Agglomeration And Dispersion Evolution Model And Algorithm Research For Spatial Structure Of Urban Logistics Network

In recent decades, through experiences and researches, two characteristics of urban logistics activities and facilities which are polarization and sprawl, have been proved to be very important and significant. The shift from concentration to deconcentration or suburbanization for them is also very significant. The spatial distribution and combination relation of logistics activities and(or) facilities could be referred as the spatial pattern of urban logistics network. It could be a strong economic support, and of great use for optimizing industrial structure. And also it could act on the the development of city’s spatial structure. It will be of great importance for the design and optimization of urban logistics network, or the scientific and reasonable layout of logistics facilities in the city, if we could research the evolving mechanism and laws for the agglomeration and sprawl evolution of urban logistics network’s spatial pattern indepth, and build evolution models on the basis of abstraction of evolving mechanism and influence factors, to simulate and analyze the evolution process and trend quantitively. Thererore, major research contents in this paper include the following four aspects.To research the evolution mechanisms, this paper built a framework based on agglomeration economy theory and complex network theory. The following conclusions have been drawn. Agglomeration and dispersion are two basic and major evolution formation of urban logistics network spatial structure. They obey the law of the unity and opposites. The basic economy motive force driving the agglomeration evolution and dispersion evolution are agglomeration economy, and diseconomy, respectively. Their interactions result in three evolutionary modes, which are agglomeration, agglomeration coexist with dispersion, as well as dispersion. The complexity evolving mechanisms could be concluded as the preferential mechanism, the growth and updating mechanism, and the mutation mechanism. Moreover, some characteristic parameters in complex network theory and some other index could be used to describe and analyze the agglomeration dispersion characteristics. According to changing rules, the complexity evolution process could be divided into four stages, the initial stage, the agglomerated stage, the sprawl stage, and the advanced network stage.In order to study the evolutionary mechanisms of spatial structure of urban logistics network, a weighted local world evolution model was proposed based on the agglomeration degree, considering the macro factors and the operating characteristics. By simulation analysis, the network features were researched, and agglomeration degree parameters’impacts on the evolution which changed over time was also analyzed. The results reveal that the network’s degree distribution doesn’t obey the power law distribution, and it has higher clustering coefficient and lesser average path length, which shows a few hub nodes predominate in the urban logistics network and the network has strong anti-risk ability and quick response ability. The results also indicate that when the weight factors are changed over time, the evolution mechanism is more realistic, because it could overcome excessive influence of the node degree and reflect impact of all the related macro factors reasonably.In the agglomeration evolution, economic machanism is critical and distance is very important. Taken them as driving forces, this paper proposed an evolution model attempting to mimic real urban logistics network’s growth. Numerical simulations reveals that the distributions of degree, strength and edge weight all show a good power law behavior, revealing the uneven distribution of volume in the network. And nonlinear correlation between the strength and degree indicates that rich get richer. Besides, the variation tendency of un-balanced distribution coefficients for logistics volume and spatial distribution of logictics nodes, both reveal that the agglomeration evlution process of spatial pattern of urban logistics network is a complex and dynamic evolving route.Through the analysis for the essence of sprawl evolution, we regard it as a diffusion problem in complex networks. Thus, a threshold model was constructed to study the dispersion evlution. This model also take the minimum cost as the evolving objective. The effction of ecomical mechanism and limit of geographical distance are taken as evolving forces. Two situations are considerd:the node is for transition, and the node is an original point or a destionation. Algorithm was also designed, with the prupose of simulation analysis. The basic macro characteristics were discussed. Meanwhile, the utility change before and after dispersion evluiton, as well as the concentration and deconcentration characters were also been analyzed.To study the agglomeration and dispersion evolution mechanisms, process and characteristics of urban logistics space pattern, also to reveal the dynamic mechanism of the evolution driven forces, insight into evolution forces was provided first, taken the effect of economic benefit target of urban logistics activity into consideration. Then, an extended and weighted BA model was established, based on the complex properties of urban logistics network spatial structure. Simulation analysis proves that the constructed model could uncover the agglomeration and dispersion evolution process and characteristics. Research conclusions are as follows. Agglomeration economy is the driving force of agglomeration evolution, namely, agglomeration economy urges the concentration (or unbalance)of urban logistics spatial pattern. Agglomeration diseconomy is the driving power for the dispersion (or deconcentration, spawl) and balance of urban logistics spatial pattern. The concentrative degree or the unbalance of urban logistics spatial structure weakens along with the reduction of agglomeration economy influence. In the evolving process, the effect of agglomeration economy is more obvious. The logistics nodes could reflect the polarization-spawl evolution characteristics better than the logistics channels.Aside from the above, a case study was used to verify the feasibility and validity of models. Simulaitons for Chegndu’s spatial structure evolution of logistics network between 1990s and 2003 and between 2003-2009 were carried out to validate the feasibility of agglomeration medel, and dispersion model, respectively. Evolution Simulution of spatial pattern of Chengdu’s logistics network was also conducted to reveal the evolution raw and mechanism. All the results of simulation prove that models are feasible and effective.