1. Introduction
In cities, land is at the root of urban development, relating to the coordinated development of economic, social, environmental, and other factors. With rapid urbanization and growth of the urban population, urban land supply is under increasing pressure. The land resource supply has become an important factor restricting urban development [
1,
2]. For most cities around the world, urban sprawl is becoming the main way to ease urban land tensions. However, the side effects of urban sprawl, including cultivated land reduction, traffic congestion, and environmental pollution, have further limited urban development [
3,
4,
5]. The most reasonable approach for urban sustainable development is to optimize urban spaces and improve land use efficiency (efficiency is usually defined as output in relation to input, and land use efficiency is used to calculate the Gross Domestic Product (GDP) per square meter [
6,
7]), in addition to making urban land more functional [
8,
9].
According to the
Code for Classification of Urban Land Use and Planning Standards of Development Land (
GB 50137–2011) [
10], urban construction land (the generic name for residential land, public facilities land, industrial land, storage land, diplomatic land, road plaza land, municipal public facilities land, green space, and other special land) can be subdivided into multiple categories. Research on specific urban construction land prediction is increasing gradually. Nevertheless, a comprehensive study predicting the total urban construction land and specific urban construction land is yet to be carried out. Moreover, the research methods and content regarding specific urban construction land prediction need to be improved. Urban–industrial land is the main type of urban construction land [
11,
12], and is also the main space for urban non-agricultural activities [
13]. China has experienced urban land expansion alongside rapid urbanization. There was 38.59 million hectares of construction land by the end of 2015, including 31.42 million hectares of urban-industrial land and rural residential area. Accounting for 37.4% of the total land supply in China, land supply in the eastern region has reached an annual increasing rate of 3.9%. There is an increasing pressure on urban land supply [
14]. In order to adapt to the new normal of economic development, optimize the land supply structure, support new industry development, and ensure the rational and healthy development of industrial land, China has issued a series of policy documents on industrial structure optimization. In October 2015, the Chinese government put forward the development concept of “innovation, coordination, green development, opening up, and sharing,” clearly aiming to shape the new patterns of regional coordinated development. Beijing, Tianjin, and Hebei have been required to plan industrial land as a whole, and control sprawling of urban-industrial land [
15]. It is of great significance to explore urban-industrial land efficiency.
Meanwhile, China has improved the status of highway traffic through a series of measures since 2003. The investment in highway and waterway infrastructure had reached 0.23 trillion dollars by the end of the year 2003. Meanwhile, highway mileage increased by 8260 km, and rural road mileage increased by 210,000 km [
16]. Rapid development to manage highway traffic has effectively alleviated the tense situation of transportation in China. However, the planning and construction of the existing national highway network still faced some problems [
17]. First, the coverage of the national highway network was not comprehensive [
18]. More than 900 counties across the country did not have access to national highways, and 18 new cities with populations of more than 200,000, along with 29 administrative centers at the prefectural level, had not been connected to the national highways [
19]. Second, the transportation capacity is insufficient. The traffic capacity of some highway channels is tight, and the traffic congestion is serious, which does not meet the needs of rapid traffic growth [
20]. Third, the highway network efficiency is relatively low. The route of the national highway is discontinuous and incomplete, the links between the national highway and other modes of transportation are inadequate, and the network benefit and efficiency are difficult to bring into play [
21]. Therefore, it is necessary to consider the role of the national highway in guiding regional spatial distribution, to optimize the network structure of highways in the eastern region, to strengthen the construction of links between the east and the west in the central region, to expand the coverage of the national highway in the western region, and to coordinate urban and rural development. Putting forward the concept of “accessibility”, which refers to the size of the interaction potential between two geographical nodes in the traffic network, Hansen (1959) proposed that it was not only related to the spatial and temporal barrier between two nodes, but also their quality and scale, and he studied the relationship between urban land use change and accessibility by using the potential model. The potential model can be used to calculate the interaction potential between the regional nodes [
22,
23]. It is of great necessity to examine in more detail the accessibility of highway networks.
As one of the emerging economic agglomeration areas in eastern China, many cities in the Beijing–Tianjin–Hebei (BTH) urban agglomeration have faced problems, such as scattered layouts and low efficiency of urban-industrial land (their Gross Domestic Product per square meter of urban-industrial land is low) [
24]. Since the implementation of the Beijing, Tianjin, and Hebei coordinated development strategy in 2014, the industrial transfer projects of Tianjin and Hebei have blossomed [
25]. Tianjin has 15 industrial transfer projects, with a total planning area of 1030 km
2, of which only 36.8% (379 km
2) was in line with the overall land use planning. Meanwhile, 11 cities and 170 counties in Hebei Province have more than 270 industrial transfer projects, with a total planning area of 19,500 km
2, exceeding the total area of Beijing (16,400 km
2). The state-owned construction land supply of Beijing was 4100 ha in 2016, which was 12.2% lower than that in 2015. The industrial land sales of Tianjin and Hebei also declined from 2015 [
26]. Hence, the dilemma between the blind expansion of undertaking industrial transfer projects and actual demand for land use was obvious. On the other hand, from the perspective of regional scale, passenger transportation in the BTH area was heavily dependent on highways, with railways accounting for less than 10% [
27]. The public transit sharing rate in the center of big cities in the BTH urban agglomeration has been increasing over the past decades. In Beijing, where the public transit share rate was 50%, and the car share rate was 32% in 2015 [
27]. Based on the above analysis, transportation integration and industrial upgrading were the priorities of coordinated development in the BTH urban agglomeration [
28].
For the sustainable development of cities, it is essential to explore the relationship between land use and urban traffic (traffic in urban areas) from the perspective of spatial layout and traffic planning, and to analyze the coordination between them. According to the synergy theory between urban land use and urban traffic, with the continuous coordination development of these two systems, the relationship between traffic supply and demand could change and a matching mechanism of supply and demand would be formed [
29]. Therefore, this paper aims to examine the coupling coordination relationship between urban–industrial land use efficiency and accessibility of highway networks of cities. In specific, in the context of the Beijing–Tianjin–Hebei (BTH) urban agglomeration, this paper is designed to: (1) examine urban-industrial land use efficiency of BTH cities; (2) estimate accessibility of highway networks of BTH cities; and (3) identify the coupling coordination relationship between urban–industrial land use efficiency and accessibility of highway networks.
5. Results and Discussion
5.1. Urban–Industrial Land Use Efficiency
We firstly assessed the urban–industrial land use efficiency, including the economic efficiency, social efficiency, ecological efficiency, and comprehensive efficiency, of all cities in the BTH urban agglomeration, as shown in
Figure 3.
Overall, there were significant differences among economic, social, and ecological efficiencies, found through comparing the efficiency values in
Figure 3a–c. The economic efficiency of the urban-industrial land ranged between 0.026 and 0.453, and the social efficiency of the urban–industrial land of all cities in the BTH urban agglomeration ranged between 0.026 and 0.343. In comparison, the ecological efficiency of urban–industrial land only ranged between 0.006 and 0.070, far less than the values of economic and social efficiency. These results are consistent with the long-term city development pattern in China, in which various local governments have given priority to economic and social development, while the ecological environment has been neglected. Meanwhile, the low value of ecological efficiency also reflects that the urban industrial–land in the BTH urban agglomeration is currently under great ecological pressure, which should be urgently alleviated in future development.
Meanwhile, there were large differences among the economic, social, and ecological efficiencies of different cities. Obviously, Beijing and Tianjin outperformed other cities in all economic and social aspects, with economic efficiency values of 0.453 (Beijing) and 0.233 (Tianjin), and social efficiency values of 0.343 (Beijing) and 0.219 (Tianjin). These values were far higher than those of 11 cities in Hebei Province. Moreover, Beijing had the highest ecological efficiency value of 0.07, at least two times the values of all other cities, including Tianjin city. For ecological efficiency, it is observed that the values of Qinhuangdao and Chengde were much higher, which reflects the preservation of the ecological environment of these areas during development by local governments. However, other cities demonstrated low values. This means Beijing and Tianjin still had the highest urban–industrial land efficiency, while other cities had low urban–industrial land efficiency. The apparent differences in economic, social, and ecological efficiency have further resulted in large gaps in comprehensive efficiency (
Figure 3d).
To understand the causes of the current patterns of urban–industrial land use, we explored the economic, social, and ecological efficiencies based on the components listed in
Table 4. Undoubtedly, with advanced manufacturing and modern service industries as pillar industries and intensive land use levels, Beijing, Tianjin, Shijiazhuang, Qinhuangdao, Tangshan, and other cities had high levels of urban–industrial land use economic efficiencies. Beijing, Tianjin, Baoding, and Langfang showed high levels of urban–industrial land use social efficiencies. Beijing, Qinhuangdao, Shijiazhuang, and Chengde showed high levels of urban–industrial land use ecological efficiencies. Nevertheless, due to the particularity of industrial industries, the ecological environment in Baoding, Tangshan, Langfang, and other cities was faced with difficulties in governance. Furthermore, as central cities in the BTH urban agglomeration, Beijing, Tianjin, and Shijiazhuang had high levels of urban–industrial land use comprehensive efficiency.
Overall, in the current era, the advocated pattern of the BTH urban agglomeration is that Beijing should upgrade its industrial pattern through transferring primary and secondary industry to Tianjin and cities in Hebei Province [
4]. This policy is aimed at conserving land resources, promoting Beijing’s industry sustainability, and driving the economic and social development of Tianjin and Hebei’s cities. However, enterprises have only set up subsidiary companies in Tianjin and Hebei’s cities, without practical operation of these new companies [
25]. This, on the one hand, has pulled down the ecological efficiency of Tianjin and Hebei’s cities, and on the other hand, has significantly deteriorated the economic and social efficiency of Tianjin and Hebei’s cities without real economic and social output. However, the current industry upgradation policy of the BTH urban agglomeration has gone astray because of the imbalance of urban–industrial land use. More critically, the current implementation of the industry upgradation policy has not only failed to conserve land, but has also severely aggravated land resource waste [
90].
5.2. Accessibility of Highway Networks
Urban economic development affects the spatial direction of traffic flow [
91]. By giving economic weight to the shortest travel time, the weighted average travel time based on time distance can weaken the spatial blocking effect of geographical location on accessibility, and strengthen the relationship between economic development and accessibility. Moreover, urban accessibility can be measured through urban scale and economy [
92]. Therefore, measuring the urban scale and economy grade of the cities in the BTH urban agglomeration, as well as comparing their size and classifying their grades, is the basic premise for understanding the accessibility of each city. Each node city’s urban scale and economy grade
Mj was calculated using the ArcGIS 10.2 Natural Breaks Classification method, and we divided the value of
Mj into five classes (
Table 5).
Table 5 presents the urban scale and economy grades of all cities in the BTH urban agglomeration [
93].
As shown in
Table 4, there were five types of urban scale and economy grades in the BTH urban agglomeration, where Beijing and Tianjian were the two cities listed as the economic radiation center. Among all the cities in Hebei Province, Shijiangzhuang and Tangshan were the two characterized as having advanced economic agglomeration, with relatively higher urban accessibility, followed by Baoding and Handan. Cangzhou, Langfang, Qinhuangdao, Xingtai, and Zhangjiakou were cities with the characteristics of primary economic agglomeration, and Chengde and Hengshui were the economically backward areas with the lowest urban accessibility.
The highest urban scale and economy grades, which were 3.192 and 2.648 respectively, were shared by Beijing and Tianjin. Following Beijing and Tianjin, the score of the urban scale and economy grades of Shijiazhuang and Tangshan, which were important bases for the manufacturing and emerging industries in the BTH region, were 1.117 and 1.002, respectively. From the regional distribution, as the top cities of economic development in the BTH region, Beijing and Tianjin shared an urban scale and economy grade of 1, and were located in the core area of the BTH region. Cities that shared the urban scale and economy grade of 2, which included Tangshan in the coastal areas and Shijiazhuang in the west wing, were not only distributed widely, but also showed obvious regional characteristics. Regional differences indicated that Tangshan and Shijiazhuang have been becoming economic centers of the east and west wings of the BTH region. It was noteworthy that the grade of Cangzhou’s urban scale and economy was only 4. However, compared with Handan, the resident population and economically active population of Cangzhou was far less than that in Handan, although their gross domestic product and percentage of tertiary industry were roughly the same. Therefore, the low population size has become the restrictive factor of Cangzhou’s urban scale and economy. Baoding and Handan, whose urban scale and economy grade was 3, were located in the south area of the BTH region.
The cities that shared an urban scale and economy grade of 4, which included Cangzhou, Langfang, Qinhuangdao, Xingtai, and Zhangjiakou, were mainly located in the northwest, northeast, and southeast areas of the BTH region. Cities that shared an urban scale and economy grade of 5, which included Chengde and Hengshui, were mainly located in the north and south areas of the BTH region. Those cities sharing low urban scale and economy grades were almost always located in mountainous areas, and their location conditions are very poor. With an urban scale and economy grade of 5, Hengshui is a barrier to urban agglomeration in the south area, and to core urban agglomeration in the BTH region. Such a regional difference showed that the radiometric force from the core urban agglomeration in the BTH region to Xingtai and Handan in the south area was relatively weak. Generally speaking, spatial differences in urban scale and economy grade characterization revealed that the farther a city is away from the center of regional economy, the weaker the city’s external force and economic radiation ability. That is, the urban space and economic radiation ability between cities was distance diminishing.
We further used the city’s scale and economic level index (
Mj) to reflect the weight index, and measured the accessibility index (
Ai) of each node city in BTH urban agglomeration, by using the improved potential model, as shown in
Figure 4.
With a good geographical location and good economic conditions of the surrounding cities, the Ai of Beijing reached 1.416—optimal in the BTH region. However, with an urban scale and economy grade of 4, the Ai of Qinhuangdao (Ai = 0.039) was the lowest in the BTH region. This indicates that the greater the distance between the city and the regional economic center, the more obvious the influence of geographical location on the level of accessibility. In addition, the Mj and Ai of Baoding and Langfang were heterogeneous. The Ai of Langfang, with an urban scale and economy grade of 4, was higher than that of Baoding, with an urban scale and economy grade of 3. That the Ai of Baoding was slightly higher than that of Handan indicated that the spatial function and economic influence of the surrounding cities of Langfang were larger than that of the surrounding cities of Baoding.
Based on the above analysis, using the ArcGIS 10.2 Natural Neighbor Interpolation method, we obtained the regional accessibility (
Ai) spatial pattern and characteristics in the BTH region (
Figure 5). From the perspective of geographical location, the
Ai of the whole BTH urban agglomeration was relatively high. The accessibility spatial distribution showed an expanding trend from Beijing to peripheral cities; namely, the farther a city was away from the center of regional economy, the weaker the
Ai of the city. The geographical location conditions and the city’s urban scale and economy grade influenced accessibility of the city. The results indicate that the greater the distance between the city and the regional economic center, the more obvious the influence of geographical location on the level of accessibility [
93].
5.3. Coupling Coordination Relationship between the Urban–Industrial Land Use Efficiency System and Accessibility of Highway Networks System
Table 5 exhibits the coupling and coordination types between the urban–industrial comprehensive land use efficiency and accessibility of the highway network. Overall, the coupling coordination relationship showed a good coupling degree in Beijing, Tianjin, Qinhuangdao, Langfang, and Cangzhou. However, the coordination degree of these cities was relatively low. The degree of coupling coordination distinguishes between the benign and the destructive effects of the coupling action. The coordination degree was better than the coupling degree, which indicated that the urban–industrial land use efficiency system and accessibility of highway networks system were not mutually improved, and remained in hysteresis state.
According to the coupling and coordination states between the urban–industrial comprehensive land use efficiency (
U1) and accessibility of the highway network (
U2), we divided the hysteresis statuses of these two systems into three levels: (1)
U1 >
U2, urban-industrial land comprehensive efficiency (
U1) hysteresis; (2)
U1 <
U2, accessibility of the highway network (
U2); and (3)
U1 =
U2, synchronous development (
Table 6). The urban–industrial comprehensive land use level generally lagged behind the highway network development level in cities of the BTH urban agglomeration, except in Tangshan.
In this study, the urban–industrial land use efficiency could affect urban functions, such as the urban ecological environment and accessibility of the urban highway network [
94]. It also has implications for policy making in the fields of geography, economics, and land use planning [
95]. This is because urban–industrial land, as a carrier for industries, is the link between enterprise and urban land. In cities, the land use pattern is always a result of urban development, while the transportation pattern can be a simultaneous outcome—and moreover an important driver—to urban development. In particular, enterprise and the local government are two objects which determine the evolution of urban land use and transportation. Both geographical location and operation of enterprises determine the distribution of the population and employment, which further constitutes the structure of urban land use in practice. Meanwhile, land use structure affects traffic demand and road investment decisions by the government.
The traffic network, through improving location accessibility and promoting the evolution of the regional land use pattern along its route, can lead to the formation of new land intensive areas and promote the development of a polycentric urban spatial structure [
26]. As evidenced in this study, the urban traffic network has a strong spatial attraction effect on land development along the route, and the intensity of land use in the surrounding areas of the traffic trunk line follows the law of distance attenuation. The influence of the urban road traffic network on land use structure is mainly manifested by the spatial attraction and spatial differentiation effect of traffic lines on urban land evolution. The interrelationship between cities is not only related to the level of their own infrastructure, but also to the level of social and economic development, and the scale of cities in other node cities.
However, according to the national land utilization conveyance data from 2015, several problems have occurred in many cities of the BTH urban agglomeration, such as scattered layout, irrational structure, and low utilization efficiency of urban-industrial land. The central government has indicated that the BTH urban agglomeration should adhere to the new development concept of innovation, coordination, and green, orderly unblocked non-capital functions of Beijing, and promote the integrating development of industry with the implementation of the BTH region coordinated development strategy [
96]. However, in the process of industrial transfer, a number of industrial parks, such as industrial compounds, demonstration areas, and industrial agglomeration areas, have been established and left over in the BTH urban agglomeration. Saving and intensifying industrial land use in those cities has not been successful, and the low level of repeated construction causes low efficiency and extensive waste of industrial land. Therefore, saving and intensifying urban-industrial land use should be the main focus of coordination development in the BTH urban agglomeration. With the serious situation of low urban–industrial land use efficiency, we suggest that the BTH urban agglomeration should tap the potential of urban land in stock. Beijing should strictly control the scale and development intensity of construction land and promote the city’s functioning; Tianjin needs to reasonably control the scale of the central urban area and enhance the comprehensive carrying capacity; and Hebei should strengthen the industrial docking and coordination, and leave enough industrial land space to undertake the industrial base.
7. Conclusions
In this paper, an urban–industrial land use efficiency evaluation system was established, and the economic, social, ecological, and comprehensive efficiency of urban–industrial land of the cities in the BTH urban agglomeration were evaluated. The accessibility of the highway network in the BTH urban agglomeration was analyzed by using an improved accessibility evaluation method. We established a coupling coordination model to identify the relationship between urban–industrial land use efficiency and accessibility of the highway networks of cities in the BTH urban agglomeration. The results show that urban–industrial land use efficiency of cities in the BTH urban agglomeration showed significant differences, with central cities in the BTH urban agglomeration showing a high level of urban–industrial land use efficiency. Beijing had the best accessibility, and Qinhuangdao had the lowest accessibility within their geographical locations. The urban–industrial land use efficiency system and accessibility of the highway networks system were not mutually improved, and remained in hysteresis status. The urban–industrial comprehensive land use level generally fell behind the highway network development level in cities of the BTH urban agglomeration, except Tangshan.
Most of the existing research in this area focuses on the coupling and coordination relationship between industrial structures and industrial land use efficiency. For the BTH urban agglomeration, most studies have focused on the analysis of the coupling and coordination relationship of specific industries, such as manufacturing and productive services, the coupling and coordination relationship between industrial agglomeration and the ecological environment, and the coupling and coordination relationship between transportation and the regional economy. However, the three key areas of traffic integration, ecological environmental protection, and industrial upgrading and transfer are important to the coordinated development of the BTH urban agglomeration, and have been largely neglected. This paper examines the coupling and coordination relationship between urban–industrial land use efficiency and accessibility of highway networks, and addresses the gap in the existing literature. An improved accessibility evaluation method, which can enrich the accuracy of the accessibility method, is used to explore the accessibility of highway networks of cities in the BTH urban agglomeration.