Equity impacts of the built environment in urban rail transit station areas from a transit-oriented development perspective: a systematic review

Equity theory traces its roots to the foundations of social exchange theory (Littlejohn and Foss 2010). It serves as a pivotal framework for understanding how policy interventions influence distributive justice (Venter et al 2018), frequently by examining cost-benefit ratios for individuals (Guerrero et al 2017). The prevailing paradigm within equity theory defines spatial equity as the complex interaction of institutions, policies, discourses, and practices that shape spatial structures and influence interpersonal transactions, ultimately determining geographical equity outcomes and disparities (Dikeç 2001, Soja 2009). In the context of modern urban research, elucidating a precise definition of equity and formulating empirical measurement criteria grounded in philosophical concepts remain formidable challenges. Nonetheless, notable studies have made significant strides in addressing these complexities. Israel and Frenkel (2018) systematically explored the intricate interrelationships between spatial factors and individual identity, which shape equity within socio-economic and political dynamics. Fainstein (2014) asserted that the study of fairness necessitates the consideration of three essential factors: democracy, equity, and difference. Adli et al (2019) advanced the argument that equitable resource distribution presupposes the absence of resource agglomeration in geographic space. Given that many societal resources inherently possess distinct geographical distribution characteristics, overlooking these attributes during the evaluation of resource fairness may prove inadequate. Thus, it becomes essential to examine the impact of spatial elements within the built environment on equity theory and to systematically measure this influence. This constitutes a vital aspect of equity theory research. By analyzing the complex relationship between spatial characteristics of the built environment and the principles of equity theory, researchers can develop a more comprehensive understanding of the factors that influence the distribution of resources and the resulting impact on social justice. This approach ensures that the assessment of equity remains comprehensive, nuanced, and aligned with the dynamic nature of contemporary urban environments.

The current research has approached the definition and description of URT station areas from various perspectives within the context of TOD. The most widely accepted definition predominantly stems from the built environment viewpoint, conceptualizing these areas as compact buffer zones, easily accessible by walking distance from transfer stations (Ding et al 2019, Niu et al 2021, Park et al 2018). Extensive empirical research has been conducted based on the 5Ds framework of the built environment (Density, Diversity of Land Use, Design for pedestrian and transit, Distance to transit, and Destination accessibility) proposed by Ewing and Cervero (2001). Some scholars emphasize the functional coordination between regional rail transit and land use within station areas, aiming to achieve synergy and maximize the efficiency of both (Park et al 2018, Su et al 2021). These scholars describe distinct yet shared characteristics, including high density, pedestrian-friendliness, mixed-use development, and a focus on transit hubs. Their objectives include reducing motor vehicle trips while promoting walking, cycling, and public transportation (Ibraeva et al 2020, Nasri and Zhang 2014). Regarding the scope of investigation, scholars have introduced two distinct methodologies for delineating station areas within URT systems. One approach employs the Ds principle, which categorizes URT station areas into TOD and non-TOD categories (Ewing and Cervero 2010). Within this framework, TOD areas are characterized by greater pedestrian accessibility, increased development density, and mixed-use land patterns. This concept is frequently applied in studies of European and Asian cities (Vale 2015, Zhang et al 2021). The second method, as proposed by Bertolini (1999), follows a node-place model, categorizing transit hubs into various prototype types. This framework resonates with the notion that TOD involves the integration of public transit stations and the surrounding land use, with a particular emphasis on their mutually beneficial interactions. These categorization methods, despite potential variations in description. Their shared objective is to reduce car dependence while promoting walking, cycling, and transit use (Papa and Bertolini 2015, Su et al 2022). This research scope offers valuable insights for understanding TOD within various urban contexts. One approach focuses on the built environment, which is commonly applied in Australian and American cities. The other approach follows node-place model, which is frequently used in European and Asian urban studies.

Equity within URT station areas encompasses the fair and unbiased distribution of public resources, opportunities, and advantages to all residents and workers. This concept of equity is derived from the definition provided by Cohen et al (2006). Within the context of the built environment surrounding these station areas, the assessment of equity can be measured by considering the accessibility, affordability, and spatial quality of key elements such as housing, transportation, public spaces, and various amenities (Appleyard et al 2019). The notion of social equity in the built environment becomes particularly relevant when disparities in the distribution of significant benefits from TOD impede specific social groups from accessing essential opportunities, goods, services, and communal engagement essential to their daily lives (Wolch et al 2014). Consequently, measuring equity within the built environment under the TOD framework requires the equitable distribution of resources and opportunities within the physical environment. This emphasis underscores the principles of equal differentiation and providing equal opportunities, further encouraging compact mixed-use development (Abdi and Lamíquiz-Daudén 2022). This holistic approach fundamentally entails providing diverse means to access employment, community establishments, and public facilities, tailored to the needs of low- and moderate-income groups. This enables universal access, regardless of income, race, age, gender, immigration status, or disability, to urban resources, including public services, job opportunities, and affordable housing, all within close proximity to rail sites (Renne et al 2016). Such objectives can be achieved through strategic policies and targeted interventions that prioritize inclusive development, active community engagement, and the principles of environmental justice (Liao and Scheuer 2022). This comprehensive approach ensures that the benefits of URT station areas are accessible and advantageous to all, promoting a more just, inclusive, and equitable urban environment.

URT station areas encompass the physical infrastructure and surroundings of URT stations, including land use, density, diversity, design, accessibility, connectivity, and other relevant factors. Such areas significantly impact on ridership, residents' travel preferences, behavior, and urban spatial configurations. In line with the principles and objectives of TOD, the built environment within URT station areas must adhere to high-density, diverse land use. This will effectively enhance the quality of public spaces and ensure high accessibility to various destinations (Appleyard et al 2019, Canales et al 2019, Liao and Scheuer 2022). Extensive literature confirms that the equity of the built environment within URT station areas is influenced by surrounding contextual factors (He et al 2020, Lahoorpoor and Levinson 2020). These factors can be categorized into three primary groups: 1) environmental factors; 2) social factors; and 3) economic factors.

Firstly, environmental elements encompass features that are shaped or modified by human intervention within spaces and locales. These elements include buildings, streets, parks, green areas, and transportation facilities. For example, the number of transfer lines (Zhang and Wang 2014), distance to the city center (Gan et al 2020), terminal stations (Chen et al 2019), and road density (Yu et al 2022) constitute essential environmental variables that profoundly influence the equity of residents' accessibility to rail transit.

Secondly, social elements encompass critical components that significantly contribute to enhancing the quality of the environment for urban residents. These elements include population density, land use diversity, building density, walkability, spatial vibrancy, and the size of pedestrian catchment areas. Numerous studies have highlighted the pivotal role of residential zones, commercial areas, and land use diversity as key factors influencing land use density and diversity (Choi et al 2012, Tissayakorn et al 2021a, Zhao et al 2014). Researchers approach this issue from perspectives such as job-residence balance and land use mix to explore the impact of the built environment in URT station areas on the equitable distribution of employment opportunities for residents. Furthermore, some scholars have found a positive correlation between the size of pedestrian catchment areas and passenger volume within a specific range (Jun et al 2015, Zhao and Shen 2019).

Finally, economic factors predominantly encompass essential components that significantly contribute to the economic development of station areas, including housing prices, land value, housing affordability, employment density, and residents' income (Kepaptsoglou et al 2017). Examining employment density as an illustrative example, the majority of research endeavors have centered on exploring the pivotal role of URT stations in fostering job opportunities within nearby communities. Additionally, researchers have investigated the intricate relationship between URT station areas and the distribution of high-paying occupations (Canales et al 2019, Tornabene and Nilsson 2021).

In order to fully comprehend the potential mechanisms and causal relationships between the impacts of built environment equity outcomes across diverse social groups, this study conducts a systematic analysis of the pathways through which the built environment influences equity within station areas. These pathways consist of interconnected processes, wherein various elements of the built environment contribute to anticipated outcomes or societal changes (Frank et al 2019). The analytical framework used in this study includes four integral components: built environment elements, action modes, action outcomes, and equity impacts (Cartier et al 2015, Rabl and Peuportier 1995). By utilizing the defined action modes and outcomes, this study categorizes the impact trajectory into three distinct categories: direct, indirect, and cumulative impacts (figure 1).

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Direct impacts refer to outcomes that arise directly from actions taking place simultaneously and in the same location. These impacts primarily involve the impact of plans, interventions, or policies on the outcome of specific factors, such as travel time, travel costs, and travel behavior, where the causal relationship is obvious and clear-cut. However, this approach does not address negative externalities in space or social justice, such as accessibility, habitability, and cost (Martens 2016). For example, some studies utilize household income levels to measure the value of shorter commutes. This approach tends to overestimate the value for middle- and upper-income groups, while ignoring the significance of travel for low-income groups and the social costs associated with reducing demand (Blumenberg and King 2019). Thus, correct identification of equity issues may be hindered.

Indirect impacts, on the other hand, refer to the secondary effects of inhabitants' behavioral activities on the built environment. These impacts have a broader range of outcomes compared to direct impacts and are crucial in identifying potential unintended consequences or ripple effects of interventions on outcomes or populations other than the intended beneficiaries. Therefore, it emphasizes assessing the benefits of the built environment from the perspective of space users, the built environment, and the economy and society. For instance, changes in the built environment of a station site have implications for traffic, environmental and health impacts, real estate values, employment, and accessibility to opportunities. Accessibility, in particular, is a key focus in modern research (Bocarejo and Oviedo 2012, Manaugh and El-Geneidy 2012), and is widely used in empirical research to examine the social equity of marginalized groups (Kwan and Kotsev 2015). accessibility not only affects transportation mobility but also emphasizes the availability of opportunities (Curl et al 2011, Lucas 2012), has a significant impact on the spatial equity of rail stations.

Cumulative impacts refer to the synergistic effects of human activities on the built environment, integrating various direct and indirect impacts on the environmental system. These cumulative impacts contribute to the external costs and benefits of the system within the framework of benefit-cost analysis of direct impacts. For example, cumulative impacts on equity in station areas result from both natural and human factors, such as integrated environmental climate change, population clustering, and accessibility (Hidalgo et al 2013, Weisbrod et al 2016).

The current literature on the impact of URT station areas on social equity in the built environment primarily concentrates on two aspects: affordable housing prices and equal access to public services (Supplementary Material 4_Table S1).

One key aspect of concern is the relationship between housing prices in station areas and urban renewal, as well as land values (Cervero and Murakami 2009, Wu et al 2020, Yang et al 2020).The economic benefits of new TOD projects can lead to the displacement of disadvantaged populations and alter neighborhoods (Dawkins and Moeckel 2016, Delmelle 2021, Padeiro et al 2019). A study in Arizona, USA, found that only one-third of TOD projects provide affordable housing options for low-income households (Ewing et al 2022). Nevertheless, some studies argue that the development of station areas does not necessarily lead to gentrification. In Johannesburg, for instance, inclusive policies were implemented in TOD projects to cater to the needs of people from various income and racial backgrounds. These policies included the provision of public services, transit systems, and affordable housing (Harrison et al 2019). This approach successfully prevented the displacement of residents associated with middle-class gentrification. Most current studies leverage quantitative approaches, such as characteristic price models, spatial autoregressive models, and spatial error models, to determine the extent of rail transit's influence on house prices. Yu et al (2018) utilized characteristic price models and spatial Durbin models to examine the impact of the Austin Metro on commercial real estate prices. Their findings revealed a significant effect within a buffer zone of 0–0.5 miles. While the appreciation in property values in station areas often leads to higher housing costs and potential gentrification, a strategic policy framework can mitigate certain dimensions of social inequality.

Another aspect of focus in the literature is the equity of public service opportunities in station areas, especially in terms of access to public transportation. High housing prices in TOD areas create a paradox between transportation costs and housing affordability (Renne et al 2016). Low-income households, in contrast to medium-income groups, spend a significantly higher proportion of their income on public transportation (Ahmed et al 2008). As a result, vulnerable groups tend to live further away from the TOD site (Vermeiren et al 2015). Moreover, the equity of public services related to TODs in developing countries may rely on local development levels. In some developing regions or countries, public transportation may not be the most affordable mode of travel for impoverished communities (Ann et al 2019). The increase in rent or house prices may outweigh the travel cost reduction. In addition to socioeconomic factors, some studies have analyzed the gentrification of the built environment surrounding TOD in the context of residents' sense of community (He et al 2021). While some studies have shown that transportation-related public services in TODs exacerbate disparities in opportunities, further research is needed to explore these issues in terms of community awareness.

The primary focus of the literature on the built environment in URT station areas revolves around the investigation of travel behavior and modes. The objective of this study is to provide a comprehensive overview of travel equity, based on the most credible and representative sources in the field (Supplementary Material 4_Table S2).

First and foremost, early research has shed light on the impact of the quality and layout of the built environment around URT stations on travel behavior. The integration of a walkable built environment with public transit in TOD communities has demonstrated the potential to promote more sustainable travel behavior (Chatman 2013, Ewing and Cervero 2001). Cervero and other scholars have found that residents in TOD communities dedicate more time to walking during their commutes and leisure activities compared to those residing in car-centric neighborhoods (Cervero and Gorham 1995, Cervero and Kockelman 1997). A majority of existing studies have assessed the impact of the built environment on travel behavior by focusing on specific indicators related to land use characteristics or transit attributes in station areas. Notably, the Ds indicator of TOD has been utilized to evaluate land use features in station areas, revealing a significant correlation between land use diversity and increased metro ridership (Pan et al 2017, Zhou et al 2019). Conversely, numerous studies have examined the effect of transit attributes on ridership at the station level. These studies, which utilize travel behavior surveys, consistently demonstrate a positive correlation between the quantity and accessibility of transit facilities and ridership levels (Wang et al 2016) . The number of bus routes connecting to subway stations has also been found to contribute to increased subway usage among residents (Ewing and Cervero 2010). In an attempt to improve the hypothesis of a linear correlation between the built environment and travel behavior, which was identified in previous studies, recent research has utilized deep learning models to investigate the nonlinear associations within travel behavior (Shao et al 2020).

Secondly, the exploration of travel mode selection reveals an intricate interplay between residential isolation, gentrification around stations, and the complex relationship with TOD and travel behavior (Cao 2015). Given the ingrained travel habits of members within TOD communities, certain scholars have delved into the cause-and-effect connection between the built environment and travel mode preferences, particularly from the perspective of travel attitudes. For instance, it's noteworthy that even though individuals with middle and upper-income levels may be enticed by the array of amenities offered within the station vicinity or the potential appreciation in real estate value, their travel patterns may remain relatively unaffected upon relocating to a TOD community (Cervero and Day 2008). Moreover, certain studies investigating individual travel attitudes approach the influence of the built environment on travel mode choices through the lens of residential self-selection. A case in point is the study conducted by Nasri and Zhang (2014), in which they compared employment opportunities in TOD areas to non-TOD areas in Washington, D.C., and Baltimore. They observed that TOD residents with smaller household sizes exhibited lower car ownership rates and higher proportions of households without any cars. In many research endeavors, the utilization of Metro Smart Card Data and built environment attribute data offers a means to scrutinize the intricate interaction between passenger flow and the built environment. This approach not only identifies residents' travel patterns but also provides methodological support for optimizing the URT station areas' built environment, thereby fostering equitable and sustainable development of TOD.

Finally, scholars have thoroughly examined the influence of the built environment on travel equity, with a specific focus on private car usage. One of the primary objectives of TOD is to reduce dependence on private vehicles and promote non-motorized transportation modes. Current discussions regarding the reduction of car usage in URT station areas primarily revolve around the impact of spatial and social factors on residents' travel behavior, directly reflecting the equity of travel options available to them (Rau and Sattlegger 2018). On one hand, residents' travel behavior and car ownership rates are influenced by their residential surroundings and transportation infrastructure. Elements such as population density, land use diversity, well-designed pedestrian and cycling facilities, robust public transit networks, and shared mobility solutions play pivotal roles (Borgers et al 2008). Existing research indicates a negative correlation between TOD and vehicle ownership (Huang et al 2019). However, the impact on car usage may still be uncertain. Some studies suggest that car usage in station areas may not experience significant reductions and might even exhibit a counterintuitive trend, where the distance from households to the nearest bus station is negatively linked to household car travel (Al Otary et al 2022). Moreover, the impact of TOD on car usage seems to vary depending on social and spatial factors. This suggests that living closer to attractive metro stations may be more successful in reducing household car usage (Zhang 2017). On the other hand, TOD is regarded as a potent strategy for reducing car usage (Cervero and Day 2008) and improving the sustainability of the transportation system by increasing public transit ridership. A multitude of scholars have investigated the relationship between the built environment of station areas and public transit ridership (Kepaptsoglou et al 2017) to improve transportation efficiency and provide guidance for land use planning in station areas. Specifically, within the spectrum of built environment factors, both population density and employment density have a positive impact on rail transit ridership (Kuby et al 2004, Zhao et al 2014); land use diversity is positively correlated with public transit ridership (Gutiérrez et al 2011); and street network connectivity significantly increases ridership (Tissayakorn et al 2021b).

Currently, ongoing research is focused on investigating the impact of the built environment of TOD on perceptions of equity. This research aims to identify the factors that influence travel satisfaction and develop methods for measuring it. It considers residents' psychological feedback regarding the level of equity within the spatial environment (Chen et al 2019). These factors include the overall perception and appeal of the travel environment, as well as specific features. Additionally, this research explores the emotional experiences during travel, considering factors such as perceived benefits, opportunities, and other contributing factors (De Vos et al 2015) (Supplementary Material 4_Table S3).

In the realm of research on determinants of travel satisfaction, several studies have found a negative correlation between the distance from one's residence to rail stations and travel satisfaction (Zhai et al 2021, Zhao and Li 2019). However, the strength of this correlation is still uncertain, as residents in URT station areas do not consistently show significantly higher levels of trip satisfaction compared to those living further away from railway stations. This suggests a potential influence of perceived transit on overall trip satisfaction. Furthermore, research has expanded beyond factors solely related to travel, examining the broader impact of the TOD built environment on satisfaction. For instance, the effect of bus stop accessibility on residential satisfaction might be influenced by perceived accessibility rather than solely by spatial proximity to the bus stop (Olfindo 2021). Other studies have examined the psychological feedback outcomes in the context of TOD. These studies have investigated perceptions of station environment design, including acceptable walking distances, safety, and overall friendliness (Pongprasert and Kubota 2019).

In the field of transportation, existing research on measuring travel satisfaction utilizes two main approaches: satisfaction scale measurement and data collection through questionnaires or social media. Cognitively-oriented satisfaction scales, such as the Satisfaction with Travel Scale (STS), assess nine dimensions of well-being (Ettema et al 2011). Alternatively, Bergstad et al (2011) proposed a set of five STS dimensions that specifically focus on cognitive assessments related to daily travel satisfaction, rather than specific travel patterns. Emotional satisfaction scales, such as the Swedish Core Mood Scale (SCAS) (Eriksson et al 2013), Emotional Well-Being (EWB) (Friman et al 2017), the Positive and Negative Affect Schedule (PANAS) (Vella-Brodrick and Stanley 2013), trip enjoyment ratings (Páez and Whalen 2010), and happiness ratings (Lancée et al 2017), are commonly employed in research. Questionnaires and social media measurements are also employed to assess passenger satisfaction and travel service quality. These methods consider factors such as travel convenience, safety, comfort, service, and orderliness to construct comprehensive evaluation index systems (Akbar et al 2021). For instance, Wang et al (2022) developed an evaluation model for rail transit service quality using Sina Weibo social media data, text analysis technology, and the K-Means clustering algorithm, demonstrating its feasibility through a case study of Chongqing URT station areas. Various statistical analysis methods are used to explore the correlation between variables and the extent of influence. Regression models are instrumental in identifying the key influencing factors that drive changes in well-being when examining the effects of multiple independent variables on travel well-being. However, Structural equation Modeling (SEM) is less effective in revealing the underlying causal mechanisms between different factors and distinguishing direct and indirect relationships. Factor analysis is suitable for simplified analysis and is often used in conjunction with regression analysis or SEM. In essence, SEM provides a broader framework that encompasses regression, factor analysis, and other methods. This allows for a scientifically rigorous analysis of variable interactions and their overall impact (Cheng 2001) .

In general, both domestic and international scholars have primarily focused on evaluating the sense of happiness derived from travel through a subjective perception lens when assessing the fairness of the built environment. However, there is a notable lack of research specifically dedicated to examining subjective happiness related to the built environment in URT station areas.

By reviewing the existing literature, it is evident that the built environment in station areas has a positive impact on the health of TOD communities through two main mechanisms: improving environmental quality and promoting physical activity (Supplementary Material 4_Table S4).

TOD plays a crucial role in addressing various challenges related to climate change, such as improving environmental quality and reducing energy consumption and carbon emissions in transportation (Akbar et al 2021). This role becomes particularly significant in areas such as mitigating the urban heat island effect, reducing air and noise pollution, and curbing carbon emissions. While previous studies have identified specific characteristics of the built environment in TOD, such as high population density and integrated public transportation systems, that influence the urban heat island effect (Deilami et al 2016), there is a lack of comprehensive research that examines the cumulative impact of various building environment features.

In terms of air quality, most studies have found lower levels of air pollutants in TOD areas compared to non-TOD regions. Furthermore, rail transportation has been shown to significantly improve air quality and reduce air pollution associated with automobile emissions in surrounding areas (Gu et al 2019, Liang and Xi 2016). Nevertheless, some studies suggest that the increase in public transportation in TOD areas does not have a significant correlation with air pollutants (Beaudoin 2018). Regarding noise pollution, TOD areas tend to experience higher levels of noise pressure, which may exclude individuals who are sensitive to noise and vibrations (Renne 2009). Although the negative effects of noise pollution may not significantly impact the benefits of TOD (Noland et al 2014), these effects are still noteworthy. In terms of carbon emissions, research indicates that living in TOD areas, which are characterized by their proximity to employment and public services, reduces daily travel demands and car ownership (Alawadi et al 2021), leading to increased use of public transportation (Ibraeva et al 2021). These factors effectively reduce travel distances, ultimately contributing to lower regional carbon emissions (Nahlik and Chester 2014). Additionally, residents in TOD areas are more likely to use public transportation for commuting instead of driving (Park et al 2018). Several studies demonstrate that proximity to transit stops significantly reduces long-distance travel, thereby lowering carbon emissions (Choi 2018). This highlights the importance of improving the convenience and accessibility of public transportation to positively influence carbon emissions.

The investigation of the built environment in station areas has also emphasized its impact on physical activity. Cross-sectional studies consistently show that the use of public transportation is associated with higher levels of moderate-to-vigorous physical activity and a reduction in sedentary behaviors (Knell et al 2018). The design of land use and infrastructure that supports non-motorized travel within TODs has been found to improve physical activity levels (Frank et al 2022). Researchers have utilized a range of methodologies, such as ecological models, frameworks for Health Impact Assessment (HIA), and walking audit tools, to thoroughly assess the intricate connection between the attributes of the built environment and physical activity. Current studies suggest that active mobility is strongly correlated with factors such as accessibility, public transport infrastructure, walkability, and street connectivity (Cervero and Murakami 2009).

Accessibility refers to the ease and convenience with which individuals can access services from their original locations. It is determined by the ratio of supply to demand for travel impedance between the source and destination (Polzin et al 2014, Rekha et al 2017). In addition to examining spatial equity in accessibility, numerous studies have focused on residents' access to specific urban public services, such as healthcare and educational facilities, parks, and green spaces. Various location-based methodologies exist for evaluating accessibility, including the shortest distance method, minimum travel cost method, cumulative opportunity method, kernel density method, two-step floating catchment area (2SFCA) method, and the gravity model (GM) method (Boone et al 2009). Recent scholarly attention has focused on individual spatio-temporal accessibility, considering factors such as the operating hours of public service facilities, individual transportation mode preferences, and real-time traffic conditions at the time of departure, among other variables.

The existing literature on the correlation between the built environment of Transit-Oriented Development (TOD) and accessibility has primarily focused on the impact of 3Ds or 5Ds attributes on accessibility, as well as spatial accessibility analysis methodologies (Supplementary Material 4_Table S5).

Initially, Cervero and Kockelman (1997) introduced the foundational principles of 3Ds design criteria for TOD, which included density, diversity, and design. Subsequently, Cervero and Murakami (2009) expanded on these principles by adding two more factors, destination accessibility and transit distance, to establish the comprehensive 5Ds principles for evaluating the built environment of TOD. Research suggests that the density of stations and the mix of land use significantly enhance spatial accessibility. However, further investigation is needed to understand the effects of multi-attribute interactions on accessibility. For example, spatial accessibility and the utilization of public services can be improved by minimizing travel distances between the origin and destination through enhanced street connectivity and efficient access to public transportation (Kamruzzaman et al 2016). It is also important to recognize that even high-density URT station areas may face challenges in improving spatial accessibility if there is an insufficient number of corresponding public service facilities (Tyler 2017). Therefore, a critical inquiry into the specific contributions of various elements in the built environment to advancements in spatial accessibility remains an essential and unanswered question.

When examining into spatial accessibility, Schlossberg and Brown (2004) meticulously scrutinized key elements like street networks and intersections, thus assessing the walkability of subway stations. Similarly, Li et al (2017) adopted a dual approach, utilizing both the topological and utility-based methods to dissect attraction accessibility and radiation accessibility of subway stations, respectively. On the other hand, Bivina et al (2020) quantified accessibility based on actual walking distance. While these studies each emphasize specific facets of accessibility, resulting in a complex metric, an ideal measure should encompass all relevant components. However, even the most comprehensive indicator system may not fully encapsulate all barriers to access (Cavallaro and Dianin 2020). An innovative approach involves integrating pedestrian microsimulation through Agent-Based Modeling or Cellular Automata with quantitative urban spatial analysis techniques. This approach offers distinct advantages in analyzing the directional movement of dense pedestrian flows. This approach effectively utilizes multivariate data to maximize the measurement of accessibility to URT stations.

The existing literature highlights the exploration of environmental equity issues in URT station areas across various research levels. This line of inquiry delves into the social exclusion and differentiation faced by the TOD community post-construction, influencing the achievement of TOD planning objectives. Despite the significant attention given to the TOD built environment in various academic disciplines, there is still a lack of conclusive evidence regarding the relationship between the built environment and equity, particularly within the context of sustainable development and equitable urban construction (Cervero and Kockelman 1997). Consequently, understanding the effectiveness of the TOD built environment in promoting well-being and health for all residents, particularly vulnerable groups, presents a complex challenge in current research. Therefore, further theoretical exploration is needed to comprehensively interpret equity within URT station areas as a comprehensive strategy for urban planning and design.

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  How to define the built environment equity in URT station areas?

The concept of equity has different meanings across various disciplines. In economics, the concept of Pareto Optimality is discussed. In political science, the focus is on patterns of inequality. Sociology emphasizes the importance of equitable distribution of resources, while geography examines spatial-temporal accessibility. Operations research, on the other hand, addresses site selection and allocation models. Although there is no uniform definition of equity (Harvey 1973), all interpretations emphasize the concepts of fair distribution and the object being distributed. Existing research on equity in station areas of the built environment often interprets the concept based on the research purpose, resulting in four predominant perspectives (table 2).

Hence, ensuring fair resource distribution that meets the needs of every resident is pivotal for TOD community construction aimed at fostering equal living environments. Much of the present research adopts the principle of compensating for disadvantages to mitigate spatial inequity, requiring planners to grasp both the allocation of physical space resources and the importance of time and function in resource flows. In future research, we propose to examine equity concerns within the built environment of station areas through a lens of rights and opportunities compensation. This interdisciplinary approach aims to amalgamate insights from diverse fields, fostering collaboration among relevant departments and ultimately deepening our comprehension of equity within rail station environments.

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  How should we construct a comprehensive framework for researching the equity of the built environment in URT station areas?

To develop a comprehensive framework for researching the equity of the built environment in URT station areas, it is crucial to consider several factors. Firstly, it is necessary to identify key factors and specific indicators that influence the equity of the built environment in station areas. In the United States and Europe, the Equitable Transit-Oriented Development (ETOD) model takes precedence, emphasizing the creation of dense, mixed-use, pedestrian-friendly developments around URT hubs. The goal is to ensure that individuals from all walks of life, irrespective of income, race, ethnicity, age, gender, immigration status, or ability, can equally enjoy the advantages of such developments. Furthermore, beyond investments and policy considerations, the surrounding community's development plays a crucial role in shaping the equity of the built environment in station areas. By fostering inclusive communities, rail station areas can contribute to vibrant, prosperous, and resilient neighborhoods, enhancing residents' equal access to urban life.

In the field of transportation, the development and assessment of ETOD strategies have become a priority for government agencies and research institutions. For example, Chicago's ETOD progress from 2016 to 2019 showed positive impacts on traffic, economic growth, reduced car ownership, and increased job opportunities and affordable housing units. However, challenges such as racial inequalities, uneven neighborhood growth, and population displacement have also emerged. In response, Chicago's TOD regulations were revised in 2019 to prioritize equity. The ETOD policy plan serves as a critical urban development strategy to combat segregation, foster community wealth, enhance public health, and address climate concerns. This policy plan evaluates ETOD from six dimensions, employing 24 specific indicators that cover aspects such as race, the creation of affordable housing, and car ownership (table 3). This comprehensive evaluation framework aims to provide a solid theoretical basis for assessing residents' transportation attributes, population demographics, housing affordability, and accessibility in TOD regions. It's important to note that this index system primarily targets the vicinity of Chicago's CTA and Metro URT stations; its broader applicability requires validation. Additionally, uncertainties such as natural disasters, public health events, and social security incidents can impact the economy, public health, transportation services, and housing demand, especially in the surrounding areas of the sites.

Source: Reorganized from page 59 of https://elevatedchicago.org/wp-content/uploads/2022/05/ETOD-Full-Policy-Plan-with-Appendices-6–15–21.pdf

In addition to evaluating resource allocation and spatial distribution in URT station areas, it is crucial to incorporate the principle of compensating for socio-spatial attributes in the assessment and subsequent planning efforts. This paradigm shift goes beyond simply measuring the availability of resources. It emphasizes the importance of understanding who benefits from these resources, revealing patterns of spatial allocation, and uncovering the underlying mechanisms within station areas. This approach delves deeper into the essence of resource flow. Therefore, it is essential to depict disparities in resource accessibility among different regional groups and introduce an equitable dimension to comprehensively understand planning dynamics. This study proposes three overarching dimensions for evaluating equity within the built environment of URT station areas: equity of the physical environment, equity of social groups, and equity of economic externalities (figure 2).

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The first dimension, equity of the physical environment, focuses on the objective characteristics and spatial attributes of the station area. The goal is to determine whether the distribution of physical resources and amenities within the areas is equitable and inclusive.

The second dimension, equity of social groups, recognizes the presence of diverse communities and demographic cohorts interacting within the station area. It aims to uncover inequalities in resource access and utilization among different social groups, considering factors such as socio-economic status, ethnicity, and accessibility requirements.

The third dimension, equity of economic externalities, involves examining the broader socio-economic impacts stemming from the station area. It evaluates how the presence of the URT station influences economic development, property values, job opportunities, and the overall well-being of the community. It assesses whether these effects are distributed equitably among stakeholders.

By considering these three perspectives collectively, planners and policymakers can gain a comprehensive understanding of the intricate relationship between spatial equity and the TOD model within station areas. This multifaceted approach allows for a more comprehensive assessment of the impact of the built environment on various aspects of equity. It leads to better-informed decisions and policies that promote a more equitable and sustainable development of URT station areas.

The literature presents compelling evidence that contemporary TOD projects offer tangible benefits to low-income communities in diverse urban landscapes. These benefits include improved access to public services, significant savings in commuting time and costs, and overall enhancements in health and safety standards. However, it is important to acknowledge that the distribution of these advantages is not uniformly equitable, highlighting a critical gap in the examination of factors that influence equity within the TOD built environment. This omission is also evident in the field of causation analysis. Furthermore, it is apparent that these benefits primarily accrue to the higher segments of the low- and middle-income groups. This socioeconomic bias in the distribution of TOD benefits emphasizes the urgent need for a more comprehensive and nuanced exploration of the underlying factors driving these disparities. Therefore, it is crucial to critically evaluate the selection of variables and the construction of models to effectively capture and elucidate the causal relationships at play. Such methodological precision is essential for a robust and equitable assessment of the true impact of TOD projects on disadvantaged communities.

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  Select more representative variables to construct a TOD built environment equity evaluation model

To address this issue, it is necessary to select more representative variables and construct a TOD built environment equity evaluation model. This study builds on the existing literature review and presents a conceptual framework that outlines the determinants of equity assessment within the built environment of station areas. This framework recognizes equity as a societal phenomenon influenced by various factors with direct and indirect effects. It emphasizes the importance of distinguishing different mechanisms and manifestations through which these elements exert influence. The conceptual framework incorporates three levels of factors: environmental, social, and economic (figure 3).

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The first level focuses on the environmental determinants from the perspectives of spatial and health equity. It places emphasis on material elements enhancing the quality of residents' living environment and natural environmental components. It highlights attributes such as the walkability and health degree of URT station areas, alongside the accessibility and availability of essential amenities.

The second level revolves around social determinants, with the assessment goals of perceived fairness and social group equity. This dimension amalgamates individual and group stances concerning macro-policy mechanisms, considering individual environmental perceptions and socio-economic status as factors influencing the degree of equity outcomes. It involves intermediary factors such as inclusivity in daily activities, satisfaction with interactive travel, the distribution system for public services, and the minimum living security system.

The third level considers economic elements aligned with the objectives of regional economic balance. Through market regulation strategies, policies are introduced that enhance public service quality and the efficiency of providing secure housing by fostering competition, choice, and private sector participation.

In summary, this framework synthesizes the three hierarchical levels of equity within the TOD built environment in station areas. It demonstrates that equitable determinants are constructed through the intervention of environmental, social, and economic factors, with the overarching goal of achieving equitable and sustainable development in TOD station areas while optimizing the welfare of marginalized populations.

Additionally, future research and policy should incorporate more detailed indicators regarding the characteristics of the TOD built environment. For example, assessing the impact of spatial matching between public transportation routes and destinations on resident's travel behavior and travel patterns in travel equity (Deboosere et al 2018). Additionally, examining the impact of First/Last Mile built environment quality on overall satisfaction with rail travel is critical for perceived equity (Park et al 2021). Besides, it is necessary to determine how to weigh and coordinate various equity impacts to develop a comprehensive evaluation model that reflects the implications of equity in the TOD built environment.

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  Identify the causal relationship between the built environment and equity of URT station areas

Understanding the causal relationship between the built environment of TOD and equity is of utmost importance in order to examine how the physical design and characteristics of urban spaces impact the health, well-being, and opportunities of diverse populations. This area of exploration presents significant challenges but is crucial for unraveling the causal link between the spatial attributes of station areas and the realization of equity, thereby enhancing the scientific robustness of research findings and informing urban planning practices that foster an equitable urban landscape. One approach to establishing causality is through experimental or quasi-experimental designs that aim to isolate the effects of the built environment on various outcomes, such as physical activity, social interactions, or environmental qualities. However, the practical application of such designs encounters significant difficulties. Consequently, the investigation of the causal relationship between the built environment and equity in a research context presents several formidable challenges.

Firstly, equity is a complex concept that encompasses multiple dimensions, including income, race, gender, and social class. Balancing all of these dimensions in research while isolating the impact of the built environment from other factors contributing to inequality is a challenging task.

Secondly, the relationship between the built environment and equity is bidirectional, meaning that changes in equity also affect the built environment. For instance, communities with more resources and political power may be better able to advocate for improvements in the built environment. However, explaining reverse causality in research is difficult.

Thirdly, confounding factors such as historical URT station development, spatial imbalances, and economic-political dynamics introduce additional variables that make it harder to assess the impact of the built environment on equity.

Fourthly, data constraints pose a challenge to conducting rigorous studies that account for variables and establish causality in the relationship between the built environment and equity. Inadequate geographic detail and other limitations in available data hinder comprehensive analysis.

Lastly, the ethics and complexity of experimentation present challenges in studying the causal link between the built environment and equity. Many interventions for improving the built environment require complex, long-term changes, making randomized controlled trials ethically or logistically unfeasible. The scarcity of longitudinal data and natural experiments that capture the dynamic, long-term impact on equity further complicates the establishment of a causal link. As a result, studies often rely on observational data or quasi-experimental designs to address these limitations.

In future research, it is essential to broaden the scope of investigation and enhance the thoroughness of examining equity within the built environment of URT station areas. This can be achieved by focusing on three key aspects.

To begin, it is imperative to foster interdisciplinary synergy by integrating a diverse range of disciplines, such as urban planning, geography, and economics. This collaborative endeavor shall be instrumental in elucidating the fundamental principles that support equity within station areas. Concurrently, this approach facilitates a comprehensive exploration of the mechanisms that govern equity in the built environment, encompassing three pivotal perspectives: environmental, social, and economic. The current body of research highlights the expanding scope and significance of rail transportation studies, as more experts and scholars recognize its pivotal role in promoting sustainable urban development. Nonetheless, there is a significant gap in evaluating the built environment from the perspective of TOD construction. Present evaluations predominantly emphasize spatial elements or predefined targets, often failing to fully capture the complexities of equity in the built environment. While existing studies deploy equity evaluation metrics such as the Lorenz curve and Gini coefficient to assess specific allocation targets, they seldom scrutinize the appropriateness of equity theory for these targets. As a result, future research should focus on exploring the intricacies of built environment equity theory within station areas. This will provide a strong theoretical basis for a comprehensive equity evaluation model.

Subsequently, this study delves deeply into the distinct influences of built environment equity in URT station areas. TOD not only alters residents' daily travel habits but also impacts their leisure and shopping activities. Consequently, it is imperative to expand research on the effects of residents' activity patterns in URT station areas. Building upon both linear and nonlinear hypotheses, this study augments the discourse by introducing discussions on the heterogeneity of the built environment and the various types of activity patterns. This expansion aims to provide a more comprehensive understanding of the intricate interplay between the built environment and activity patterns in promoting equity within urban spaces. Future research should also explore the mechanisms of behavior change related to TOD, such as the reduction of car usage and shifts in travel patterns. Including psychological outcomes in studies of perceived equity could provide further insight into the well-being and attitudes of individuals affected by the built environment. Research may investigate the impact of TOD on perceived equity and explore variables such as the availability of healthy food environments, social media usage, and information communication tools before and after resident relocations to elucidate potential mechanisms of impact.

Technological innovation plays a pivotal role in improving our comprehension of the impact of the TOD built environment on equity. The future trajectory of TOD is deeply intertwined with the advancements in next-generation technology. The transportation paradigm must shift to embrace connected, shared, and digital mobility. This requires well-defined planning mechanisms and technical initiatives aimed at fostering the intelligent evolution of the built environment within TOD areas. Future research endeavors should comprehensively consider the synergistic effects of interventions such as Connected-Automated Vehicles (CAVs), Mobility as a Service (MaaS), and information and communication technologies (ICTs) on the equity of station areas. The integration of emerging technologies, such as customer digital twins, decentralized identities, and digital humans, holds the potential to create novel virtual spaces and communities. These advancements can facilitate meaningful interactions with physical spaces, ultimately advancing the objective of achieving equity in URT transit areas.