Causal effects between bus revenue vehicle-kilometers and bus ridership

To create a sustainable urban transportation environment, transportation authorities often implement incentive strategies to attract new public transportation users. Various studies have shown that increasing bus revenue vehicle-kilometers (BK) is a key factor for increasing bus ridership (BR). In this situation, BK affects BR. However, an increase in BR may lead to an increase in BK. Then, BR affects BK. The existence and directions of causal effects between BK and BR are the research questions addressed in this study. Since 2007, the Taichung City Government has been adding and adjusting bus routes to build a dense bus network. In 2011, it implemented a free fare bus policy. BR grew from ca. 2 million per month in 2007 to 10 million per month in 2015. Moreover, BK increased by ca. 3 times during that same period. This provides an opportunity to understand the causal effects between BK and BR. Extended convergent cross mapping (ECCM) can be used to assess the causal relationship between two variables without modelling. We adopted this method to distinguish the influences of BK and BR. According to the results, increasing BK attracted more public transit passengers during the period 2007 to 2010. However, there was no effect of BR on BK. After implementation of free fare bus policy in 2011, increases in BK led to increases in BR and increases in BR drove incremental growth of BK. The result was a virtuous cycle of mutual influence between BK and BR. In conclusion, increasing BK, such as route length or number of scheduled runs, can promote the usage of public transportation. Moreover, authorities can implement free fare bus policy to attract potential users and force incremental growth of BK.