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152 This appendix is intended to provide transit professionals a description of the traffic engineer- ing profession as it relates to implementing transit-supportive roadway strategies. It uses the somewhat narrow terminology of traffic engineering even though most professionals use the broader terminology of transportation engineering, which conveys a more multimodal perspec- tive. However, because transportation engineering also applies to transit, the narrower term of traffic engineering is used to denote the operation of the surface transportation system for pedestrians, bicycles, all street-operating types of transit, trucks, and automobiles. The first section covers why traffic engineering standards exist, how they are applied, and available opportunities to vary from the standards to achieve better project outcomes. The second section covers reference documents that provide both standards and guidance on traffic engi- neering practice. The third section describes common types of analysis tools used in planning, operations, and design. The final section provides a primer on how traffic signals operate and how transit operations can be integrated into traffic signal operation. A.1 Traffic Engineering Practice Traffic engineers have a long history of operating the surface transportation system for all modes of transportation. This practice has resulted in standards, guidance, state practice, and local practice. Traffic engineering practice has evolved over the years and continues to evolve to this day. This section is intended to help transit professionals understand the traffic engineering point of view in order to better communicate and, ultimately, to provide a better transportation system for all users. Agency Stakeholders It is important to understand that the transportation system can be viewed from many perspec- tives other than that of the agency (e.g., transit agency, city public works department, state DOT). These perspectives can be seen as competing unless one steps back and realizes that the broader view of transportation is that of the user. Users are less interested in agency and jurisdictional boundaries and more focused on completing a trip that often crosses agency and jurisdictional boundaries and may shift modes one or more times over its course. It is therefore desirable to establish multi-agency operations working groups to exchange information and work together on common problems in order to achieve mutually beneficial operational strategies. User Perspective Transportation users consist of many types, including some who change transportation modes (e.g., motor vehicle, bicycle, pedestrian, transit) two or more times in making a trip (e.g., drive to A P P E N D I X A Understanding Traffic Engineering Practice (for Transit Professionals)
Understanding Traffic Engineering Practice (for Transit Professionals) 153 a park-and-ride lot, take transit to a stop near their destination, and walk to their destination). Public-sector traffic engineers typically work within their agency jurisdiction and may focus on a specific modal perspective such as bicycles. Transit is clearly one of several system users. Ultimately, the user is best served by a collaborative approach between all agencies responsible for the operation of the transportation system. Standards, Guidance, and State or Local Practices Traffic engineering practitioners works with three basic types of documents: standards, guidance, and (depending on the agency or jurisdiction) state or local practices. These docu- ments are discussed in more detail in Section A.2. However, it is important to understand from the start the differences in weight and interpretation of these types of documents. The MUTCD (FHWA 2009), for example, is a set of standards that also provide guidance and options. Stan- dards generally have no room for variation or interpretation by the engineer, other than possibly through a formal design exception process. Guidance is essentially recommendations for best practice, with room for interpretation on their applicability to specific locations. State standards typically exist for use on state facilities but may also apply to other facilities when funds originate with the state or are passed through the state. Local standards are typically how local agencies interpret guidance for their jurisdictions. Multimodal Perspectives As vehicular traffic has become more congested and options for improving capacity limited, practitioners have become more focused on multimodal uses of the system, especially in areas where non-automobile modes are encouraged. This evolving area of practice is changing agency standards to provide designs that are more accommodating of non-automobile modes and working toward developing complete networks for all modes. While land use goals may favor increased density and a multimodal system, transportation goals continue to be dominated by automobile mobility measures. In some cases, the need to maintain set levels of automobile mobility is at odds with creating an environment friendly to other models of travel. For instance, to meet auto-based operations standards at an intersection, a roadway agency may need to add turn lanes or widen the roadway approaches, introducing longer cross- ing distances and potential conflict points for pedestrians. As another example, some roadway agencies may find it challenging to implement transit signal priority if it results in some delay increases for automobiles. Increasingly, however, projects needed to meet automobile mobility standards may not be supported by the community or may be counterproductive to creating a multimodal environment. To respond, many agencies are starting to look at multimodal performance measures and alternative methods of measuring performance. Considering multimodal performance allows practitioners to promote improvements that will enhance the transportation system for all modes and have more flexibility to pursue projects that provide the most benefits to users throughout the day rather than just focusing on vehicle performance during peak conditions. In addition, a focus on Complete Streets is emerging in the traffic engineering world. Com- plete Street policies intend to âdevelop integrated, connected networks of streets that are safe and accessible for all people, regardless of age, ability, income, ethnicity, or chosen mode of travelâ (Seskin and McCann 2013). As the practice evolves, more jurisdictions are recognizing that beyond Complete Streets it is important to develop complete networks. While it may not be feasible or cost-effective to design every street to accommodate all modes, a ânetwork context considers all usersâ expectations of the entire networkâ (Active Transportation Alliance 2012). A network approach to Complete Streets recognizes that instead of trying to make each street
154 A Guidebook on Transit-Supportive Roadway Strategies perfect for every traveler, communities can âcreate an interwoven array of streets that emphasize different modes and provide quality accessibility for everyoneâ (Seskin and McCann 2013). More transportation agencies are adopting policies aimed at considering all users in trans- portation projects, encouraging street connectivity for all modes, and establishing operations performance standards with measurable outcomes. Complete Street/network policies provide an opportunity for traffic engineers to work with other transportation professionals to pursue multimodal projects such as transit-supportive roadway strategies. This heightens the importance of creating and maintaining connections between traffic engineers and transit agency staff, which is one of the objectives of this guidebook. A.2 Reference Documents The following are standard reference documents that guide traffic engineering practice. Manual on Uniform Traffic Control Devices The MUTCD is the most authoritative U.S. reference for traffic engineering practice regard- ing traffic signals, traffic signs, and traffic markings. The MUTCD is published by the FHWA under 23 Code of Federal Regulations (CFR), Part 655, Subpart F. The purpose of this CFR is to prescribe âpolicies and procedures of the FHWA to obtain basic uniformity of traffic control devices on all streets and highwaysâ (23 CFR 655.601, Purpose). State transportation agencies are required to either: â¢ Adopt the national MUTCD as their standard; â¢ Adopt the national MUTCD, along with a state supplement that may specify which of several allowable options are to be used; or â¢ Adopt a state traffic control device manual that is based on the national MUTCD and is in substantial conformance with the national MUTCD. The MUTCD is updated periodically to reflect new technologies, traffic control tools, and traffic management practices. When a new national MUTCD is published, states have 2 years to adopt the updated document. The MUTCD provides four types of information: â¢ Standards. Provided in bold text in the MUTCD, these are hard requirements with no room for interpretation. For example, Section 4D.27, Preemption and Priority Control of Traffic Control Signals, states that âDuring the transition into preemption control . . . the yellow change interval, and any red clearance interval that follows, shall not be shortened or omittedâ (FHWA 2009). Standards may use the word shall to denote a ârequired, mandatory, or specifi- cally prohibitive practiceâ (Section 1A.13). The MUTCDâs traffic signal warrants are standards that state that the need for a traffic signal âshall be consideredâ if the warrant criteria are met. â¢ Guidance. Provided in italic text in the MUTCD, guidance describes recommended best prac- tices that provide some room for interpretation. Some guidance is less specific and requires local interpretation and engineering judgment, such as âTraffic control signals operating under preemption control or under priority control should be operated in a manner designed to keep traffic movingâ (Section 4D.27). Guidance may use the world should to denote a ârecommended, but not mandatory, practice in typical situations, with deviations allowed if engineering judgment or engineering study indicates the deviation to be appropriateâ (Section 1A.13). â¢ Options. Provided in standard text in the MUTCD, options are âpermissive condition[s] and carr[y] no requirement or recommendationâ (Section 1A.13). Option statements sometimes
Understanding Traffic Engineering Practice (for Transit Professionals) 155 provide modifications to standard or guidance statements to provide flexibility for jurisdictions to fit specific, local needs. For example, Section 4D.26, Yellow Change and Red Clearance Intervals, provides a standard practice for developing all-red intervals at traffic signals. An option is provided that states âThe duration of a red clearance interval may be extended from its predetermined value for a given cycle based upon the detection of a vehicle that is predicted to violate the red signal indicationâ (FHWA 2009). The words shall and should are not used in option statements. â¢ Support. Provided in standard text in the MUTCD, support statements provide information and do not âconvey any degree of mandate, recommendation, authorization, prohibition, or enforceable conditionâ (Section 1A.13). The MUTCD is divided into nine parts that cover signs, markings, traffic signals, and traffic control devices. Sections of particular relevance to bus operations are: â¢ Chapter 4C, Traffic Control Signal Needs Studies; â¢ Section 4D.27, Preemption and Priority Control of Traffic Control Signals; and â¢ Chapter 8, Traffic Control for Railroad and Light Rail Transit Grade Crossings, which includes details on light rail signal displays that are also an option for certain types of bus operations. The full version of the MUTCD is available online at mutcd.fhwa.dot.gov. It is possible to experiment with new concepts that are not currently provided in the MUTCD. This process for experimentation is defined in Section 1A.10, Interpretations, Experimentations, Changes, and Interim Approvals. In summary, jurisdictions requesting approval for experi- mentation of a new traffic control device are required to submit their request to the FHWA. If approved, the requesting jurisdiction may install the experimental traffic control device, evaluate its performance, and provide regular reports to the FHWA. If granted, interim approval allows for interim use of the device pending official rulemaking. Requests are considered based on âthe results of successful experimentation, results of analytical or laboratory studies, and/or review of non-U.S. experience with a traffic control device or applicationâ (Section 1A.10). For example, bicycle signals are an area of currently evolving practice that has been partially accepted for limited use by FHWA. Jurisdictions seeking permission to use bicycle signals must comply with the conditions set forth by the FHWA for their interim use and maintain an inventory list of where bicycle signals are installed. AASHTO Publications The American Association of State Highway and Transportation Officials publishes A Policy on Geometric Design of Highways and Streets (AASHTO 2011), which is commonly known as the Green Book because of the documentâs color. This is the definitive guidance for state departments of transportation. It is also used by many local agencies by reference or by using or adapting its guidance. The Green Book has evolved over the years from a largely auto- and truck-oriented document to a more multimodal document. Much of its guidance was developed to ensure ade- quate geometric standards for higher speeds and vehicles, including multi-unit trucks. Its influence on urban practice has been significant, especially on roadways operated by state departments of transportation. However, many local agencies have modified this guidance to reflect the more confined space available in urban environments. The Context Sensitive Solutions approach is the application of appropriate designs in urban or otherwise constrained environments. AASHTO also publishes design guidelines for pedestrian facilities (AASHTO 2004), bicycle facilities (AASHTO 2012), and transit facilities (AASHTO 2014). Chapter 4 of TCRP Web-Only Document 66 provides possible changes to the AASHTO Transit Guide resulting from the research conducted by TCRP Project A-39.
156 A Guidebook on Transit-Supportive Roadway Strategies Another AASHTO publication is the Highway Safety Manual (HSM, AASHTO 2010), which can be used to quantitatively assess and predict crash frequency and severity based on traffic volumes and roadway characteristics. This manual also provides crash modification factors (CMFs), which quantify the change in average crash frequency expected with a geometric or operational modifica- tion to a roadway. CMFs are provided in the HSM for a variety of roadway strategies, and additional CMFs are available online at the FHWAâs CMF Clearinghouse (www.cmfclearinghouse.org). The current state of research is limited with regard to the safety implications of transit-supportive road- way strategies; however, the HSM can be used to evaluate changes in roadway or intersection char- acteristics introduced in conjunction with these strategies, and the CMF Clearinghouse includes some transit-related CMFs, such as for implementing transit signal priority and restricting turning movements at transit-serviced locations. Highway Capacity Manual The Highway Capacity Manual 2010 (Transportation Research Board 2010) is frequently used by traffic engineers to evaluate roadway operations and is also often referenced by roadway agencies when setting their operational standards for roadways. As with other traffic engineering references, the HCM has evolved over time to provide more analysis methods and performance measures for non-automobile modes. Typical performance measures defined by the HCM for urban streets and roadways are: â¢ Delay. Delay is defined generally as âadditional travel time experienced by a driver, passenger, bicyclist, or pedestrian beyond that required to travel at the desired speedâ and more specifically as control delay, which is âdelay associated with vehicles slowing in advance of an intersection, the time spent stopped on an intersection approach, the time spent as vehicles move up in the queue, and the time needed for vehicles to accelerate to their desired speed.â â¢ Speed. â¢ Volume-to-capacity ratio. Volume-to-capacity ratio can be thought of as the percentage of an intersectionâs or intersection approachâs capacity that is in use or in demand. â¢ Level of service (LOS). LOS assigns values of a specified performance measure one of six ranges, represented by the letters A through F, âwith LOS A representing the best operating conditions from the travelerâs perspective and LOS F the worst.â For intersections, automobile LOS is based on control delay; for roadways, automobile LOS is based on average speed; and for non-automobile modes, LOS is based on the modeâs LOS score. â¢ Modal level-of-service scores. These scores blend multiple factors into a measure reflecting average modal user satisfaction with a defined set of conditions; for example, the transit level-of- service score incorporates pedestrian access, bus stop amenities, bus frequency, bus reliability, bus speed, and onboard crowding as factors (Transportation Research Board 2010). State and Local Design and Operations Standards State departments of transportation and local roadway agencies typically develop their own design manuals that reflect their agencyâs perspective of the traffic engineering guidance provided in the Green Book and other sources. For example, these manuals may specify that certain options provided in the national guidance should not be used by the agency, or they may specify design standards that are higher than the minimums provided in the national guidance. These roadway agencies also specify operations standards for their roadways, which specify the minimum operation considered by the agency to be acceptable for a particular type of roadway. Most roadway agencies have traditionally used either automobile LOS or volume-to-capacity ratio for their standards. However, as roadway widening becomes more challenging and expensive to implement and, in some cases, conflicts with state and local goals regarding livability and sustain- ability, some jurisdictions have begun to adopt alternative measures (e.g., measures of travel time
Understanding Traffic Engineering Practice (for Transit Professionals) 157 reliability or vehicle miles traveled) or to consider a broader range of measures (e.g., person delay, multimodal LOS) as part of their decision making. A.3 Analysis Tools A common theme expressed in the transit agency surveys conducted for this project was that transportation engineers are typically easy to work with when one comes prepared with a traffic analysis that demonstrates how a proposed transit-supportive roadway strategy will likely affect roadway operations. A variety of analysis tools exist for evaluating roadway operations, ranging from simple to complex in terms of both information required and computational complexity, and roadway agencies will often specify what tool to use in what situation. Typical tools are: â¢ Regional transportation planning models. These models are often used to identify long- range transportation needs by mode and are based on assumed land use, population, and employment patterns. They can also be used to assess how traffic patterns may shift between facilities and modes when changes are made to the transportation system (e.g., converting a general travel lane on a street to a bus lane). â¢ HCM and HCM-like methods. These tools implement the HCM or other methods for evaluating roadway operations by using computer software to perform the calculations. Users typically have to provide detailed information about traffic demand patterns, roadway char- acteristics, and traffic signal timing, although default values may be substituted in some cases where these data are unknown. These tools are often used to demonstrate that a proposed project will meet a roadway agencyâs operational standards. â¢ Simplified planning analysis tools. These tools are typically simplified versions of HCM methods that are implemented in the form of tables, spreadsheets, or computer programs and that require relatively few data inputs but produce correspondingly less-precise results than other methods. They are often used to quickly evaluate a large set of alternatives for sufficiency and to produce loose performance measure estimates when a more precise answer is not needed. â¢ Microsimulation. These tools model the movements of individual roadway users (e.g., autos, buses, pedestrians, bicyclists) and produce the most-precise results of any tool. Their accuracy depends in great part on how well the model is calibrated to existing conditions. They require significant amounts of time and data to implement and so are usually not used for evaluating large numbers of alternatives. Instead, they are often used to confirm the results of a less- precise tool, to address situations not directly addressed by HCM methods (e.g., traffic signal operations with transit signal priority), and to generate visualizations of roadway operations. To assess the complexities of multimodal operations, microsimulation is often needed in conjunction with traffic signal optimization tools. Traditional optimization tools can often be used as a starting point for traffic signal operation. However, these tools cannot optimize for multimodal operations since the competing operational objectives for each user type must be prioritized based on local needs. Microsimulation can be used to evaluate alternative operational strategies (e.g., transit signal priority, curb extensions) in terms of a broader range of outcomes (e.g., reduced person-delay, improved transit reliability) than is possible with optimization tools that solely optimize vehicle delay. A.4 Traffic Signal Timing Concepts The following is a summary of signal timing concepts from NCHRP Report 812: Signal Timing Manual (Urbanik et al. 2015). Signal timing is the process of selecting appropriate values of timing parameters to implement in traffic signal controllers and associated traffic signal system
158 A Guidebook on Transit-Supportive Roadway Strategies software. Appropriate signal timing programs ensure that signal timing parameters are appro- priate over the life of the traffic signal system. While effective signal timing is necessary, it will not automatically sustain a successful signal timing program. A signal timing program includes all aspects of traffic signal implementation, operations, and maintenance consistent with com- munity needs. A successful program requires agency staffing and maintenance funding that is consistent with the level of service planned. Signal timing typically needs to be reviewed and be updated when traffic volumes and patterns change or when community priorities change. The signal timing produced by software largely reflects the system user priorities (generally some version of vehicle delay) built into the softwareâs signal timing optimization model. These priorities may or may not fit the needs of the actual operating environment or users (including pedestrians, bicycles, and transit). Signal Timing Approach An outcome-based approach to signal timing (summarized in Figure A-1) allows the practitioner to develop signal timing based on the operating environment, users, user priorities by movement, and local operational objectives. Performance measures are then used to assess how well the objectives are being met. Once the objectives and performance measures are established, timing strategies and timing values can be developed. The final steps of the process involve implementa- tion and observation (i.e., determining if the timing strategies and values are working) as well as monitoring and maintenance in order to sustain operations that meet the operational objectives. This process was developed with an understanding that there is not a one-size-fits-all method for signal timing. The approach is described in more detail in NCHRP Report 812: Signal Timing Manual, but brief descriptions of the eight steps in the outcome-based process and associated considerations are provided here. Step 1: Define the Operating Environment Signal timing should reflect the character of the timing location, so the outcome-based approach begins with an assessment of the operating environment. The operating environment Source: NCHRP Report 812: Signal Timing Manual, 2nd Edition (Urbanik et al. 2015). Figure A-1. Signal timing outcome-based process.
Understanding Traffic Engineering Practice (for Transit Professionals) 159 goes beyond physical location characteristics and includes goals of the local operating agency and its regional stakeholders. Step 2: Identify Users The process continues with the identification of primary users at the focus intersections. This approach allows all users (people on foot, including seniors and persons with disabilities, bicyclists, transit vehicles and passengers, truck drivers, and motorists) to be considered in the signal timing process. This is consistent with the multimodal perspective discussed earlier in this appendix. Step 3: Establish User and Movement Priorities Priorities should reflect the local operating agency and regional stakeholder goals for mobility. Priorities should be established by movement for the primary users by location and time of day. For example, in a central business district, pedestrians might have the highest priority, while in a suburban environment, through-vehicle movements on an arterial might have the highest priority during peak hours and a lower priority off-peak. Step 4: Select Operational Objectives Once priorities are established, the process requires the establishment of operational objectives (e.g., pedestrian safety, vehicle mobility) by location and time of day. Non-vehicleâoriented operational objectives are often more difficult to assess because of their qualitative nature; how- ever, performance measures can be selected for qualitative as well as quantitative assessment. Step 5: Establish Performance Measures Traditional optimization tools generally focus only on simple vehicle-oriented performance measures because they are easy to quantify and, therefore, easy to optimize. However, vehicle stops and delays may be less important than transit and pedestrian performance in a central business dis- trict or other existing or developing areas with significant pedestrian, bicycle, and transit activity. The practitioner needs to make appropriate adjustments to the traffic signal timing process to account for the operating environment and user priorities. Step 6: Develop Timing Strategies and Timing Values Once operational objectives and their associated performance measures have been determined, the process continues with the development of signal timing strategies (such as minimizing cycle length or favoring arterial through traffic) and with the selection of appropriate timing values. The options for signal timing may be restricted by standards or guidance. For example, the MUTCD provides guidance on the pedestrian clearance interval and provides standards for determining the duration of the yellow change interval and red clearance interval (FHWA 2009). Step 7: Implement and Observe The next step is implementing the signal timing values and making final adjustments to the timing parameters based on field observation (since it is important to understand that analytical tools do not capture all the subtleties of actual field conditions). However, it is equally important not to make changes based on a single field observation since traffic characteristics can vary from hour to hour and day to day. Step 8: Monitor and Maintain After implementation, a successful program requires ongoing monitoring and maintenance. Collecting periodic (at least annual) volume data at a midblock location on each arterial or
160 A Guidebook on Transit-Supportive Roadway Strategies subsystem is suggested to determine if shifts in traffic characteristics may have occurred and further investigation is necessary. A good maintenance management system can identify commu- nication and detection issues, which are often a significant contributor to poor signal operation. Signal Timing and Transit-Supportive Roadway Strategies Signal timing is a complex process that allocates time and space to various roadway users based on the operating environment and movement priorities. When choosing to implement a signal-related transit-supportive strategy, collaboration is required between traffic and transit engineers. The technical requirements include understanding and providing the necessary capa- bilities to implement preferential treatment. This is typically done by first developing a concept of operations. The concept of operations is a nontechnical document that essentially describes what the needs of the system are and how they will be met. Once a concept of operations is agreed to by all stakeholders, the technical requirements to implement the concept of operations can be developed. It should be understood that there are many practical realities to be addressed in the process of developing preferential treatment of transit. Compromises may need to be made based on all users of the system, including pedestrians. Pedestrians are present in many operating envi- ronments where transit priority is warranted and present certain constraints based on mobility requirements that must be met (e.g., MUTCD-specified pedestrian clearance times). In addi- tion, motorized vehicle operations may be affected by transit-supportive strategies. These and other considerations are discussed in Chapter 4. In order for transit priority to be successful, important capabilities generally need to be pro- vided by the transit system. These include: â¢ Integration of a transit automatic vehicle location system to determine if the transit vehicle is running late. This is important because priority for on-time vehicles is unnecessary and limits the signalâs ability to provide priority to late transit vehicles arriving in the same time period. â¢ Ability to communicate a vehicleâs estimated time of arrival to the traffic signal system. This ensures that the traffic signal is able to process the request in a timely manner. The potential transit detection technology may be constrained by existing technology used for emergency vehicles. That is but one example of the need for a concept of operations and ultimately the development of specific technical requirements. The capabilities of the traffic signal system may also be an important consideration in how transit-supportive strategies may be implemented. Each traffic signal system has capabilities that are defined by the vendor of the system. It is generally not possible to significantly change the capabilities of the traffic signal sys- tem. Again, the concept of operations process allows stakeholders to come to an understanding of what is possible, either within the current capabilities of the system or by modifying the sys- tem. Significant modifications to or replacement of the system may require substantial financial resources from stakeholders. In summary, preferential treatment of transit at traffic signals is something that requires stakeholders to come to an agreement on what is possible and how it might be implemented. The process can be relatively simple or relatively complex based on the realities of each stake- holderâs current capabilities.