## 5 Results and Discussion
### Nonlinear Effect between Built Environment and Bus-Metro-Transfer Ridership
Figure 6c depicts the positive impact of commercial ratio on the bus-metro-transfer ridership. When the ratio is less than 0.1, it has a trivial effect on transfer ridership. When the commercial ratio increases from 0.1 to 0.22, transfer ridership keeps increasing. Transfer ridership remains stable, when the commercial ratio reaches the threshold. For urban planners, these trends can be considered to maximize bus-metro-transfer ridership. Figure 7a,b show the nonlinear effects of street density and land-use diversity on bus-metro-transfer ridership. Figure 7a shows that the increase in street density has a positive impact on transfer ridership, when the street density is between 6 km and 11 km per square kilometer. Specifically, when the street density is between 6 km and 8 km per square kilometer, the climbing rate reaches the maximum. Similarly, other studies obtained the positive influence of the road density and public transport services on bus and metro [7,9,32]. Figure 7b shows that there is a significant nonlinear effect of land-use diversity on bus-metro-transfer ridership. When land-use diversity is above 0.7, transfer ridership increases rapidly to the peak. The trend becomes stable, when the threshold is reached. Figure 6: The effects of land-use attribute variables for trip attraction on transfer ridership. (**a**) Employment density (thousand/sq. km); (**b**) industrial ratio; (**c**) commercial ratio. Interaction Effects of Bus-Metro Connection Characteristics and Closeness Centrality on Transfer Ridership  
This study, further, investigates the interaction impact of bus-metro connection characteristics and closeness centrality on bus-metro-transfer ridership. Figure 8 demonstrates that the interaction influence of bus-network density and closeness centrality on transfer ridership is multivariate and nonlinear. As shown in Figure 8, when bus-network density ranges from 12 km to 14 km per square kilometer, and closeness centrality is more than 0.1, transfer ridership keeps sharply increasing. The transfer-ridership growth peaks at closeness centrality of 0.2 and bus-network density of 14 km per square kilometer. Figure 9 depicts the interaction effects of bus-stop density and closeness centrality on transfer ridership. The result indicates that when the density of bus stops is 6 to 8 per square kilometer, and closeness centrality is between 0.18 and 0.2, we can get the maximum transfer ridership. This finding can help planners to reasonably arrange the number of bus stops, according to the location attribute of metro stations. Figure 10 illustrates the combined effects of the network repetition ratio of bus-metro and closeness centrality on transfer ridership. We found that when the repetition ratio ranges from 0.2 to 0.3, closeness centrality is more than 0.16, and transfer ridership remains a high value, respectively. The finding can provide significant suggestions for the commercial planners of transportation network companies. Figure 8: The interaction influence of bus-network density and closeness centrality on transfer ridership. Figure 7: The effects of other land-use attributes on transfer ridership. (**a**) Street density (km/sq. km); (**b**) land-use diversity. Figure 11 describes the influence of average transfer distance and closeness centrality on transfer ridership. The result shows that when the average transfer distance grows from 0.2 km to 0.25 km, and closeness centrality is more than 0.17, transfer ridership keeps stable growth. Transfer ridership reaches the peaks at the station, with a closeness centrality of 0.19. The above results indicate that the setting of a bus-metro connection should be considered, together with the network attribute of the metro stations. This result implies that the bus-metro connection characteristics around stations depend on the stations' positions. Figure 10: The interaction influence of network repetition ratio of bus–metro and closeness centrality on transfer ridership.