## 5 Results and Discussion
### Nonlinear Effect between Built Environment and Bus-Metro-Transfer Ridership
Within the closely connected range of the bus network and the metro network, the bus can transport people, in areas that cannot be radiated by railways, to metro stations. Figure 4: The effects of bus–metro connection characteristics on transfer ridership. (**a**) Bus-network density (km/ sq. km); (**b**) average transfer distance (km); (**c**) network repetition ratio of bus–metro; (**d**) bus-stop density (counts/sq. km). Figure 4d displays the significant nonlinear association between bus-stop density and bus-metro-transfer ridership. When the bus-stop density is 2 counts to 7.5 counts per square kilometer, the bus-stop density has a positive influence on the transfer ridership. For each additional station within this interval, an average of 116 transfer passengers will be added. When the bus-stop density exceeds 5 per square kilometer, the transfer ridership keeps decreasing with the increase in bus-stop density. It has almost no effect on the bus-metro-transfer ridership, when the bus-stop density increases to 11 per square kilometer, and the transfer ridership will tend to be stable. Figure 5 depicts the marginal impact of network attributes, based on metro stations on transfer ridership. Figure 5a suggests that closeness centrality has a positive influence on the bus-metro-transfer ridership. When closeness centrality is less than 0.15, there is almost no change in transfer ridership. By contrast, when closeness centrality increases from 0.15 to 0.18, transfer ridership increases, by about 5000 passengers. Once it is above 0.18, the corresponding transfer ridership slightly decreases. This result is reasonable. The stations of closeness centrality of more than 0.18 are central stations. These central stations are the travel destination of most passengers. In other words, only within a certain interval of closeness centrality does it have a significant impact on bus-metro-transfer ridership. In comparison with traditional studies on bus-metro-transfer ridership, this result is not fully explored. Figure 5b demonstrates that the distance to the central station has a positive impact on transfer ridership. The farther away from the central station, the more bus-metro-transfer ridership there is. Due to the punctual characteristic and large capacity of the Shanghai Metro, many passengers work in the city center and live in the suburbs. During morning peak hours, passengers commute by the combined mode of bus and metro. Figure 6 illustrates the nonlinear influence, of the four independent land-use attribute variables for trip attraction, on the transfer ridership. Figure 6a shows the threshold effect of employment density on transfer ridership. When the employment density is less than the threshold, it has a positive impact on transfer ridership, which is consistent with many previous studies [9; 57]. When the employment density is above the threshold, increasing the number of jobs has almost no impact on transfer ridership. The result shows that a reasonable distribution of jobs around metro stations will improve the attractiveness of bus-metro transfer, but over-densification may lead to traffic chaos and overload, thus reducing people's willingness to travel by public transport [29]. Figure 5: The effects of network attributes of metro stations on transfer ridership. (**a**) Closeness centrality; (**b**) distance to the central station (km). Figure 6b,c illustrate the nonlinear effects of the industrial ratio and the commercial ratio on the bus-metro-transfer ridership. As shown in Figure 6b, the industrial ratio has a positive impact on transfer ridership. When it grows from 0.05 to 0.09, transfer ridership increases by about 205 passengers, in an almost linear way. By contrast, when the industrial ratio is above 0.2, the change of industrial ratio has no effect on transfer ridership. However, the over-densification of industry will lead to traffic congestion, which will bring inconvenience for people to travel by public transport.