Each era of change and technological innovation is accompanied by the development of new urban functions, structures, and forms [
1]. With the advancement of globalization, cities and regions have become more complex [
2] and are now the basic spatial units for participation in global competition [
3]. Urban connections range from “space of places” to “space of flows” [
4], and urban space continues to break through administrative boundaries to form new functional areas and relationships [
5]. In this process, time, space, and their relationship are redefined, and more and more countries and cities are aware of the construction of spatial order and the reshaping of urban spatial structure in the new era. At the same time, the daily activities of residents have undergone changes. “Cross-border mobility” and “twin-city life” have become increasingly frequent. Border regions have gradually transformed from “depressed areas” dominated by the economy of their administrative regions into key spaces for regional development. Western countries have produced rich theoretical research and have practical experience in this area. Herzog (1990) discussed the landmark case of the Mexican–U.S. border, arguing that the emerging cross-border urban space is a specific product in the context of globalization [
6].
Since the 2000s, urban agglomerations have become the main form of China’s new urbanization. At present, China has approved a total of 19 urban agglomeration plans, and continues to insist on building a coordinated urban development pattern based on urban agglomerations and metropolitan areas. In 2008, the concept of city integration was first proposed by China and promoted as a national strategy in the Outline of the Reform and Development Plan for the Pearl River Delta Region [
7]. City integration refers to two (or more) neighboring cities with conditions for integrated development in terms of economy, administration, ecological environment, culture, etc., that break administrative barriers and form an urban area with gradually increasing radiation and competitiveness [
8]. At present, the integration strategy, with the theme of cross-border coordination and cooperation, has become an important choice in promoting coordinated regional development [
9]. The Guangzhou-Foshan metropolitan area is one case. As modes for cross-border cooperation, the Guangzhou-Foshan metropolitan area has initiated numerous planning practices in the Pearl River Delta and completed the first global intra-city planning in China by the end of 2021. Currently, China’s city integration has developed into a deepening stage, changing from the earlier competing model supported by industries and transportation facilities, etc., and exploring a multi-dimensional integrated development model. However, in actual practice there remain problems such as divergent local interests, uneven and backward levels of joining areas, and effective links that have not yet been formed.
2. Regional Coordinated Development
Border areas are places where numerous interactions and divergences intersect and are intertwined [
22,
23]. Types of border are divided into international, provincial, and local from top to bottom, according to the regional administrative level. According to its dominant attributes in development, the border is divided into natural, administrative, and economic boundaries [
24]. The study of cross-border areas in Western developed countries emerged in the Middle Ages and has gradually shifted from the early focus on border morphology to functional analysis. The European Commission considers border areas to be “the spaces that deserve the most active attention in the European integration process” [
25]. Over the years, Western countries have conducted in-depth discussions on the characteristics, functions, urbanization processes, and types of border areas at different scales, between the United States and Mexico [
6], Singapore and Malaysia [
26], Eastern and Western Europe [
27], the interior of the Luxembourg, Basel, and Geneva metropolitan areas [
28], and Minneapolis and St. Paul [
29], etc. They have continuously explored cross-border integration development models, which have shown an overall diversified and dynamic development trend, with a profound impact on city–region relations.
Regional integration and city integration advanced the connection and cooperation between cities in terms of function, form, and pattern [
30]. Since the beginning of the 21st century, Western developed countries, especially European countries, have provided a practical model for exploring cross-border integrated development [
31]. The EU is actively exploring the opening of borders and functional integration within metropolitan areas and promoting the implementation of cross-border activities to achieve economic linkages, cross-border commuting, facility sharing [
32], cultural integration [
33,
34], multi-level governance [
35], and even meta-governance [
36] in cross-border areas. For China, the development of cross-border regions has also become a regional theme. In the 1990s, Yunnan, Guizhou, and Guangxi provinces [
37] developed inter-provincial border areas in Shandong Province, as early cross-border areas in China. In the 21st century, under the wave of globalization and regional integration, with the breakup of market segmentation and trade barriers in geographic regions, cross-border regions in China developed rapidly. On an international scale, Cheng (2022) studied the three spatial evolution modes of unilateral expansion, bilateral expansion, and cross-border integration between China and neighboring countries against the background of the construction of the “One Belt and One Road” cross-border economic corridor [
38]. On a regional scale, there is the economic cooperation among the Yangtze River Delta cities [
39]; the industry connection and development in the border area of Gansu, Sichuan, and Qinghai [
40]; the cross-border governance mechanism in Guangdong-Hong Kong-Macao Greater Bay [
41]; frequent population flow in the Hong Kong-Shenzhen boundary area, etc. [
42]. On the other hand, under China’s new urban pattern, the form and function of borders are re-characterized. As the degree of functional integration of urban areas increases, spatial mobility across borders is frequently enhanced, which eventually helps to form integrated regions at a new territorial scale [
43].
City integration promotes the high-frequency inter-city mobility of people, and high mobility is one characteristic of urban agglomerations. The movement of people across provinces and cities is a key driver for accelerating and reshaping China’s urbanization process and changing the urban spatial pattern. Since the boundaries are no longer a barrier to spatial and temporal effects, new changes take place in the cross-border area of residents’ activities, from fixed to flexible, from urban to regional, from local to mobile, and from hierarchical to networked [
44]. In the past 20 years, the research on residents’ activities in the cross-border area has become increasingly diversified [
45]. Western scholars have carried out abundant research but not on cross-border travel, and there is little comprehensive research at the multi-scale. Domestic studies have mostly focused on specific spatial scales, ignoring the role of “people” in them, and there are fewer empirical studies identifying the results of cross-border travel and changes in regional spatial structure. Some existing studies have focused on the characteristics, spatial patterns, and mechanisms of urban travel behavior (cross-city commuting trips, high-frequency one-day round-trip trips, and high-frequency two-location round trips) and explored their effects on the agglomeration and diffusion of urban space [
46,
47,
48,
49]. For example, Shen (2003) found that the cross-border travel rate from Hong Kong to different cities differs significantly depending on the timing of cross-border travel, and policy, time cost, and population economy are important factors affecting cross-border travel [
50]. Castells (2004) argued that cross-border travel is represented by inter-city capital investment flow, technology flow, material flow, etc. [
51].
When urban integration accelerates, people’s travel trajectories and travel decisions across regions become more complex. A number of scholars have studied the characteristics, spatial patterns, and driving forces of the cross-city mobility of urban residents at the regional scale, and explored ways to implement sustainable cross-border cooperation and coordinated regional development [
52,
53]. Taylor et al. (2010) [
54], Gan et al. (2021) [
55], and Wang et al. (2017) [
56] summarized the expansion law of people’s travel range and its impact on the connection between cities. Niu (2018) [
57] found that different types of cross-city functional connections influence the spatial and temporal patterns of residential travel. At the urban scale, some scholars have explored the influencing factors of cross-border travel from the perspective of spatial activities. For example, Zhao et al. (2003) [
58] analyzed travel from the perspective of urban allotments and urban spatial distance. Tang et al. (2018) [
59] found that cross-city travel and regional activity space, through the mutual enhancement of technology, affect the flow of information and people, which in turn reshapes the urban spatial structure.
In the process of city integration in China, urban and regional spaces are facing drastic reconstruction and reorganization, with incrementally oriented, mixed land use close to work and home. Population flow in two or more cities increases flows to proximate cities within a region. Cross-border travel accelerates coordinated regional development and regional integration. However, the development of cross-border regions in China inevitably displays different characteristics, as the policy context and development stage are different from those of Western developed countries. Therefore, within the context of city integration, it is now necessary to explore the spatial patterns of cross-border regions by analyzing the spatiotemporal characteristics of residents’ cross-border travel behavior and the mechanism of its impact on spatial evolution during the urban transition period in China.
3. Spatial Pattern of Cross-Border Area in China: A Case of the Guangzhou-Foshan metropolitan area
This study explored the spatial evolution of the Guangzhou-Foshan cross-border area and the driving forces behind it during 1985–2020. After detection of the spatial evolution of the Guangzhou-Foshan region, we determined its development pattern to be spreading expansion, with Liwan District as the central core and connecting to several sub-centers. The cross-border area in the Guangzhou-Foshan region represented a compact, extremely integrated degree and a well-matched functional space. From the perspective of the residence and employment space in the border area of Guangzhou-Foshan, the distribution was relatively concentrated and frequently interacted. There were obvious local hotspots such as Nanhai Guicheng, Eastern New Town, Liwan District, and Tianhe District.
Second, from the analysis of the cross-city travel of residents in the border areas in 2019, instead of the previous one-way attraction pattern caused by the spread of residence, a large number of cross-city trips for leisure and entertainment purposes emerged, and the one-way unbalanced flow, “Foshan to Guangzhou”, changed to two-way circulation. The cross-city travel modes of residents in border areas were summarized into the following three types: Mode 1 signifies living and working in different cities, and having leisure activity in the neighboring city. Mode 2 indicates living in a city but working and recreating in a different city, and Mode 3 represents living and working in one city, but experiencing social life and recreation in another city. Mode 3 was predominant among border residents living in either Guangzhou or Foshan, that is, the current cross-city travel of Guangzhou-Foshan residents was mostly for leisure and entertainment purposes, which was a more significant change from the original commuter-driven cross-border behavior.
Third, the spatial analysis of the border area revealed the spatial evolution patterns of the Guangzhou-Foshan cities from 1985–2020, which transformed from a circle of independent core radiation to a contiguous compact development. The urban integration of Guangzhou and Foshan began in the 20th century, moving from non-governmental cooperation to planning-led development and then to multiple cooperation. The growth points of the two cities continued to expand toward the borderline in the direction of integrated development, while the growth axis was mainly in the direction of the city border, urban arterial roads, and subway lines. In the context of city integration, the spatial and temporal characteristics and patterns of people’s activities in the border areas are becoming increasingly complex, and the administrative boundary effects are becoming increasingly blurred.
Encyclopedia
