Does bus bunching happen inevitably: The counteraction between link and stop headway deviations?

• Bus bunching occurs much fewer in observations than in conventional models. • Counteraction between link and stop headway deviations can resist bus bunching. • Superposition between link and stop headway deviations destroys headway regularity. • Counteraction is more prominent than superposition with buses moving forward. • The counteraction effect owes to drivers’ reactions to bus travel time deviations. Bus headway deviations are the major causes of bus bunching, which is detrimental to bus service reliability and leads to excessive waiting time for passengers. Generally, it is believed that bus bunching is an inevitable result once a service disturbance occurs, as long as the route is long enough. However, in this study, it goes as low as 5% of all trips from the empirical data, contradicting the expectation in theory. This study explores the underlying reasons for this contradiction based on empirical data from two representative bus routes in China. The results show that headway deviations on links and at stops mostly counteract with each other, constraining the occurrence of bus bunching. Under this counteraction effect, the path headway deviations remain within a stable range, normally bounded by the planned headway. It indicates that the bus system has a self-repair ability to resist headway disruptions, which explains why bus bunching happens at a considerably lower frequency in practice than in theory. We further discuss how the counteraction effect works. The insights gained from this study can shed light on preventing bus bunching and generating new ways to recover bus service regularity after a headway disruption. If we can adequately adjust the control methods to make the link and stop headway deviations counteract with each other, the bus system can automatically mitigate the headway deviations.