## 5 Evolution of Passenger and Freight Transportation Network  
The evolution of the urban passenger flow network in each year is shown in Figure 3. In 2000, the second highest value areas were located in Chengdu and Chongqing, while the higher value regions were located in Sichuan province, Xi'an, Guiyang, Zunyi, and other cities, forming a passenger flow network structure dominated by Southwest China. In 2006, areas like Luzhou, Nanchong, Guang'an, Guiyang, Qiannan, and Qiandongnan had stronger passenger flow network correlation strength, and the sub-high value region gradually expanded. Meanwhile, Guizhou and Guangxi province had stronger overall correlation strength and gradually formed a spatial pattern of passenger flow network with Chengdu-Chongqing urban agglomeration as the center and Xi'an and Guiyang as the sub-centers. In 2012, the correlation strength of Chengdu, Meishan, Luzhou, Nanchong, Guang'an, Chongqing, Xi'an, Zunyi, and other regions further enhanced; located in the high-value area, the second-highest value area migrated to Chengdu-Chongqing urban agglomeration, Central Guizhou urban agglomeration, Zhanjiang, and Haikou. The higher-value area gradually expanded, extending to Guanzhong urban agglomeration. In 2018, the main high value area was located in Chengdu, Chongqing, Guiyang, and the second-highest value was located in Chengdu-Chongqing urban agglomeration, Central Guizhou urban agglomeration, and Xi'an. A higher value is moving to Yunnan; further models show a passenger flow network spatial pattern with Chengdu, Chongqing, and Guiyang as the main centers and Chengdu-Chongqing urban agglomeration, Central Guizhou urban agglomeration, and Guanzhong urban agglomeration as the sub centers. What forces are driving the evolution of this passenger network? The development of the passenger flow network is inseparable from economic, political, population growth, and other factors. It is these factors that gradually promote the development of urban agglomeration, and the development of urban agglomeration further affects the population flow. In order to transfer the surplus economic development capacity of the eastern coastal areas to the western region, so as to improve its economic development level, the State Council put forward the western development policy in 2000. During this period, most of them focused on the basic needs of the people and were in the stage of infrastructure construction. The gross domestic product (GDP) and resident population of Chengdu and Chongqing were in the forefront. In 2006, the State Council, through the Development of the West \"eleven-five\" plan, sped up the economic development of the western region. At this time, the western region was in a slow growth stage, and the GDP of Chongqing and Chengdu were still in first and second place. However, the resident population of Chongqing decreased, the resident population of Chengdu increased, and the GDP of Xi'an and Guiyang increased significantly. In 2012, the State Council approved the \"12 th Five Year Plan\" for the development of the western region, which made overall planning for the spatial development pattern of the western region, and it supported 11 key economic zones such as Chengdu Chongqing Economic Zone, Guanzhong Tianshui, and Guangxi Beibu Gulf to take the lead in development and 8 resource rich zones such as Panxi Liupanshui to develop intensively. The development of the border open areas was accelerated. At this time, the economy of the western region was in a rapid growth stage. The proposal of the  
Figure 3: Evolution of passenger transport network, (**a**) Passenger network in 2000, (**b**) Passenger network in 2006, (**c**) Passenger network in 2012, (**d**) Passenger network in 2018. \"the Belt and Road\" initiative in 2013 has brought new opportunities for the development of the western region.