Research on the Synergy Degree of Aboveground and Underground Space along Urban Rail Transit from the Perspective of Urban Sustainable Development
Aboveground and underground spaces along the urban rail transit (AUSURT) are integral parts of the urban space. Collaborative planning of AUSURT can help to improve the efficiency of urban space use and boost urban sustainable development (USD). This paper attempts to conduct a study of the synergetic degree of AUSURT on the spatial level. From the perspective of USD, a synergy system of AUSURT is created firstly, which consists of four sub-systems: land use, traffic condition, population
... n, population effect and underground space. Then, taking the Phase-I Project of Shenzhen Rail Transit as the example, this paper uses a sequential synergy degree model to calculate the order degrees of the sub-systems in 1999, 2005 and 2015. The system's synergy degrees from 1999 to 2005, from 1999 to 2015 and from 2005 to 2015 are obtained next. The study shows that the order degrees of the sub-systems increase at varying speeds, and all develop to an ordered state, while the system's synergy degree rises to 0.505 in 2015 from 0.179 in 2005, which will boost USD. Above all, the results of the study can reflect the laws of the synergetic evolution of AUSURT, providing scientific judgment and rational decision-making references for the collaborative planning. happy life by taking the slogan of "equality, vitality, harmony and stability"  . In the economic aspect, efficient and intensive urban economic development by innovative and effective technological means is insisted upon  , so as to evenly increase the productivity of various urban economic sectors and give vitality to sustainable economic growth. In regards to the resource aspect, cities should consume natural resources in a reasonable way while providing a high-quality life for the citizens; give priority to using environment-friendly, low-carbon and recyclable resources; and strive to improve resource use efficiency [9, 10] . On the environmental aspect, USD pays equal attention to natural environmental protection, on the one hand, and cultural heritage and development, on the other hand, with the goal of creating a healthy natural ecosystem and an abundant and harmonious cultural system  . The functional optimization of urban space is one of the basic conditions of USD. In other words, USD should be safeguarded by reasonable function division, efficient land use and synergetic spatial layout. At the entire urban system level, various material elements are zoned according to different function divisions, which exert urban overall efficiency by means of effective space organization, so as to create a good environment for city operation. At a partial region level, the development of underground space provides new support for urban development. Especially in the central urban area and along the rail transit, the integration of aboveground and underground space constitutes the inseparable urban three-dimensional space, unleashing the largest potential of land resources and promoting urban space quality. As the core of urban public traffic, rail transit not only reinforces the connection among different function divisions [12, 13] , but also promotes the joint development of aboveground and underground space along its line. Therefore, it is a key step in achieving the objectives of USD [14, 15] . The development of urban rail transit speeds up the spatial exchange of resources, populations and economy  and changes the urban spatial arrangement and land use pattern in the surrounding area. The collaborative planning of aboveground and underground space along the urban rail transit (AUSURT) also helps to create a double-layered network above and below urban ground, realize the optimal configuration of urban material elements and rational urban spatial arrangement, promote intensive and economical use of urban land , improve urban space use efficiency and boost urban sustainable development  . However, while many big cities in China are energetically promoting rail transit construction, the development of aboveground and underground space is lacking in rational collaborative planning due to too urgent and too hasty urban development and construction, consequently leading to a chaotic and dysfunctional urban space, disordered urban morphological structure, environmental degradation, etc. These problems increase the risks of inappropriate urban land use, causing a horrendous waste of social and natural resources, and hinder the progress of USD. In view of the above, this paper attempts to conduct a study of the synergetic degree of AUSURT from the perspective of USD. A quantitative analysis (as this is the Introduction section, the contents aim at inducting the question and emphasizing the necessity of this study, such that we analyze problems in the subsequent sections; therefore, what is expressed here as "appropriate methods" actually refers to the "sequential synergy degree model") is carried out using appropriate methods in order to understand its development status and tendency. On this basis, feasible measures are recommended to provide reference for future collaborative planning of AUSURT. Obviously, this study mainly focuses on the functional optimization of AUSURT, and the economic development is the topic of this paper. However, social, cultural and environmental developments, which are the important parts of USD, should be mentioned, although they are not the topics of this paper. Analysis of Synergy System of AUSURT Synergetics was founded in 1975 by the German theoretical physicist Professor Herman Hawken, which gradually developed into an important branch discipline of the self-organization theory and has been widely applied in the fields of natural science, such as physics, chemistry and biology, and the fields of economics and social sciences     . The theory studies how each component of the system cooperates to produce the space structure, time structure and function structure at the macro level. Sustainability 2016, 8, 934 3 of 22 In addition, it agrees that the synergistic action of sub-systems will form the overall macroscopic movement that is better than the sum of the parts movement, which will produce the synergy effect that is "1 + 1 > 2"  . The synergy theory contains not only the coordination between sub-systems in the process of evolution, but also the kinds of possible competition between sub-systems to achieve collaboration. In fact, competition is the basic premise and condition of collaboration and the most active power for the system evolution. That is because it has to achieve a certain basic condition between the system and its sub-systems in the process of collaboration, while the competition, internal and external, of the system indeed exists in the process of self-organization. As a result, the system is always in a process of unceasing evolution and constant self-organization. Even though it reaches the synergistic effect, it still keeps evolving in order to achieve a better result, which will promote the system to a higher order. The synergy theory is an important tool and method to study systems science, and it reveals the common principle of the change in the updating process of the system development  . Now, it has found more application domains, such as tourism planning , logistics system , traffic network  and urban planning [28, 29] . Especially in the field of urban planning, it is used to study the coordinated development of urban-rural and the synergetic development of urban agglomeration    . However, most of these research works are about the concept, and there is no investigation to evaluate their synergy degree by a qualitative and quantitative method. In summary, the synergy theory, taking the system made up of two or more objects interrelating with each other as its subject, studies the process and the ability of accomplishing certain tasks by cooperating and coordinating with each other. The AUSURT is such a system, which realizes its synergy through the mutual cooperation and competition between each factor. Therefore, there is a certain valuable feasibility and applicability to use the synergy theory to study the system of AUSURT. In order to carry out this study, a synergy system of AUSURT is created firstly. This system takes society, economy, environment and other elements as the material basis for its survival and development within a specific region along the rail transit. It aims to achieve the objective of the intensive use of land resources through the interaction between these elements and gradually develops itself into a well-structured organic whole, which shows a complex time-space characteristic. The synergy system maintains a complex multi-dimensional relationship with the external environment, and at the same time, the internal elements nonlinearly interact with one another. This study focuses on the spatial level to probe into the synergy issues of AUSURT, so these factors considered are different from those considered by urban planning and development. Once again, it is necessary to state that the function synergy of AUSURT is the focus of this study, and it is reflected in land use and traffic along the rail transit. Therefore, it is the economic value that mainly drives our inquiry, and this study can only address USD from an economic perspective. In combination with the requirements of USD, the synergy system is divided into four sub-systems: land use, traffic condition, population effect and underground space. The reasons why these factors are chosen are that: on the one hand, these factors have close relations with the research object and can show the synergic relationship of AUSURT to a certain level; on the other hand, they can also represent the human social behavior in the evolution process of AUSURT by a two-dimensional scale, which reflects the evolutionary trend and feature of AUSURT. For the rest, such as cultural and environmental concerns, they do affect the synergic evolution of AUSURT, but we could not put them into this simple mathematical equation, because they are difficult to address by this study and need a different research approach and method. In addition, the order degree of each sub-system can be, according to the sub-system characteristics, represented by means of one or more order parameters. By determining the order parameters of the sub-systems, an order parameter system of the synergy system of aboveground and underground space along the rail transit can be established.