## 4 Results
### Results of the Evaluation of Sustainable Development Factors
#### 4.2.2 Regional Development Level  
(1) Density of Roads  
An area with high road density is suitable for the development of underground space, and the demand for underground space is stronger. On the one hand, the existence of roads can promote the surrounding economic activities, so road-intensive areas are often accompanied by higher land utilization, reduced available construction space, and an incapacity for further development of the local economy. The development of underground space can provide more development space to meet the growing urban population and commercial needs. On the other hand, the traffic flow on the ground in high-density road areas is large, and traffic congestion occurs frequently. Traffic facilities, such as underground tunnels and parking lots, are planned in the underground space to divert ground traffic, so as to improve the problem of traffic congestion. Based on the Euclidean distance between roads in Nantong City, these plans reflect differences in the suitability of underground space development across different regions. The evaluation results are shown in Figure 15. The road density is relatively high across most areas of Nantong. Among them, the road density is the highest in Chongchuan District and the northwest of Rugao City, and a few areas are not suitable for development. (2) House Prices  
High-price areas indicate that land resources are more valuable, and thus the construction of basic service facilities brings lower economic benefits. The utilization of underground space is not affected by surface land prices, allowing for the transfer of commercial streets and infrastructure to underground spaces. This enables the rational planning of urban space and the formation of an organic urban structure with integrated functions both above and below ground, thereby maximizing economic benefits. Using the Shell Housing search network, housing prices in the Nantong City neighborhood were obtained. Based on these data, the high and low suitability for underground space development was categorized, and the results are shown in Figure 16.  
Figure 14: Suitability distribution based on ecological reserve constraints. (3) GDP per capita  
The impact of GDP per capita on the demand for underground space development is mainly reflected in two aspects: government funding and residents' disposable income, as well as business and infrastructure needs. Residents in areas with high GDP per capita usually have higher disposable incomes, and the government has more funds for infrastructure construction and urban development. Therefore, there will be more funds to invest in the planning, construction and maintenance of underground space. At the same time, a high GDP per capita represents a higher consumption capacity, and there is a greater demand for high-quality commercial and service facilities, making these areas more suitable for the development and construction of underground space. According to the statistical yearbook  
Figure 16: Suitability distribution based on house prices. Figure 15: Suitability distribution based on road density. of Nantong City, the distribution of GDP per capita in each district can be obtained, and the suitability zoning is carried out, as shown in Figure 17.  
(4) Population Density  
Land resources are limited in cities, and high population density areas indicate a high demand for urban infrastructure and land resources, so it is more necessary to carry out rational planning and development of underground space to alleviate land pressure. The population of townships and streets is obtained from the Nantong City Statistical Yearbook, and the population is visualized as density.