National Academies Press: OpenBook

Transit Signal Priority: Current State of the Practice (2020)

Chapter: Chapter 5 - Conclusions and Further Research

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Page 68
Suggested Citation:"Chapter 5 - Conclusions and Further Research." 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:"Chapter 5 - Conclusions and Further Research." 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:"Chapter 5 - Conclusions and Further Research." 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:"Chapter 5 - Conclusions and Further Research." 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:"Chapter 5 - Conclusions and Further Research." 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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68 C H A P T E R 5 This study sought to provide an up-to-date review of the current state of bus TSP deploy- ments in North America to help professionals better understand benefits, challenges, and les- sons learned regarding bus TSP, and to identify notable practices on the part of transit agencies that might contribute to successful TSP implementations and effective TSP operations and optimization. The results of the literature review, survey, and case examples provide useful information about bus TSP and TSP deployments in North America, including their architectures, business rules, and parameters, as well as performance monitoring and operations and maintenance. This concluding chapter provides a summary of the main findings of this study and suggests some topics for future research. Main Findings • History and Prevalence of TSP – The potential to use traffic signals to improve transit operations was identified as early as 1962. – TSP deployments are becoming increasingly common and notably more complex— allowing for more nuanced and specialized business rules and parameters that can be applied to specific bus routes, directions, intersections, days of the week, and times of the day. – 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. – Most survey respondents (20 of 25) have one TSP deployment, but four respondents have two or more. Deployment groups vary in size from just one route to 11 or more routes, and from 30 or fewer intersections to 61 or more. • Requesting Priority – Fifty-eight percent of deployment groups in the survey used unconditional TSP. That is, for these deployment groups, buses were allowed to request priority regardless of their operational state (e.g., schedule adherence, headway adherence, or passenger load). – Requesting priority when buses are late is the most common type of conditional priority among surveyed transit agencies, and the amount of lateness required to trigger a priority request is between 1 and 6 minutes. – A few transit agencies allow their late buses to continue requesting priority until they are back on (or close to) schedule. – A handful of surveyed transit agencies incorporated other conditions into their priority request business rules—for example, including passenger loads in the priority request conditions. Conclusions and Further Research

Conclusions and Further Research 69 – More than half of the deployments allowed buses to request priority at intersections with near-side bus stops either before or after leaving the stop. Business rules and parameters for buses that requested priority before leaving the stop had to take into account the anticipated dwell time at the stop. • Granting Priority – Extended and early green lights are the most common form of priority given to buses (28 and 16 out of 31 deployment groups, respectively). Fifteen deployment groups used a combination of extended and early green lights. – Green light extensions range from 1 to 30 seconds, and early green lights are activated between 1 and 60 seconds early (median 10 seconds). • Restricting Priority – Survey respondents very often had additional restrictions on when TSP could be granted and requested. For example, transit agencies often used intersection lockouts (e.g., a cer- tain number of seconds or cycles have to pass before the intersection will grant priority again). Intersection lockouts allowed the intersection to get back on its normal cycle and reduce the impact of TSP on cross traffic. – Other common restriction business rules included limiting TSP by day of week, time of day, type of service, and so on. • TSP System Architectures – Most TSP systems at surveyed transit agencies used a decentralized architecture (22 of 30 responses, or 73%). In a decentralized architecture, buses communicate directly with signals (or other intersection communication points) rather than through a back-end transit or traffic management center. – Half of surveyed transit agencies (15 of 30) have integrated their AVL systems with their TSP systems. Other integrations (e.g., with APC or fareboxes) were far less common. – Although other system integrations were not common, these integrations enable transit agencies to have complex business rules in place (e.g., when two priority requests conflict, grant priority to the bus with the most riders). • Performance Monitoring, Operations, and Maintenance – Surveyed transit agencies regularly monitor the functioning of their TSP systems by tracking the number of priority requests sent and granted or by evaluating outcome measures such as travel time and on-time performance. – However, even though transit agencies monitored system function, few transit agencies have regular processes in place to reevaluate TSP business rules and parameters. Most surveyed transit agencies evaluate business rules ad hoc (i.e., in response to problems or at the request of a local partner agency). – A notable challenge for transit agencies is ongoing TSP operations and maintenance— likely related to many transit agencies reporting not having operations and maintenance budgets for TSP. – A lack of dedicated funding for TSP also reduces the possible amount of study or reevaluation of business rules and parameters to achieve the optimal benefit from TSP. – Most transit agencies (21 of 31 deployment groups, or 68%) quantify benefits from TSP by evaluating travel times, schedule adherence, and travel time variability. However, transit agencies often saw quantifying benefits as a significant challenge. • Coordination with Local Partners – Most deployment groups (24 of 30, or 80%) are used by a single transit agency and are not shared with other transit providers. – Transit agencies report a mix of perspectives on the success of partnerships and coopera- tion with local partner agencies. However, the importance of open, robust communications with local partner agencies was frequently cited as a critical factor to TSP success. Clearly defined responsibilities of each stakeholder help all parties know their roles, and docu- mented agreements help solidify and sustain those roles over time.

70 Transit Signal Priority: Current State of the Practice • TSP Benefits – Surveyed transit agencies found TSP to be generally effective at achieving desired outcomes—especially in reducing intersection delay, reducing travel times, and improv- ing schedule adherence. Improving headway adherence and reducing headway vari- ability were rated as the lowest benefit, but this may be attributable to the fact that most survey respondents implemented TSP to improve schedule adherence, not headway adherence. – The transit agencies that reported the most benefit from TSP had business rules that supported a combination of priority types. For example, agencies that allowed both extended greens and early greens reported more benefits than those that allowed extended greens only. – However, many of the TSP deployments included additional priority treatments (e.g., dedicated bus-only lanes) that likely also benefited buses. The confounding effects of other priority treatments make it difficult to parse out exactly how much benefit is caused by TSP systems. – Some case example agencies provided their TSP evaluations to the study team. King County Metro found reductions in intersection delay of between 2% and 14% for its deployment group focusing on travel time reductions, and 30% improvements in headway adherence for its deployment group focusing on schedule adherence. • TSP Challenges – Several surveyed transit agencies found measuring the TSP system’s benefits a challenge— often related to a lack of quality baseline data, the confounding effects of additional priority treatments, or unclear methodologies for evaluating benefits. – Other challenges included maintaining long-term TSP operation in the context of chang- ing priorities and financial pressures, coordinating with jurisdictions that manage traffic signals, and determining optimal business rules and parameters. Suggestions for Future Research • Tools and Manuals for Bus TSP Comprehensive guides for bus TSP are quickly becoming outdated as transit and signaling systems and supporting communication technologies continue to evolve. Moreover, interest in bus TSP does not appear to be waning, and transit agencies and their local partner agencies need more tools and resources available to help make TSP deployments effective at achieving desired outcomes. A TSP toolkit, manual, or guidebook could include “menus” of business rules and parameters and their associated effects on transit and traffic. Such a toolkit could also include an overview of key TSP architectures and technologies, and checklists or step- by-step guides to support feasibility studies, implementation, operations and maintenance, evaluation, and optimization. • Standard Training for TSP There is little available training (e.g., workshops or seminars) on bus TSP. Creating new training opportunities for both transit and traffic professionals could help advance the state of TSP in the industry and support common TSP language and mutual understanding of the challenges, benefits, and tradeoffs that come with TSP deployments. Such a workshop could be provided by the National Transit Institute, the National Highway Institute, or some other entity. • Quantification of TSP Benefits There are many different types of benefits possible from a TSP deployment, and different deployments often have different goals. Transit agencies seeking simple answers about the

Conclusions and Further Research 71 effectiveness of TSP will be frustrated by the relative lack of standardized, empirical infor- mation. More research is needed to better define the types of benefits from TSP, standard- ize measurement approaches to quantifying different benefit types, and actually calculate benefits from different deployments that sought to achieve specific benefits using relevant business rules and parameters. • Use of TSP to Stabilize Headways With the increase in the number of bus network redesigns in North America and a renewed focus on high-frequency service, transit agencies are being challenged by bus bunching and gapping (creating headway instability), which is more likely to occur on high- frequency routes. Research is already under way to examine different approaches to use TSP to help stabilize headways (e.g., Anderson and Daganzo, 2020); however, this research needs to be continued and extended to fully understand the potential for TSP to help improve headway stability. • Use of TSP to Improve Schedule Adherence Many transit agencies use TSP hoping to improve on-time performance. However, achiev- ing improved on-time performance requires a combination of operational strategies, op- timized bus schedules, and TSP systems that are correctly configured. Even the number of time points, the location of bus stops, and the established scheduled speed can influence the number and effectiveness of priority requests. Again, research in this area is already un- der way, but that research needs to be extended and then developed into practical steps or guidance so that transit agencies can fully leverage their investments in TSP to achieve desired outcomes. • Use of TSP at Near-Side Stops Conventional wisdom suggests that requesting TSP at intersections with near-side bus stops may be problematic and not beneficial. However, more than half of the deployment groups allowed buses to request TSP at near-side stop intersections. More research is needed to understand the best approaches to utilizing TSP with near-side stops and to increase the industry’s capability to implement near-side stop priority successfully. • Bus Operator Reactions to TSP Very little research exists to understand the interplay between bus operators, TSP systems, and TSP benefits. Because bus operators may experience signal cycles they are not used to or do not understand, it may be critical that transit agencies engage their bus operators very early on in TSP implementation to avoid any loss of TSP effectiveness attributable to operator behaviors. • TSP Deployment Marketing and Communications There is little research on community perceptions of TSP—especially during deploy- ments. Transit agencies would benefit from additional research to understand how to com- municate a TSP implementation project to communities and passengers to achieve the strongest possible level of support for TSP projects while also reducing confusion about what TSP actually does. • Impacts of Technological Advances As transit and traffic technologies advance, these systems will be able to support even more complex business rules and parameters with more flexibility and control for transit and traf- fic agencies. There currently is very little research examining what technological advances might do to enhance TSP effectiveness, to provide additional benefit while reducing impacts on other road users, or to reduce the overall cost of implementing TSP. • Advanced Signaling Approaches and Transit Benefits Traffic signal progression and TSP could work together in ways that are not in common practice (e.g., granting priority to buses that fall out of the green band). Advanced approaches

72 Transit Signal Priority: Current State of the Practice to better align coordinated and even adaptive signaling systems with TSP need more investi- gation to fully understand their potential benefits and challenges. • Best Practices for TSP Operations, Maintenance, and Optimization The transit industry needs more guidance on best practices for operating, maintaining, and optimizing TSP systems after implementation. Whether this guidance is part of the aforementioned updated tools and manuals for bus TSP or not, the guidance should include budget estimates for operations and maintenance costs, suggested methods for monitoring TSP system function, and protocols for optimizing TSP business rules and parameters in conjunction with local partner agencies.

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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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