# How Does the Built Environment Affect Mechanical Parking Space Planning: A Case Study in Xi'an City
## 6 Discussion and Conclusions  
The non-linear relationship between the mechanical parking spaces ratio (MPS ratio) and the built environment is constructed by using the GBDT model. Moreover, the ALE plot is used to simulate the influence of variables on the mechanical parking spaces ratio and to determine the influence threshold. In addition, the SHAP value is used to reveal the local feature of built environment variables on the MPS ratio. The local feature means the influence of variables on a sample which is a buffer for a parking lot. Due to the different spatial locations of the different buffers, the built environment around them is different, which makes the effect of the factors on the MPS ratio show different effects. Based on these studies, some valuable insights are provided for the planning of mechanical parking lots  
Figure 6: Contribution of different variables obtained from SHAP values over space: (**a**) distance to CBD; (**b**) land use mix; (**c**) architecture type; (**d**) population density; (**e**) bus stop density; (**f**) fee. and spaces. The results make three contributions to the parking analysis. First, the results reveal the non-linear influence of built environment variables on the mechanical parking spaces ratio. Second, the relative importance of variables on the mechanical parking spaces ratio helps planners make the mechanical parking lots and spaces planning. Third, the use of SHAP values to describe the spatially local impacts of the built environment can reveal the spatial variability of variable impacts. From Figure 2, the land use mix and the distance to CBD play an important role in the planning of mechanical parking spaces and lots. The construction and maintenance costs of the mechanical parking spaces are higher than those of conventional parking spaces. Therefore, the mechanical parking spaces or parking lots in the necessary locations can not only meet the parking demand but also save costs. In this study, it is more necessary to build mechanical parking spaces or parking lots in an area where the land use mix is higher than 0.83 and the distance from the city center is less than 7 km. However, according to the local effect diagram in Figure 6, the construction of mechanical parking spaces or parking lots can also be considered for areas where the variables have a positive effect. For building types, the impact of the construction of mechanical parking spaces or parking lots can be considered when planning parking lots for building types with a higher impact, such as hotels with higher impact. According to the local effect diagram in Figure 6, the variables in some areas have a positive impact on the mechanical parking spaces ratio; therefore, it is necessary to build a mechanic parking space for the area with a large value of the variable. The variables in some areas have a negative effect on the mechanical parking spaces ratio, which requires the construction of mechanical parking spaces for areas where the built environment variables have lower values. According to the results of the study, shown in Figure 6a, it can be seen that the positive effect of the variable distance to CBD on the construction of mechanical parking is mainly evident in the city center as well as in the periphery. Thus, the closer to the city center, the more mechanical parking spaces are needed to meet the parking demand. However, the ALE graph shows that, although the distance to the CBD has the second highest impact, the impact trend is not very large. This conclusion also reflects that the CBD does not advocate a lot of parking lots, as this will increase parking demand and thus traffic congestion in the city center area. At the same time, the periphery can be a park-and-ride site and provide a reference for the installation of park-and-ride sites.