Introduction

To achieve high satisfaction indicators, a public transport system needs to provide a high level of service to its users. A high level of service not only requires that users experience low waiting times, fast travel times, and a minimum comfort standard, but also that this service is reliable – that is, it does not change significantly from day to day. These expectations explain some of the success of heavy rail systems around the world. However, heavy rail has a high infrastructure cost, ranging between US$70 and US$350 million per kilometre (Wright and Hook 2007). This cost makes rail an unattractive alternative for corridors with passenger demand under 20,000 passengers per direction per hour, and makes it very often too expensive for most developing countries. These are precisely the countries where demand for public transport is high.

To overcome these problems, Bus Rapid Transit (BRT) has arisen as an alternative to rail potentially offering the same level of service at a reduced cost. BRT bases most of its high level of service on rapidness. The Rapid in BRT stands for both the speed of the buses and their frequency. High frequencies create shorter trip times and higher capacity. This reduces waiting times and improves comfort, two critical elements of the level of service perceived by users. In the developed world, low frequency services (fewer than six buses per hour) are often operated with schedules. Under this scheme, passengers may experience not just a low wait time, but a reliable service. However, operating a medium or high frequency service on a timetable is not only challenging, but also inefficient since cycle times would be increased by extra travel times needed for buses to reach control points on schedule. Thus, most BRT services commit to an average frequency, not a schedule. Even though segregated lanes help isolate bus operations from general traffic, BRTs operation is still affected by traffic signals and demand fluctuations. It is well known that such perturbations affect the regularity of bus headways, inducing what is called bus bunching, where buses travel together. This phenomenon happens because buses trailing a longer than average headway get more loaded than usual, and therefore run more slowly than the average bus.