## 5 Results and Analysis
### Mechanism of the Nonlinear Effect of Built Environment Factors on Ridership
#### 5.4.4 Floor Area Ratio  
Figure 11a-c show the partial dependence of building volume ratio on total passenger flow and inbound and outbound passenger flow in the morning peak on weekdays, and it can be seen that the building volume ratio has a nonlinear effect on them.  
It can be seen that the floor area ratio has a nonlinear positive relationship on both total passenger flow and outbound passenger flow and a U-shaped nonlinear relationship on inbound passenger flow. For both total and outbound passenger flow, the total passenger flow saturates and peaks after the building volume ratio reaches 3 (as shown in Figure 8a). This means that, as the building density (i.e., floor area ratio) increases, the number of people using the underground also increases, but after the floor area ratio reaches 3, this growth trend begins to level off and peaks. This indicates that, as buildings become taller and denser, more people tend to choose the metro as their mode of travel, but this effect is no longer significantly stronger after a floor area ratio of 3.  
Figure 10: The nonlinear influence of road network density on total ridership and inbound and outbound ridership. (**a**) On total ridership; (**b**) on inbound ridership; (**c**) on outbound ridership.  
The inbound passenger flow decreases and then increases, with the floor area ratio between 1.5 and 2.5. This means that, at a floor area ratio of 1.5, a certain number of people choose the metro for travelling in relatively low building densities; however, when the floor area ratio increases to 2, the inbound passenger flow decreases to the lowest point, which may be due to the fact that medium-density building areas may be inaccessible, congested, or have other factors that lead to a lesser tendency to choose the metro as a mode of travelling. As the floor area ratio of 2 gradually increases to 2.5, the inbound traffic begins to increase to a maximum value and then drops to zero. This suggests that medium-density building areas are less conducive to attracting people to use the metro, possibly because Shenzhen is a working city and there may be traffic congestion or other inconveniences in these areas, whereas very high or low-density building areas are more conducive to the gathering of metro commuters.  
Therefore, in high-density (FAR > 2.5) built-up areas, the frequency and capacity of metro services should continue to be strengthened, and pedestrian and cycling connections should be optimised to ensure the convenience of the 'last kilometre' and to further enhance the metro's status as a major mode of travel. Meanwhile, in medium-density (\(1.5\leq\text{FAR}\leq 2.5\)) built-up areas, the focus should be on solving traffic congestion problems, for example, by improving road design, increasing public transport routes, or introducing intelligent traffic management systems to alleviate traffic pressure, as well as promoting flexible working hours and telecommuting to reduce the need for commuting during peak hours. In addition, taking into account the phenomenon that very high or low-density building areas are more conducive to the gathering of metro commuters, new development projects should pay more attention to a balanced distribution, avoiding the over-concentration of high-rise buildings, so as to achieve an optimal match between the spatial layout of the city and the traffic-carrying capacity and to promote the sustainable development of the overall urban transport system.  
Figure 11: The nonlinear influence of floor area ratio (FAR) on total ridership and inbound and outbound ridership. (**a**) On total ridership; (**b**) on inbound ridership; (**c**) on outbound ridership.