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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Transit Signal Priority: Current State of the Practice. Washington, DC: The National Academies Press. doi: 10.17226/25816.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Transit Signal Priority: Current State of the Practice. Washington, DC: The National Academies Press. doi: 10.17226/25816.
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1 There are many external factors that contribute to delay and unreliability of bus transit, including several factors related to traffic and traffic control devices such as traffic signals. Transit signal priority (TSP) is one tool in the toolbox for transit agencies and their local partner agencies to improve the quality of transit service. Under the right conditions, TSP can help decrease travel times, reduce travel time variability, and improve schedule or headway adherence by providing transit vehicles with priority at traffic signals. However, TSP systems are not one size fits all. TSP systems can be designed and implemented in almost as many ways as can be imagined, and each system may interact differently with local traffic and pedestrian patterns and achieve different results—in terms of both transit benefits and impacts on other travelers. Because of the wide variety of possibilities for TSP systems and strategies, having a firm grasp on the current state of the practice is critical. This synthesis aims to provide readers with an up-to-date review of the academic and professional literature on bus TSP, as well as with a summary of the current state of bus TSP practices at North American transit agencies, through a survey of 46 transit agencies and documentation of five transit agency case examples. The case examples include the San Diego Metropolitan Transit System, the San Francisco Municipal Transportation Agency, the Toronto Transit Commission, the Rhode Island Public Transit Authority, and King County Metro Transit. TSP systems are defined by their architectures, business rules, and parameters. Archi- tecture refers to what physical and virtual components and modules provide TSP and how those components and modules work together and communicate. Business rules refer to the governing logic that determines when buses can request priority (e.g., buses must be late) or when intersections can grant priority (e.g., at all times except for rush hours). Parameters refer to the specific values used by each business rule (e.g., buses must be at least 60 seconds late to request priority). This report provides additional detail about potential architectures, business rules, and parameters, and explains some of the basic tradeoffs associated with implementing TSP. According to the survey results, most transit agencies’ TSP systems (77%) are decen- tralized (i.e., buses communicate directly to traffic signals instead of to a centralized back end), but many transit agencies are investigating center-to-center and GPS-based systems. Transit agencies that use bus TSP have a wide variety of business rules and parameters built into their TSP systems. Most TSP systems (58%) allowed buses to request priority regardless of their operational state (e.g., whether on time or late). TSP systems that did have a business rule to limit priority requests from buses (42%) used a schedule adherence rule so that buses requested priority only when running late. For most TSP systems in the survey (65%), priority is given by extending a green light, activating a green light early, S U M M A R Y Transit Signal Priority: Current State of the Practice

2 Transit Signal Priority: Current State of the Practice or both. There are some exceptions, and some TSP systems used a combination of busi- ness rules with varying parameters to help make TSP work as well as possible within the unique operating environments at each intersection. More than half of the TSP systems in the survey (77%) allowed buses to request priority at intersections with near-side bus stops, with mixed results regarding the effectiveness of this practice. According to the survey results, it is quite common for TSP business rules to prevent granting priority on sequential cycles by requiring a given amount of time (e.g., 120 seconds) or a given number of signal cycles (e.g., at least one cycle) to pass between granted priority requests (52% of TSP systems had this business rule). In general, transit agencies that responded to the survey perceived benefits from their TSP systems in the form of reduced intersection delay, improved travel times, and increased schedule adherence. Case example agencies provided some instances of actual quantified benefits (e.g., King County Metro Transit observed reductions in intersection delays of between 2% and 14%). Most of the surveyed transit agencies (68%) quantify their TSP system’s benefits. Transit agencies that saw the most benefit tended to use business rules that supported a combination of priority types (e.g., granting an extended green or early green, depending on intersection and operational conditions). However, quantifying a TSP system’s benefit was the biggest perceived challenge to transit agencies. Other notable challenges included maintaining ongoing operations and maintenance—often because of changing transit agency priorities and because transit agencies may not set up individual budgets dedicated to TSP operations or to continued evaluation and optimization. Continual coordination with local jurisdictions was also a reported challenge—especially as staff, priorities, or signaling systems change. Transit agencies exhibited a large degree of creativity and collaboration in their TSP deployments. In most cases, transit agencies worked with multiple local stakeholders to implement their TSP systems, which often meant integrating with multiple signaling systems from different vendors and with different capabilities. The earliest TSP deploy- ments used home-grown technologies, but as TSP has become more common, more recent deployments are able to leverage out-of-the-box capabilities in both onboard and inter- section systems to achieve successful TSP implementation. Case examples identified several notable practices, challenges, and lessons learned. A summary of case study findings is included in Table 28 in Chapter 4. This study also identifies several potential areas for future research. For example, the transit industry needs updated and more comprehensive guidance, toolkits, and training for successful TSP implementation and ongoing operations, maintenance, evaluation, and optimization. The industry also needs to better understand the potential for TSP to be a tool to help stabilize headways and improve schedule adherence. There is very little research on bus operator perceptions of and interactions with TSP systems, and there is very little research on ways to best communicate about TSP projects with communities and local stakeholders to maximize buy-in and reduce confusion and misunderstandings about the system and its potential impacts and benefits.

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Public transit buses face many operational challenges—especially when operating on the same streets and roads as other vehicles. Buses can be slowed by traffic congestion and get repeatedly caught at traffic lights, slowing buses down and delaying both passengers on board and passengers waiting at stops farther along the route.

The TRB Transit Cooperative Research Program's TCRP Synthesis 149: Transit Signal Priority: Current State of the Practice documents the current practice of TSP, which is an important tool that increases bus speeds and reliability, thereby improving transit system efficiency and effectiveness.

Twenty-eight (61%) of the 46 surveyed transit agencies had active TSP deployments, and 13 transit agencies (28%) either are in predeployment testing or have plans to pursue TSP in the future.

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