Urban expansion is an important stage and inevitable occurrence in urban social and economic development [1
]. With the rapid development of global urbanization, the compact use of urban land has become even more important. Rational urban planning, the control of urban expansion, and the prevention of disorderly unauthorized construction are beneficial to sustainable urban development. Road networks, which are the “skeleton” of urban social development and the foundation and important driving factor for urban land expansion, are closely linked with urban expansion [3
]. Road networks, which are integral components of the infrastructure and support urban integration, connect various functional areas. Increasingly well-developed road networks bring the administrative regions of cities into closer contact and accelerate the exchange and aggregation of logistics, energy flows, information flows, capital flows, and other flows. The investment scale, area, space action, and other factors of road networks have made these features significant drivers of urban expansion [5
]. Thus, the rapid expansion of urban land increases the rigid demand for the land in road networks.
Urban expansion is a research focus of land use cover/change, urban development, ecology, and other areas, and many scholars have conducted studies on this topic, including the drivers of urban land expansion, temporal and spatial variations, simulations and predictions, ecological and environmental effects, and other effects [8
]. The main research methods include methods for monitoring spatial expansion that are based on geographic information systems (GIS) and remote sensing (RS) [11
], logistic regressions [13
], cellular automata (CA) [9
], and other analysis methods that involve models of system dynamics. Burchfield, et al. [17
] studied the extent to which urban development in the U.S. was expanding and what determined the spatial differences in expansion. These authors used remote sensing data to track the evolution of land use and noted that early public transport infrastructure increased expansion. Handy [18
] provided an overview of the theory of smart growth and made several specific propositions regarding the relationships between transportation and land use: (1) building more highways contributes to more sprawl, (2) building more highways leads to more driving, (3) investing in light rail transit systems increases densities, and (4) adopting new urbanism design strategies reduces automobile use. Additionally, the use of road networks for the promotion of urban land expansion was emphasized. Tayyebi, et al. [19
] proposed an urban growth boundary model (UGBM) that uses artificial neural networks (ANNs), geospatial information systems, and RS to simulate the complex geometry of the urban boundaries of Tehran, Iran. ANNs were used to train on “roads” as the first predictor variables of the urban boundary geometry in Tehran. Alsharif and Pradhan [13
] analyzed urban sprawl in the metropolitan city of Tripoli by using a logistic regression model and found that roads were an important driving force in the region. Thus, road networks are an important driving factor of urban expansion [20
]. However, studies investigating the specific relationship between road networks and urban expansion are uncommon.
In contrast, the effect of roads on land use is one of the most active research areas in road ecology and landscape studies [23
]. Important results have been obtained by research on road networks, and scholars have focused on the ecological effects of road networks [26
], design and optimization [27
], the response of road networks to urbanization [24
], the evolution of road networks [28
], and other aspects. Research on road networks and urbanization has increased in recent years because the world is experiencing a period of rapid urbanization [29
]. Rui and Ban [30
] examined the relationship between different street centralities and land-use types in Stockholm and found that the density of each street centrality was highly correlated with one type of land use. To study the evolution and complexity of urban street networks, Mohajeri and Gudmundsson [31
] introduced methods for quantifying the geometric characteristics of street networks and analyzed the details of the evolution of the networks of Sheffield (U.K.), Khorramabad (Iran), and Kerman (Iran) over tens to hundreds of years. Their results suggested that the spatial distribution of streets was strongly affected by the surrounding landscapes but in different ways. Lerman, et al. [32
] used spatial syntax to model pedestrian movement in urban transportation planning; their study showed that the distribution of pedestrian movement could be explained by the spatial variables that represent the properties of a street network. In brief, road network and urbanization studies have attracted increasing attention. However, the results of most of these studies are lacking in terms of the quantitative relationships between urban road networks and urbanization factors at the holistic urban scale; thus, the effectiveness of roads and their role in urbanization remains unclear.
In summary, few quantitative studies have examined the correlation between urban land expansion and road networks, and the existing results do not effectively guide practical work related to land use. Previous studies of urban expansion mainly found that road networks are one of the driving forces of urban expansion, but related studies from the perspective of road networks are lacking. Furthermore, the driving mechanism of road network density is unclear. Although a turning point and critical value of the urban expansion cycle, which are driven by the road network density, may exist, these concrete values have not been measured. The lack of quantitative research and the abundance of studies that only examined their qualitative relationship have created limitations in terms of land-use planning, decision making, and other processes.
To address these issues, this study aims to reveal the quantitative relationship between road networks and urban land expansion by establishing a quantitative model of the relationship between road networks and urban expansion in Beijing, New York, Chicago, and London. In this paper, the term “urban expansion” refers to urban land space expansion. We assume that the urban population growth, tax structure, policy, and other driving factors of urban expansion maintain relatively steady growth and do not fluctuate considerably during the study period. The objectives of this study are (1) to summarize the spatial distribution characteristics of the expansion of these four cities over nearly 30 years and the classification signatures of the urban road networks in 2014 based on RS and GIS; (2) to analyze the spatial coupling relationship between urban expansion and the road network density using spatial statistics; and (3) to model the quantitative relationship between the road network density and urban expansion using regression analysis methods to compute the eigenvalues of urban expansion. Our study is designed to help urban planners and decision-makers better understand the quantitative relationship between road networks and urban land expansion and make scientifically sound decisions for future urban development, particularly in developing countries.
The remainder of this study is organized as follows. Section 2
describes the study areas and data sources and introduces the methodology and data processing. Section 3
analyzes the results of the spatial and temporal characteristics of urban land expansion, the road network density characteristics of urban expansion, and the fitting analysis of the road network density and urban expansion. Section 4
presents the discussion, and conclusions are provided in Section 5
4.1. Spatial Coupling Relationship between Urban Expansion and Road Network Density
Over the last three decades, the trends in urban land expansion have been similar in three of the studied cities. The area of urban land expansion has gradually decreased from the center outward, while the distribution of the road network density has gradually decreased outward from the city center. A close association existed between urban land expansion and the spatial density of the road network. Some differences existed in the spatial distribution of urban expansion because of the different locations. For example, New York City, which is a port city, exhibited higher road densities farther from the sea, and urban land expansion has been distributed in coastal areas for nearly three decades. The distribution of the road network density in Beijing was higher to the southeast and lower to the northwest, tending to decrease from traditional areas to the areas surrounding the older towns. Urban expansion in Beijing has occurred from the traditional core of the city over the past three decades, and the surrounding areas were affected to various extents by spatial radiation from the center of the city.
In spatial planning or land use planning, developed countries in Europe and the Americas require a higher level of urban land facility construction. In these countries, infrastructure often pioneers urban land expansion [41
]. Local governments and related departments encourage and guide the urban construction land shafts developed along both sides of the road to take advantage of existing infrastructure facilities, which is legally mandated in some areas. However, the fundamental demand for transport has always been a primary consideration for increasing commuters [43
]. The attraction of urban transit promotes new construction in developed areas, especially in China’s largest cities. For example, the subway in Beijing City affects the housing and rental prices in the suburbs. Therefore, the construction of a road network is the driving factor of urban land expansion, and urban land expansion is the outstanding response to this network.
Through our analysis, the road network density distribution reflects the basic trends of urban land expansion. The expanded area was centrally associated with road density grades of 2–4 or 2–3. Differences in the response patterns existed during different stages of urban development or in different areas.
(1) In high-density road network areas and urban construction areas beyond the expansion threshold, i.e., urban centers with almost no available land, the high cost of construction, overloaded transportation systems, and air and noise pollution [44
] that result from poor housing and industrial and ecological factors [45
] encouraged more businesses and residents to move to the suburbs for convenient transportation, lower costs, and a better environment.
(2) Areas with low-density urban roads, i.e., areas under the inflection point where urban construction and development are at low levels, which are often urban fringe areas or adjacent areas, should be managed for spatial planning with a focus on urban development. To protect ecological land, we should focus on ensuring compact construction, preventing sprawl and illegal construction, and promoting compact urban development [46
]. Because these regions are different from older regions and are approaching the saturation point, the vast development potential makes planning an important function, especially considering the need for the strict rational use of open space.
(3) In areas past the turning point and regions in which expansion is slowing, e.g., urban intermediate zones, we should focus on monitoring plan implementation. The transition zone or the middle of an urban area is a concentrated area of neighborhoods, and we can improve land-use efficiency and reduce urban land pressure by rationally increasing plot ratios, better developing three-dimensional space, and expanding floor space, among other approaches.
4.2. Turning Points of Urban Land Expansion
A fitting study was performed to analyze the relationships between these urban road networks and urban land expansion. When the road network density was less than the threshold value, the rate of urban sprawl and the density of the road network exhibited an inverted U-shaped relationship. The turning point is when the trend changes so that the rate of urban expansion increases with increasing road network density. When the road network density does not exceed the turning point, the rate of urban land expansion exhibits a gradual upward trend; however, after exceeding the turning point, the rate of urban land expansion decreases.
Relative to the already stabilized urban expansion in cities such as New York, London, and Chicago, the turning point in Beijing was relatively low. New York, London, and Chicago, which were chosen as mature samples of global cities, placed more emphasis on the improvement of city quality, while Beijing is in a stage of rapid urban development. Its diverse needs promote the continual expansion of the urban area. Furthermore, several significant differences exist between Beijing and the other three cities in terms of urban spatial planning and the architectural styles of the neighborhoods [47
]. For example, a large number of gated communities were constructed in Beijing’s central area. Additionally, the main roads are generally wide and branching roads are relatively thin. Beijing’s road network density is expected to further increase because the concept of “narrow the roads, densify the road networks, open the blocks” has been supported for urban development in China.
The turning point for the exploration stage when using the Beijing road network of 1988 to model the road network and urban expansion from 1984–2014 was calculated as approximately 2.3 km/km2. The Beijing map of 1988, which covered the current main urban area, was downloaded from the University of Texas Libraries at the University of Texas at Austin. When using the same spatial scale, the turning point was 6.25 km/km2 as calculated by the model for the road network of 2014. Thus, the turning point of Beijing’s urban expansion has been greatly improved after more than 20 years of development.
With respect to the results in New York, London, and Chicago, the fitting accuracy in Beijing was lower because of a larger proportion of low-density areas, which mostly consist of urban fringe areas and are irregular in terms of land use, reducing the overall simulation accuracy.
The turning point is the most important feature of an urban land expansion-road network density model with relative stability. In addition, the road network density threshold is an important indication of the urban development level, particularly in developing countries. According to the turning point, a city’s development can be divided into different stages in terms of temporal and spatial development to develop relevant spatial planning based on local conditions, which could help prevent poor investment and construction in terms of urban infrastructure and improve the rationality of urban spatial planning. Thus, related studies can provide important references regarding the utility of urban expansion. To avoid the rapid expansion of urban land, rational traffic planning is an effective means of achieving the mutual promotion and coordinated development of urban land use and road networks.
4.3. Limitations and Future Perspectives
This study focused on the antecedence of road networks for driving urban expansion. The turning points and the threshold values were calculated through research on the spatial quantitative relationship between urban expansion and the road network density. A large number of theoretical and empirical projects confirmed that road networks and urban expansion are closely linked. Road networks have an inducement effect on urban expansion, that is, the distribution and speed of urban expansion. The quantitative indicators for road networks are rich and mature, which is convenient for our research. Moreover, the quantitative relationship between road networks and urban expansion can facilitate the rational utilization of urban land. The existing literature on urban expansion and road networks lacks quantitative analysis of the relationship between these factors. The results of this paper enrich the research results of urban expansion theories and methods.
However, a few limitations must be addressed in future studies. For example, this paper considered the urban spatial expansion of a single city. The presence of single or multiple urban centers can change urban land use in a range of single cities. Additionally, some suburban cities are located outside these megacities, and sprawl is likely to continue. On the other hand, this study only used one period of urban road network data. In further research, we will study the effect of the evolution of road networks on urban expansion and compare the multiple-scale quantitative relationship between road networks and urban expansion, such as single cities and metropolitan areas, to examine additional indicators of urban expansion trends by using the urban land expansion-road network density model.
In this paper, we used Beijing, New York, London, and Chicago as the study areas and comprehensively utilized the RS and GIS platforms to analyze the general characteristics of urban expansion over the past three decades based on image interpretation, information extraction, and spatial overlay analysis on urban land use. Furthermore, based on the road network data, the distribution characteristics of the urban road network density were summarized using the kernel density analysis tool. Finally, a road network-urban expansion model was constructed using regression analysis to determine the quantitative relationship between the road networks and urban expansion. In addition, the turning point for urban expansion was determined and further investigated.
The turning points of urban land expansion in Beijing, New York, London, and Chicago were 3.3 km/km2, 11.84 km/km2, 16.86 km/km2, and 21.40 km/km2, respectively, and the road network density threshold values were 18.9 km/km2, 37.8 km/km2, 57.0 km/km2, and 64.7 km/km2, respectively. When the road network density was less than the threshold value, the rate of urban sprawl and the density of the road network exhibited an inverted U-shaped relationship.
Road networks are an important foundation for urban development, and these features affect urban expansion. On the one hand, urban development requires efficient road networks for support. On the other hand, improving road networks plays a key role in affecting urban land use and guiding changes to the urban landscape. This paper demonstrated that the road network-urban expansion model plays an important role in predicting and controlling the direction and scale of urban expansion by studying the associated relationship. Urban expansion varies based on the degree of road-network development in different locations, and the observed distributions are conducive to scientific planning and rational land use in these cities and to improving land use efficiency. In summary, the quantitative relationship between road networks and urban expansion exhibited an inverted U-shaped pattern. The turning point of urban expansion is worth investigating in other cities that are experiencing rapid development to promote sustainable development and achieve smart growth in the future.