods.

(3) Design: urban areas with more street intersections are correlated with higher mobility density, shorter dwell time and distance in normal period. Intersections can support higher levels of human activity, resulting in increased incoming and outgoing behaviors. Although mobility density decreases significantly during the pandemic, the patterns of dwell time and incoming distance are less affected. Street network design with enough intersections plays a crucial role in human mobility behaviors, enhancing accessibility, and promote resilience.

(4) Transit accessibility: frequent public transport service predicts more incoming and outgoing trips and longer mobility distance in normal period. And it can mitigate the decline in non-home dwell time, and incoming distance, and facilitate outgoing distance increase in the pandemic. Therefore, promoting higher frequency of public transport services is valuable for enhancing mobility dynamics and accessibility.

(5) Destination accessibility: more accessible destinations (auto-based assessment) are associated with more area-level mobility trips, longer non-home dwell time and shorter mobility distance in normal period. It is associated with noticeable decline in mobility density, increased incoming dwell time and decreased outgoing dwell time, and increase in outgoing distance. Proportional accessibility predicts shorter outgoing distance in normal, and mitigated decline in mobility density and non-home dwell time in the pandemic. Thus, accessibility is important to enhance mobility dynamics and promote accessibility resilience. It is advisable to maintain moderately accessible destinations near neighborhoods.