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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Intelligent Transportation Systems in Headway-Based Bus Service. Washington, DC: The National Academies Press. doi: 10.17226/26163.
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Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Intelligent Transportation Systems in Headway-Based Bus Service. Washington, DC: The National Academies Press. doi: 10.17226/26163.
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Page 2
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Intelligent Transportation Systems in Headway-Based Bus Service. Washington, DC: The National Academies Press. doi: 10.17226/26163.
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Page 3

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1   High-frequency bus routes, defined as those with a headway of 10 minutes or less, often experience various operational problems, such as schedule adherence, bunching and gapping, and full load capacity. These problems are particularly severe in high- frequency service because passenger arrival patterns and interactions between buses create a positive feedback loop where small variations in bus travel time quickly develop into larger problems if left unchecked. Research has suggested that operating high-frequency routes by headway instead of schedule can improve reliability by focusing on the interactions between buses that drive instability. In headway-based service (HBS), the objective is to maintain consistent and appropriate separation between buses, which requires real-time knowledge of bus locations and some form of communication between buses. Advances in intelligent transportation systems (ITS) have greatly increased the availability of real-time transit data and intro- duced new means of communications between buses. As a result, many transit agencies in North America and elsewhere have implemented headway-based bus service in the last few years, and others have plans to implement in the future. The purpose of this synthesis was to review 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, such as collecting ridership data, the focus of this study was on applications that directly support the operation of HBS by helping agencies to monitor buses, anticipate or identify service problems, and implement various operational strategies. 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. This synthesis found many variations in current practices. First, the objective of HBS can be defined in different ways. One option is to set a specific target headway (e.g., 10 minutes), while another option is to achieve regular (i.e., equal) headways that may vary based on current conditions. Headway-based routes do not require a schedule but may have one, particularly if the route is operated by schedule in off-peak time periods or if the schedule is used to define driver shifts. Buses experience many different types of travel time variability, including traffic inter- actions, signal delay, and passenger movements at bus stops. Even if all buses start each trip at the same headway, these variations in travel time accumulate over time and cause buses to deviate from their headway objective. Agencies that operate HBS typically use one or more of the following operational strategies to correct headway deviations: • Boarding limits, which restrict the number of passengers that can board at a stop. • Holding, when buses wait for a specified amount of time after boarding and alighting passengers before departing a stop. S U M M A R Y Intelligent Transportation Systems in Headway-Based Bus Service

2 Intelligent Transportation Systems in Headway-Based Bus Service • Short turns, when buses turn around before the end of the route and enter service in the opposite direction. • Speed guidance, where bus drivers are instructed to adjust their driving to make up time or slow down. • Stage vehicles, buses that are held in reserve and can be placed into service when conditions warrant. • Stop skipping, when buses do not serve a stop to make up time. Holding has received the most attention in the literature, likely because it is easy to implement and has a clearly observable impact on headways. These strategies are not mutually exclusive, and many researchers have investigated how two or more of these strategies can be used together. Combinations often have strategies that act in different situations. For instance, they may have one strategy to slow down buses that are ahead of their target and another to speed up buses that are falling behind. Among surveyed agencies, holding and stop skipping were the most commonly used, while short turns and speed guidance were rated as the most effective. ITS technologies relevant to HBS include • Automated passenger counters (APCs), which count passengers as they board or alight. • Automatic vehicle location (AVL), which reports bus locations at regular intervals. • Computer-aided dispatch (CAD), a method for real-time monitoring and control. • Electronic fare payment (EFP), a category of fare payment that includes magnetic stripe media, smartcards, digital wallets, and smartphone apps. • Passenger information systems (PIS), which include screens at bus stops as well as voice, text, and web services. • Security systems (SS), which include text and voice messaging systems as well as cameras. • Smartphone apps, including agency and third-party applications that provide EFP and PIS features. • Transit signal priority (TSP), which allows buses to request special treatment at traffic signals to save running time. All surveyed agencies use CAD/AVL (usually implemented together) for real-time moni- toring and control, and a plurality use TSP. Real-time uses of APCs, EFP, and SS have been proposed in the literature, but the survey results suggest that these applications are not yet common in practice. Headway-based service also requires different performance indicators than schedule- based service. Research has suggested several options, including headway adherence, service regularity, headway ratio, and regularity indices. Most surveyed agencies use head- way adherence. The case example interviews revealed additional insights about the issues surrounding the deployment and operation of HBS. • Benefits: Agencies saw the main benefits of HBS as providing flexibility in operations and improving route performance on measures such as travel time variability, out-of-vehicle waiting time, and passenger load balancing. • Personnel: All agencies underlined the importance of driver and dispatcher buy-in to make HBS work. Training includes the following: – Driver training is important to explain the goals of HBS, the targets that drivers are expected to monitor, and what actions they can take to self-correct. – Dispatcher training is also important to develop a common understanding of the goals of HBS, what operational strategies can be used, and when to intervene.

Summary 3   • Costs: The costs of implementing HBS are difficult to quantify because many ITS technologies are already widely implemented (e.g., CAD/AVL) and shared across the agency. Direct costs of HBS include driver and dispatcher training as well as dedicated dispatchers (if used). Switching to HBS may also reduce operating costs by improving reliability and shortening cycle times. • Notable Practices: Notable practices included dynamically adjusting the target headway based on current conditions, providing route and stop amenities that reduce travel time variability, and providing long layover times that allow buses to recover and start their next trip at even headways. • Challenges: Challenges included schedule sliding and driver shift changes. Schedule sliding occurs when large amounts of holding are used to control headways, which slows all buses down and can result in drivers working significant amounts of overtime. Agencies also noted that drivers approaching the end of their shift sometimes ignore headway targets and dispatcher instructions to reach the relief point sooner. • COVID-19: COVID-19 was a challenge for all agencies because the large reduction in passenger demand on headway-based routes had to be balanced with the need for safe physical distancing on board. Two agencies had reverted to Sunday service at the time of the interview, while one was monitoring load factors and adding/subtracting buses as needed. Overall, HBS is a useful practice that agencies can use to improve the performance of high-frequency bus routes and potentially reduce operating costs. Successful implemen- tations have been done using existing ITS, with the main effort being the adoption of new performance indicators and operational strategies. Consistent monitoring is important for the successful operation of HBS, and driver and dispatcher training is an ongoing need. There remains potential for future research on blending data from multiple ITS technologies, evaluating passenger perceptions of different operational strategies, and managing in service.

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Intelligent transportation systems and, in particular, computer-aided dispatch and automatic vehicle location (CAD/AVL), have become quasi-universal in urban bus operations and support a variety of functions.

The TRB Transit Cooperative Research Program's TCRP Synthesis 155: Intelligent Transportation Systems in Headway-Based Bus Service synthesizes the current state of the practice of headway-based service operations and focuses on the proactive use of intelligent transportation systems technologies to optimize these services.

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