The most relevant issues related to headway variability in public transport planning, operations and quality of service are reviewed in this paper. We discuss the causes and consequences of headway variability, the alternative metrics that have been proposed to measure it, the preventive and reactive strategies to control headway variability in both research and practice, including the role of drivers and of present and future technology, and how service provision contracts might deal with headway variability through metrics and financial incentives. The most influential elements that explain headway variability along a route are the irregularity at which vehicles are dispatched, the scheduled frequency, the distance travelled or route length, the passenger demand and associated dwell times, and the number of stops. We conclude that there is a large gap between the state-of-the-art and the state-of-practice in terms of identification of headway variability issues, as well as in the development of mitigation and control measures. It is therefore paramount that future research will contribute to closing this gap by addressing organisational, contractual and technological barriers in the implementation of measures aimed at mitigating headway variability in public transport services.

Buses have an inherent tendency to bunch due to randomness in passenger demand and congestion. Many sophisticated control strategies have been developed to reduce bus bunching, however, few of them have been implemented in high frequency real services. Building upon a control strategy comprised of a rolling horizon mathematical programming model that yields the optimal holding time that minimizes user-waiting times6, we have developed real-time software and implemented it on two bus services in Santiago using different technologies to communicate the instructions to bus drivers. The results presented in this paper are encouraging, on the days the system was implemented less bus bunching was observed, which translated in fewer headway-irregularity fines. Moreover lower passenger fare evasion was observed when the bus control strategy was used