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4 Background In the past few decades, many transit agencies in North America and elsewhere have explored operating bus routes by headway instead of schedule. In headway-based operation, the schedule is removed (or at least de-emphasized), and the key objective is to maintain appropriate headways between buses. Agencies may consider HBS for several reasons, including a desire to address operational problems and passenger complaints common in high-frequency bus service, and to reduce agency costs associated with high travel time variability. Headway-based service is best suited for high-frequency bus routes, defined as those with a headway of 10 minutes or less. Such routes commonly experience several operational problems, including schedule adherence, bunching and gapping, and full load capacity. From the passenger perspective, these problems are experienced as late bus arrivals, large gaps in service resulting in long out-of-vehicle waiting times, and denied boardings when buses reach their full load capacity. These problems have a common cause: high-frequency routes are an unstable system, which means that small disturbances can quickly grow into larger problems if left unchecked. The instability of high-frequency routes has been recognized in the transit litera- ture for many years (Welding 1957; Newell and Potts 1964). Headway-based operation has several key differences with respect to schedule-based operation. In schedule-based operation, the main objective is for buses to arrive at points along the route at specific times. Performance is measured by metrics like schedule adherence, defined as whether the bus arrived within a specified time window (e.g., 1 minute early to 3 minutes late). Since the main performance metric is individual, bus drivers are largely able to self-monitor with only occasional support from dispatchers. In headway-based operation, the schedule is removed (or at least de-emphasized), and the key objective is to maintain appro- priate headways between buses. As a result, different performance measures are needed, and several options exist. If there is a scheduled headway, then headway deviation (i.e., the differ- ence between actual and scheduled headway) or headway adherence (e.g., within ±3 minutes of scheduled headway) are possibilities. Otherwise, measures like the coefficient of variation of headway can be used to track the regularity of headways on the route. Since even calculat- ing a headway requires tracking the position of at least two buses, headway-based operation generally demands real-time communication and more active involvement from dispatchers. Driver and dispatcher training has also been recommended, especially when headway-based operation is first introduced, because buy-in from these groups is an important factor in the success of headway-based operation. In the past 50 years, many ITS technologies have been widely adopted by transit agencies. Notably, CAD and AVL, typically implemented together, have improved the capabilities of dispatchers. Other technologies, such as APCs, were originally intended to automate C H A P T E R 1 Introduction
Introduction 5  pencil-and-paper data collection tasks but later found real-time uses. Still, many ITS technolo- gies are not used to their full potential. Transit agencies collect a large amount of data from APC and AVL devices, fareboxes, cameras, and so on but use only a small portion of the data for real- time monitoring and control. Additional uses of these data could improve bus performance at relatively low cost since the technologies are already implemented and the changes are mostly procedural. Objective and Scope The objective of this study was to synthesize the national and peer international state of the practice of headway-based bus service and the proactive use of ITS technologies for monitoring and control. Although ITS technologies have many applications in bus operations, the focus of this study was on applications that directly support the operation of HBS. The tasks conducted as part of this synthesis are described in more detail in the Technical Approach to Synthesis Project section of this chapter. Definition of Terms This report uses many technical terms to describe concepts and components of headway-based bus service. Because there are some variations in the terminology used by researchers and transit agencies, this report uses the following definitions for consistency: ⢠Backward headway: the headway between a bus and the one behind it, expressed in time units. ⢠Bus bunching: a phenomenon in which adjacent buses tend to form pairs; sometimes referred to as bunching and gapping because the phenomenon produces both long and short headways. ⢠Business rule: a policy that determines when to take an action, such as buses being held when the headway deviation exceeds a threshold. ⢠Control points: stops where holding is applied. ⢠Deadheading: a movement to reposition a bus without passengers. ⢠Forward headway: the headway between a bus and the one in front of it, expressed in time units. ⢠HBS: a form of bus operation where the main objective is to maintain the headways between buses. ⢠Layover time: a planned buffer period before a bus resumes service in the opposite direction; may be extended or shortened depending on operational needs. ⢠Parameter: a numerical value used in a control strategy. For example, if the business rule is to hold buses when a headway deviation threshold is exceeded, the parameter is the threshold, such as 3 minutes. ⢠Regularity: a performance measure that evaluates the similarity of headways between all buses on the route. Headways on the route at a specific point in time can be described as regular if they are all approximately the same or irregular if there are significant variations. ⢠Segment skipping: skipping several stops in a row. ⢠Target headway: a specific objective used in operational control; also referred to as the scheduled headway when it is communicated to riders. Technical Approach to Synthesis Project The synthesis was conducted in three main phases: a literature review, a survey of North American transit agencies, and case examples of four transit operators.
6 Intelligent Transportation Systems in Headway-Based Bus Service Literature Review First, a review of relevant literature was conducted to identify previous research on the synthesis topics. The literature review, contained in Chapter 2, has been organized into two main sections: ⢠The first section covers HBS, providing a general introduction, discussing common challenges, and discussing operational strategies that have been proposed by researchers and/or implemented in practice. ⢠The second section covers ITS technologies, first introducing the technologies that are avail- able and relevant to HBS and then describing current uses by transit agencies worldwide. Proposed uses are covered in the Operational Strategies subsection under Headway-Based Service to avoid duplication. Survey of North American Transit Agencies A list was developed of North American transit agencies believed to operate headway-based bus service. Agencies on this list were recruited to participate in an online survey about the use of ITS in their headway-based operations, including the technologies employed, data sources, possible actions, business rules, and performance monitoring. Twenty-four transit agencies received the survey. Sixteen complete responses were received, equaling a 67% response rate. Appendix A includes a flowchart of the survey organization and the complete questionnaire. Table 2 in Chapter 3 provides the full list of surveyed transit agencies and their deployment status. Figure 1 provides a map of those agencies. Figure 1. Map of transit agencies that responded to the survey, with status of headway-based service.
Introduction 7  The survey was divided into seven sections: ⢠Introduction and screener questions. ⢠Questions for transit agencies that do not implement HBS. ⢠Questions for transit agencies that do not use ITS technologies to support HBS. ⢠Questions for transit agencies that do implement HBS. ⢠Questions for transit agencies that do use ITS technologies to support HBS. ⢠Questions for transit agencies that do implement HBS about how they monitor the perfor- mance of headway-based bus service and perceive the benefits of this service delivery. ⢠Participant information. The synthesis team provided custom survey links to each transit agency to permit the survey to be easily shared among multiple people within the transit agency or at local partner organi- zations in order to obtain all requested information. Survey results are included in Chapter 3. Case Examples After the survey was complete, four transit agencies that have successfully implemented HBS and represent a mix of geographic locations, agency size, operating context, and opera- tional strategies were selected as case examples. Interviews were conducted with the selected agencies to better understand their operating context, system development, stakeholder collaborations, operational strategies and use cases, performance monitoring, and training. Full case example write-ups, highlighting each agencyâs notable practices, challenges, and lessons learned, are included in Chapter 4. Organization of the Report This synthesis report is organized as follows: ⢠Chapter 1 (this chapter) introduces the topic, the scope of the study, and the work performed. ⢠Chapter 2 presents a literature review of HBS, operational control strategies, and related ITS technologies. ⢠Chapter 3 describes the results of a survey of North American transit agencies. ⢠Chapter 4 contains the write-ups from four case examples selected from the survey respondents. ⢠Chapter 5 summarizes the study and key findings and concludes with a discussion of gaps in knowledge and future research needs. ⢠List of Acronyms and Abbreviations defines frequently used acronyms and abbreviations for concepts, technologies, and organizations related to bus operations. ⢠References contains a list of sources. ⢠Appendix A contains the survey instrument. ⢠Appendix B shares the survey results tables.
