2.1. Space–Time Impacts of Individual Mobility
In the 1960s, Chapin [
25] first introduced the concept that travel is a demand derived from the necessity to participate in activities, reflecting the internal needs, desires, and commitments of individuals and households. Hägerstrand [
26] then developed the two-dimensional space–time prism, where time and space were used to describe individual mobility. In time geography, the space–time prism has been conceptualized as a space–time that encapsulates an individual’s travel path and activity participation within a certain time frame [
27]. The interior of the prism is described from three aspects: “location in space,” “expansion in area,” and “continuity in time”. In the spatial dimension, researchers have projected the space–time prism onto a two-dimensional plane named “activity space”, representing the area containing potential locations for all daily travels and activities [
28]. Numerous methods are used for measuring activity spaces. The research using the activity space concept is, however, inadequate for identifying individual activity-travel participation and temporal factors.
For activity participation, Hägerstrand first makes a binary distinction: fixed and flexible activities. The author believes that residents’ daily activities can either be fixed (e.g., work and picking up children) or flexible (e.g., shopping and socializing), where a natural competition mechanism exists between these types of activities because of the required cost of time and space. This dichotomy was criticized since the extent to which an activity is spatially and/or temporally fixed may vary, and a binary scheme may not adequately capture such variability [
29]. On this basis, Kuppam and Pendyala [
30] proposed three categories of activities, namely, subsistence activities, maintenance activities, and leisure activities. Subsistence activities are activities that must be performed to maintain the basic essence of life, and an example is work. Such activities are relatively stable. Maintenance activities are performed to maintain a normal state of living; examples are shopping and caring for children, and these activities are of secondary importance. Leisure activities can be freely chosen in time and space and belong to higher-level activity needs, such as sports leisure, and cultural entertainment; these activities are more flexible than others [
31].
The time dimension of individual mobility has attracted considerable research attention. Time use is the allocation of individual time to various activities during a certain period. It can reflect the individual time allocation decisions for various activities such as a mirror reflecting residents’ daily lives. Dharmowijoyo et al. [
32] examined the relationship among travel time, discretionary activity duration, and activity space and found that travel time for subsistence activities has a stronger effect on discretionary activity duration than on the time for subsistence activities. Kuppam and Pendyala [
30] found a positive correlation between maintenance and leisure activity times, indicating that commuters who participated in more maintenance activities showed a greater tendency to participate in leisure activities. Wang et al. [
33] found a trade-off mechanism for three types of activity time. Specifically, people’s time resources are limited, and when they are more involved in a certain type of activity, they are less involved in other activities because of other unobserved properties reflecting the individual’s space and time constraints.
2.2. Mobility and Transport-Related Social Exclusion
The term “social exclusion” originated from French literature in the 1970s and mainly refers to a relationship disruption between the individual and society: the individual’s separation from the social whole [
34,
35,
36,
37]. Social exclusion was introduced into the British government’s policy procedure with the establishment of the Social Exclusion Unit (SEU) [
35,
36,
37,
38,
39]. In 1994, the United States government introduced a similar concept with the title “environmental justice” [
34]. Since the 1990s, social exclusion has been featured in social inequality and social policy discourses in most countries and regions [
40].
Social exclusion is a theoretical concept that acknowledges the undesired alienation of certain individuals from society and examines the process, causes, and consequences of alienation [
39]. In the transportation sector, social exclusion can be due to spatial factors combined with a lack of certain transportation options, preventing individuals from engaging in desired activities [
41], such as participation in employment, education, healthcare, and leisure. Kamruzzaman et al. [
42] summarized this phenomenon as “transport-related social exclusion”. European countries have focused on the interlinkages between transportation and social exclusion of specific social groups since the late 1990s. For example, the Centre for Social Exclusion in the United Kingdom maintains that transportation disadvantages may cause social problems in many key areas of society and that the core of developing social integration is the improvement of accessibility to key services and opportunities [
43]. Preston et al. [
44] recommended that we should focus on regional planning as well as individual and group differences in the research on transportation-related social exclusion. Lucas [
45] noted that personal accessibility is the main factor affecting transportation-related social exclusion. In addition, several studies have shown that high travel costs, physical barriers, geographic or distance constraints, and poor service accessibility can result in individuals being socially excluded through difficulty in accessing transportation services [
46,
47]. Church et al. [
48] proposed a conceptual framework linking transport and social exclusion and examined a series of indicators identifying transportation-related social exclusion in previous studies. In “Church’s Social Exclusion Framework”, transportation-related social exclusion was related to seven primary areas, which were economic, physical, geographic, spatial, fear-based, time-based, and facility-access. Katarzyna [
49] proposed the phenomenon of social exclusion in car-sharing services based on Church’s conceptual framework. In addition, Katarzyna put forward remedial measures (i.e., policy, markets, technology, and infrastructure) for each area of social exclusion from transportation that car-sharing services may suffer.
In the field of sociology, researchers regard social exclusion as a subjective feeling about the quality of life and design questionnaire indicators from multiple dimensions, such as physiological adaptation, economic level, social interaction, identity, and psychological integration [
40]. However, Xia and Shen [
50] found that residents may subjectively feel that they are not excluded from transportation even when they are less mobile than others. A lack of participation in activities has been identified as a vital outcome of social exclusion [
35,
51]. Significant barriers to participation in key activities may be caused by a lack of suitable transportation, a lack of accessible opportunities, or a combination of both [
52,
53,
54,
55,
56]. According to Maslow’s theory of human motivation [
57], people are thought to engage first in activities that meet physiological needs, such as earning an income through work, then those that meet needs for love and belonging, such as dining with family, and finally those that meet needs that contribute to self-actualization, such as fitness. These activities help people achieve love and belonging, personal fulfillment, and other life pursuits. Thus, the satisfaction derived from maintenance and leisure activities is essential for personal fulfillment and active living [
58,
59]. This study directly analyzed activity participation and time use at different levels of need to determine residents’ transportation-related social exclusion. The method adopted here addresses the problem encountered by Xia and Shen [
50] (i.e., individuals’ subjective assessment of whether they suffer from transportation-related social exclusion is inaccurate). Previous methods based on accessibility and activity space have also noted the results of traffic exclusion; however, He et al. [
60] showed that these methods are relatively insufficient for considering the time dimension and, therefore, lack the time dimension for analyzing the traffic-related social exclusion suffered by residents. Consequently, it is difficult to provide support for appropriate time management policies.
Transportation mode greatly influences transport-related social exclusion, which is reflected in the mobility of individuals. First, numerous studies have demonstrated the role of cars in alleviating social exclusion. Cars allow people to eliminate time and space constraints, increase travel rates, cover longer distances, and participate in more free activities [
61]. However, for groups of a lower social status or those that are economically disadvantaged (e.g., immigrants and the poor), the ownership and use of automobiles require considerable resources that are inaccessible to the groups [
62]. In contrast, the adjustment of transportation modes such as public transportation [
63], electric bicycle [
64], cycling, and walking [
65], is more beneficial for improving the mobility of such groups to reduce transport-related social exclusion. In addition, the study by Luo et al. [
66] shows that car-sharing, as a new urban travel mode, has played an essential role in the sustainable development of urban transportation systems.
This literature review demonstrates that numerous studies use activity participation and time-use to determine the constraints and inequalities that disadvantaged groups experience in daily mobility and activities. However, most of these studies were performed on a cross-section, with limited work examining long-term changes in mobility (e.g., years or more). During urban spatial transformation, with changes in economic level and the built environment, the mobility of groups with different transportation modes is affected by heterogeneity. This difference in mobility change may involve a cumulation of transportation-related social exclusion phenomena [
67], which has not been studied in detail. Therefore, it is crucial to study the dynamic relationship between transportation modes and mobility on a longitudinal level to provide a more comprehensive understanding of transport-related social exclusion.