sustainability Article Identifying Urban Structure Based on Transit-Oriented Development Yingqun Zhang 1 , Rui Song 1,*, Rob van Nes 2, Shiwei He 1 and Weichuan Yin 3 1 Key Laboratory of Transport Industry of Big Data Application Technologies for Comprehensive Transport, Ministry of Transport, Beijing Jiaotong University, Beijing 100044, China; [email protected] (Y.Z.); [email protected] (S.H.) 2 Civil Engineering and Geosciences, Delft University of Technology, 2600 GA Delft, The Netherlands; [email protected] 3 One Belt-One Road Strategy Institute, Tsinghua University, Beijing 100084, China; [email protected] * Correspondence: [email protected]; Tel.: +86-010-5168-8460 Received: 30 October 2019; Accepted: 13 December 2019; Published: 17 December 2019 Abstract: The fast development of urbanization has led to imbalances in cities, causing congestion, pollution, and urban sprawl. In response to the growing concern over the distribution of demand and supply, a more coordinated urban structure is addressed in comprehensive planning processes. In this study, we attempt to identify urban structure using a Network–Activity–Human model under the Transit-Oriented Development (TOD) concept, since TOD is usually regarded as an urban spatial planning tool. In order to explore the strengths and weaknesses of the urban structure, we define the TOD index and unbalance degree and then classify the urban areas accordingly. We take the city of Beijing as a case study and identify nine urban types. The results show a hierarchical urban structure: the city center covers most of the hotspots which display higher imbalances, the surroundings of the city center are less developed, and the city edges show higher potentials in both exploitation and transportation development. Moreover, we discuss the extent to which the spatial scale influences the unbalance degree and apply a sensitivity analysis based on the goals of different stakeholders. This methodology could be utilized at any study scale and in any situation, and the results could offer suggestions for more accurate urban planning, strengthening the relationship between TOD and spatial organization. Keywords: TOD; imbalance; urban structure; transportation network; activity; human 1. Introduction There are three core concepts of urban systems: urban form, urban interaction, and urban spatial structure [1]. The urban spatial structure is defined as the geo-location and integrated relationship of different urban elements [2,3], and it is also the internal mechanism between urban form and urban interaction. In urban planning, policy makers and the government always attach great importance to urban structure since it reflects both physical and dynamic contexts [4]. Identifying urban structure can give planners a deeper insight into the evolution of the city. The rapid development of urbanization has resulted in a prominent aggregation of people and buildings among metropolises, creating new urban growth poles or subcenters. Urban structure evolution, from a monocentric to a polycentric model, always emerges along with the economic development and the growth of urban scale. Uncontrolled development gives rise to urban issues, most of which derive from the imbalances between supply and demand [5]. Urban elements such as land and transportation belong to the static Sustainability 2019, 11, 7241; doi:10.3390/su11247241 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 7241 2 of 21 Sustainabilitysupply side, 2019 and, 11,human x FOR PEER behavior REVIEW always belongs to the demand side (see Figure1). The limitation2 of 21 of urban boundaries restricts urban land supply, thus enhancing urban densities. The lack of an efficientefficient public transportation system results in congestion and pollution, especially in some rapidly developing countries countries.. M Meanwhile,eanwhile, un unaaffordableffordable housing housing price pricess and and rent rentss in downtown areas force people to relocate to thethe undevelopedundeveloped suburbs,suburbs, generating crowdedcrowded andand lengthylengthy commutes.commutes. Owing to the economic bubble and f financialinancial crisis, the unemployment rate rate is is high high,, resulting in a vacancy problem in both residential houses and commercial houseshouses [[6]6].. All of these phenomena reflect reflect the urban disequilibrium. Supply Demand travel behavior Transportation balance integration Human Distribution Land Use/Infrastructure living/working behavior social behavior Figure 1. The relationship of supply and demand in urban dynamics dynamics.. In order to catch up to the high spe speeded of urban development, i itt i iss necessary necessary to to build build a a compact, compact, balancedbalanced,, and sustainable city. In this context, it is of great significancesignificance to analyze the relation and interaction of didifferentfferent urbanurban elementselements andand theirtheir comprehensivecomprehensive impactimpact onon urbanurban structure.structure. A variety variety of of studies studies have have been been done done on on urban urban structure, structure, and and significant significant urban urban elements elements are recognizedare recognized as forming as forming and andinfluencing influencing the urban the urban structure structure,, such suchas transportation as transportation [7,8], [road7,8], networkroad networkss [9], distribution [9], distribution of buildings of buildings [10], land [10],- land-useuse patterns patterns [11], [economic11], economic performance performance [12], [and12], humanand human behavior behaviorss [13]. [13 ]. By analyzing the the distribution distribution of of different different urban urban elements, elements, urban urban structure structure can can be beunderstood understood in multiplein multiple perspectives. perspectives. According According to the to land the land-use-use data, data, urban urban functional functional areas areassuch as such industrial as industrial areas andareas residential and residential areas areasare distinguished are distinguished [14]. Building [14]. Building densities densities or employment or employment densities densities can help can identifyhelp identify the polycentric the polycentric structure, structure, and high and- high-densitydensity places places are always are always regarded regarded as the as city the centers city centers and subcentersand subcenters [15]. [In15 the]. In field the of field urban of urban structure structure identification, identification, most existing most existing studies studies emphasize emphasize how a givenhow a urban given element urban element influences influences the urban the form urban and form, thus and,, only thus, focus only on focus the ongeo the-distribution geo-distribution of this singleof this urban single element urban element.. Their comprehensive Their comprehensive influences influences on urban on structure urban structure and relations and relations are rarely are studied.rarely studied. OOnn thethe otherother hand, hand there, there are are many many literatures literatures discussing discussing the interactions the interactions of different of different urban elements urban elementsand the most and frequentlythe most frequently used method used is method the Geographically is the Geographically Weighted Weighted Regression Regression [16–19]. However, [16–19]. However,these works these mainly works focus mainly on the focus relations on the between relations urban between elements urban and elements ignored and the ignored geo-distribution the geo- distributionand the urban and structure the urban identification structure identification process. process. In order toto fillfill thisthis research research gap, gap, we we focus focus not not only only on on urban urban structure structure recognition recognition but but also also on theon therelations relations of urban of urban elements. elements. This paperThis paper examines examines the urban the elementsurban elemen throughts through three aspects: three adaptedaspects: adaptedspace with space potential with potential activity, activity, dynamic dynamic space with space the with aggregation the aggregation of humans, of humans, and channel and channel space withspace multiple with multiple transportation transportation services. services. Places thatPlaces are that well are covered well covered by public by transportation public transportation services, serviceaggregatings, aggregating commercial commercial and social and institutions social institutions and attracting and attracting human activities, human activities are defined, are as defined urban vibrancy.as urban vibrancy. Places aggregating Places aggregating one or two one types or of two elements types of are elements defined asare potential defined urban as potential growth urban poles. growthOur aim poles. is to identify Our aim urban is to vibrancyidentify urban and potential vibrancy urban and growthpotential poles urban and growth to judge poles urban and imbalances to judge urbanaccording imbalances to the distribution according ofto di thefferent distribution urban elements. of different By analyzing urban elements the correlation. By analyzing between the correlationareas’ development between potentialthe areas’ and development their imbalances, potential we and can their offer imbalances, suggestions we for can urban offer planning. suggestions for urban planning. 2. TOD Concept and Urban Structure Sustainability 2019, 11, 7241 3 of 21 2. TOD Concept and Urban Structure In this paper,