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3 C H A P T E R 1 Background Public transit buses face many operational challengesâespecially 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. Operational delays result in increasing operational costs, and the magnitude of operational delays may vary from hour to hour and day to day, adding unreliability to a busâs schedule or headway and further deteriorating customersâ experiences on transit. Transit signal priority (TSP) is one tool available to transit agencies and their local partners to help improve bus speeds and reliability and enhance the quality of service. TSP is a technology that allows transit vehicles to modify traffic signal timing or phasing to increase transit vehicle speeds, improve transit vehicle reliability (i.e., reduce travel time variability), or both. Starting in the 1970s, advances in traffic signal, transit, and communications technology led to the development of sensors that track bus location and signal controllers that respond to real-time inputs (Urbanik and Holder, 1977). A first generation of bus location technology used fixed sensors to detect buses at specific points along the route (Riter and McCoy, 1977). Newer systems use GPS or cellular networks to record bus locations at a given time inter- val, for example, every 60 seconds. TSP originated in work on signal preemption (Evans and Skiles, 1970), which immediately stops all conflicting phases and is now reserved for emergency vehicles and trains. Developments in communications technology, from fixed sensors connected by wire to infrared, radio, and fiber-optic links, to remote traffic management centers, have allowed buses to transmit more information to traffic signals. The increased information has allowed for more sophisticated TSP strategies, which can now decide whether to grant priority based on a busâs current schedule deviation, passenger load, and so on. Given the increasing variety and sophistication of TSP system architectures and busi- ness rules, there is a need to assess the current state of practice and to develop guidance for transit agencies that currently have TSP or are considering a new implementation. The most recent state-of-practice study was 15 years ago (Smith, Hemily, and Ivanovic, 2005). Furthermore, there is relatively little documentation available about business rules and parameters used in practice. It is particularly valuable for agencies pursuing a new TSP system to know what business rules and parameters have been used elsewhere and which combinations have produced the best results for buses. This information will allow agencies to select and customize an appropriate strategy to address specific operational issues. Introduction
4 Transit Signal Priority: Current State of the Practice Definition of Terms Different TSP terminology is used by different authors and transit agencies. For consistency, the following definitions will be used in this report: â¢ TSP system: the hardware and software components and modules that work together to make TSP work. â¢ Architecture: the functional design of the TSP systemâs hardware, software, and communi- cation interfaces, including how TSP system components and modules work together. An example TSP system architecture could be that buses request TSP at intersections using an infrared transmitter. â¢ Business rules: the rules and decision logic that determine whether TSP is requested and whether TSP is granted. There are many types of business rules: bus eligibility, intersection eligibility, priority eligibility, and priority type. For example, a TSP system might allow only late buses to request priority. â¢ Parameters: the specific values associated with individual business rules. For example, the business rule may be that only late buses can request priority. The parameter is how late (in seconds or minutes behind schedule). Another example: if green extension is the business rule for the type of priority granted, the duration of the green extension is the parameter. â¢ Deployment group: a collection of bus routes, buses, and intersections that share a common TSP system with the same or similar architecture, business rules, and parameters. A TSP deployment group could be implemented across multiple corridors or geographic areas or at different times. What defines a deployment group is how the system works and what business rules and parameters are in place, not necessarily where and when TSP was deployed. Because this synthesis focuses solely on bus TSP, all references to TSP are referring to bus TSP and not to rail TSP. Objective and Scope This report aims to provide readers with an up-to-date review of academic and professional literature on TSP and a synthesis of current TSP practices at North American transit agencies. This synthesis documents â¢ Planning for and implementing TSP: Why do transit agencies pursue and implement TSP, how do they choose routes and corridors for TSP, and how are local partner agencies involved in the implementation process? â¢ The characteristics of TSP systems: What are the size and scope of TSP deployments in North America (in terms of routes, intersections, and vehicles equipped and eligible for TSP)? â¢ TSP strategies, business rules, and parameters: What TSP strategies are in place at different transit agencies, and how are those strategies converted into specific business rules and parameters? What business rules and parameters appear to be most effective? â¢ TSP system architectures: What vendor (or home-grown) systems and technologies are used to support TSP deployments? What were the implementation costs? â¢ TSP system operations and maintenance: How do transit agencies operate and main - tain their TSP systems, and what budgets are established to support operations and maintenance activities? How do transit agencies evaluate the functioning of their TSP systems?
Introduction 5 â¢ TSP benefits, challenges, and lessons learned: What benefits do transit agencies observe from their TSP deployments? And what challenges and lessons learned from current TSP deploy- ments can provide much-needed insight for the rest of the industry? Report Organization This report is broken into five chapters: â¢ Chapter 1: Introductionâincludes an introduction to TSP and a description of the scope of the study. â¢ Chapter 2: Literature Reviewâdiscusses the state of TSP research and practice and describes possible TSP strategies, business rules, parameters, and technologies. â¢ Chapter 3: Survey Resultsâpresents the results of the survey. â¢ Chapter 4: Case Examplesâpresents the five case examples. â¢ Chapter 5: Conclusions and Future Researchâprovides a summary of findings and sugges- tions for future research. This report provides the following basic introduction to TSP, giving readers the key terminology and knowledge necessary for understanding the studyâs scope and results. Introduction to Transit Signal Priority Traffic signals are a key determinant of travel time on urban streets for all road users. The poten- tial to improve bus operations by giving buses preferential treatment at signals was recognized as early as 1962 (Sperry Rand Corporation, 1972) and is further described in chapter 6 of TCRP Report 165: Transit Capacity and Quality of Service Manual (Kittelson & Associates et al., 2013). Preferential treatment for buses at traffic signals can generally be divided into two categories: â¢ Passive priority measures involve changes to traffic signal timing that benefit buses but do not detect bus presence or have any communication with buses in real time. â¢ Active priority measures make changes to traffic signal timing, either when a bus is detected or in response to a priority request sent through some communications channel. Passive and active priority measures are distinct but not mutually exclusive. Passive priority reduces the average signal delay that buses experience and may increase the likelihood that buses arrive at signals when the light is green. Passive priority can complement active priority by reducing the frequency of priority requests. Active priority can help passive priority by indicating where the bus is in real time so that the timing of a green band (a coordination strategy that allows vehicles to pass through several consecutive signals without stopping for a red light) can be adjusted as needed. For the purpose of this synthesis, TSP refers exclusively to active priority measures for bus transit. In current practice, the decisions of when to request priority and when to grant priority are based on a set of business rules. Many business rules have associated parameters that can be tuned to fit the local context. The following sections give examples of common hardware and software system architec- tures, the types of priority that buses can receive, and typical business rules and parameters. System Architecture In this synthesis, system architecture refers to the functional design of the TSP system, including the components and modules and how they work together. All TSP systems include
6 Transit Signal Priority: Current State of the Practice a detector, a priority request server, and a signal controller. The detectorâs role is to identify when there is a bus that wants priority and estimate when it will arrive. The priority request server plays a coordination role between the detector and the signal controller by processing the priority request, checking business rules if applicable, and deciding what changes to make to signal timing. The signal controller receives an updated timing plan from the priority request server and physically controls the lights displayed to vehicles and pedestrians. There are two main types of system architectures: distributed and centralized. In a distributed architecture, all decisions are made at the local (intersection) level. In the simplest form of a distributed architecture system, only the signal infrastructure has equip- ment enabling TSP. Signals have some means of detecting bus presence and attempt to grant priority whenever a bus is detected (Hounsell et al., 2004). Because there is no communication with buses or the transit management center, this architecture supports only unconditional priority, where all buses are treated the same. Newer distributed architectures have TSP equipment on both buses and signals. Each bus contains a priority request generator that checks the conditions for requesting priority and generates a request if the conditions are satisfied. Conditions can include type of bus, direction of travel, route, and many other factors related to service and bus status. The priority request is sent to the priority request server located in the traffic signal cabinet, which passes the request to the traffic signal controller. In a centralized architecture, priority decisions are sent through a centralized management center; however, several variations exist. In one common variation, there is little computa- tional power on either the buses or the traffic signals. Buses usually have an automated vehicle location (AVL) system and are in communication with a centralized transit management cen- ter. When a bus is approaching a signal, the priority request generator located in the transit management center decides whether or not to request priority. Priority requests are sent to the priority request server, located in the traffic management center (which may be separate from or combined with the transit management center). The traffic management center then processes the request, decides what (if any) signal timing modifications to make, and forwards these to the signal controller. For more details about and standards for signal priority architectures, readers are directed to the joint publication of the American Association of State Highway and Transportation Officials, Institute of Transportation Engineers, and National Electrical Manufacturers Asso- ciation, called NTCIP 1211: National Transportation Communications for ITS ProtocolâObject Definitions for Signal Control and Prioritization (2014). Types of Priority An extended green (also called phase extension) extends the signal phase used by the bus. This action is most useful when a bus is detected near the end of a green phase and would arrive after the signal turns red. There are different ways of implementing green extensions: some- times a fixed extension is added (e.g., 10 seconds), and in other cases a form of check-out detec- tion is used so that the phase can end as soon as the bus has cleared the intersection. An example of a fixed extension is shown in Figure 1a, where the bus can save some signal delay as long as it sends a priority request before the typical end of green. Early green (also called red truncation) is used when a bus arrives during a red phase. The traffic signal starts the next green phase earlier than the green phase would normally occur in the cycle, allowing the bus to progress sooner. Early greens are typically constrained by safety business rules and may only reduce signal delay for the bus instead of eliminating it. An example is shown
Introduction 7 Figure 1. Diagrams illustrating types of priority. (a) Extended green (b) Early green (c) Special bus-only phase (d) Phase insertion (e) Sequence change (f) Phase skipping
8 Transit Signal Priority: Current State of the Practice in Figure 1b, where the bus is arriving near the end of the opposing phase and could save some signal delay by requesting priority. Note that this priority request would have to be sent well in advance for the signal to safely terminate the opposing phase and pass the bus through as shown. A phase insertion provides a special phase for the bus. This action may be used to provide a queue jump (i.e., allowing buses to use an available lane to go around the line of stopped cars and depart first) or to allow buses to make a nonstandard movement, such as a left turn into a transit center (Advanced Traffic Management Systems Committee and Advanced Public Transportation Systems Committee, 2002; Ryus et al., 2016). This phase may be indi- cated by a special, bus-only signal to avoid confusing other vehicles. Two examples are shown, in Figure 1c and Figure 1d. Figure 1c represents a special bus-only signal. Typically, this phase would be just long enough for the bus to clear the intersection. Figure 1d shows a slightly longer phase being inserted. If the inserted phase is also available to other vehicles, this variant may be preferred. In many systems, it is important for signals to maintain a consistent offset with respect to other neighboring intersections. If this is the case, the signal will typically return to normal timing by shortening other phases to compensate for the extra phase. It is a common practice to return to normal signal timing within one cycle (Kittelson & Associates, 2007). A sequence change (also called phase rotation) rearranges signal phases so that the busâs phase starts sooner than it would normally occur. This action is typically associated with multiphase signals and may be less disruptive than an early green because the phase lengths do not change, only the order. An example is shown in Figure 1e. Phase skipping skips one or more conflicting phases to return to the busâs phase earlier (Urbanik et al., 2015). An example is shown in Figure 1f. TSP under adaptive signal control. Adaptive signals have no fixed signal timing and continu- ously adapt to real-time observations (Advanced Traffic Management Systems Committee and Advanced Public Transportation Systems Committee, 2002). For this type of signal controller, a bus requesting priority is another vehicle input that will be considered when updating the timing plan. The strategies listed above can also be implemented by this type of signal control. Business Rules and Parameters An important category of business rules consists of rules that determine when buses are allowed to request priority. Early TSP deployments were unconditionalâbuses requested priority at (or were detected by) every TSP-equipped signal (Evans and Skiles, 1970; Ludwick, 1975; Urbanik and Holder, 1977). Even in this scenario, priority is not granted 100% of the time. Virtually all TSP systems have business rules related to safety, including â¢ Clearance intervals, typically the duration of a yellow light or flashing hand that allows vehi- cles or pedestrians to finish crossing the intersection, are never shortened to grant priority. â¢ Minimum green times, which provide a minimum phase duration based on driver reaction time, the expected queue of vehicles, and/or the pedestrian crossing time, must be satisfied. â¢ Preemption, in which transit priority will not be granted when signals are preempted by other road users (e.g., an emergency vehicle or train or vehicle using an at-grade rail crossing), will not be overridden by a TSP priority request. Conditions for requesting and granting priority can be subdivided into rules pertaining to bus operations, service attributes, and intersection operations. The first category, bus operations, includes business rules related to requesting priority on the basis of
Introduction 9 â¢ Minimum schedule deviation, â¢ Minimum headway deviation, â¢ Minimum passenger load, and â¢ Bus door status (e.g., open or closed). The second category, service attributes, includes business rules related to the transit service the bus is providing. Examples include â¢ Time of day, â¢ Direction of travel, and â¢ Type of service (express versus local). The third category, intersection operations, includes business rules related to the characteris- tics of the intersection and signal. Examples include â¢ Minimum time or cycles since last TSP request, â¢ Requirement to maintain coordination with other signals, and â¢ Maximum side street queue. Technical Approach to the Study The study team accomplished the project objective and scope through three tasks: â¢ A scan of the industry and relevant literature â¢ A survey of North American transit agencies using TSP â¢ In-depth case examples for five North American transit agencies Each task is described in more detail in the following sections. Literature Review First, the study team reviewed relevant professional and academic literature to identify pre- vious research on the synthesis topic and to understand the prevalence of TSP among North American transit agencies. The literature review is detailed in Chapter 2. Survey of North American Transit Agencies Second, the study team developed a list of North American transit agencies believed to be using TSP and distributed an online survey instrument to 79 randomly selected agencies from this list. Fifty-five eligible transit agencies confirmed receipt of the survey (several transit agencies were deemed ineligible because they had incorrect contact information, they did not actually have TSP, or they had language barriers). Forty-six of the 55 eligible transit agencies completed the survey, a response rate of 84%. Survey results are presented in Chapter 3. Case Examples After the survey was complete, five transit agencies were selected as case examples that together formed a representative sample of geographic regions, agency size (as measured by vehicles operated in maximum service, or VOMS), and TSP system maturity. The five case examples are located in Chapter 4 and are listed in Table 1.
10 Transit Signal Priority: Current State of the Practice Table 1. Case example agency key operational statistics. Case Example Agency Region Bus Peak Vehiclesa TSP Implement Year No. of Deployment Groups TSP Intersections TSP Buses San Diego Metropolitan Transit System San Diego, CA 512 2014 1 50 130 San Francisco Municipal Transportation Agency San Francisco, CA 674 1998 1 450 900 Toronto Transit Commissionb Toronto, ON, Canada 1,920 c 1991 1 200 2,000 Rhode Island Public Transit Authority Providence, RI 195 2014 1 59 223 King County Metro Transit Department Seattle- Tacoma, WA 1,155 1999 3 or more 200 1,500 NOTES: aBus peak vehicles includes all fixed-route bus modes. bData for the Toronto Transit Commission were obtained from the Toronto Transit Commission website (2019). cThe Toronto Transit Commission has 1,920 buses; fewer would be used in peak service. SOURCE (unless otherwise specified): National Transit Database (2017).