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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Page 25
Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
×
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Suggested Citation:"Chapter 1: Background and Motivation." National Academies of Sciences, Engineering, and Medicine. 2014. Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report. Washington, DC: The National Academies Press. doi: 10.17226/22289.
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Chapter 1: Background and Motivation Traffic congestion continues to grow significantly in North America and throughout the world. Agencies tasked with managing traffic control systems are frequently challenged with moving traffic in congested conditions and situations where the traffic demand exceeds the capacity of the system. The results of the 2005 and 2007 indicate Traffic Signal Operation Self-Assessment surveys, the majority of agencies involved in the operation and maintenance of traffic signal systems are stretched thin and challenged to provide adequate service to drivers in their jurisdictions. Oversaturated traffic systems are the most complex and difficult traffic control problems. Under oversaturation, typical traffic control strategies do not work as efficiently as necessary, particularly since the objectives need to be decidedly different when mobility is restricted (e.g. “move someone, somewhere” rather than “give everyone equity treatment per cycle”). Many practitioners argue or conclude that “there is nothing that can be done when there is simply too much traffic”. This research project found that under many typical oversaturated conditions, mitigation strategies can be applied that have an appreciable effect on over-all system performance. There are two important clarifying factors. First, it is important to consider three different regimes (loading, mitigation, and recovery) of operation under which performance is measured and evaluated. Secondly, during oversaturated conditions, performance must be measured against different objectives than those that are appropriate during undersaturated operation. In particular, the objective to minimize user delay must be substituted with the objective to maximize system throughput. Mitigation of oversaturated conditions frequently involves trade-offs between the storage of traffic queues from the oversaturated movements to other less utilized movements. This practice might be described by the idiom “borrowing from Peter to pay Paul”. Counter-intuitively, the same control strategy that provides user-optimal delay minimization in under-saturation can in some cases work against the minimization of total delay when one or more approaches become oversaturated. It may be necessary to induce cycle failures and residual queuing on side streets in order to maximize the flow rates on oversaturated movements in the main direction(s) of flow. This change in operational policy or strategy trade-offs may be challenging to communicate to organizations and citizens. Development of strategies to handle oversaturated conditions is not a new concept. The research team found a variety of work in both diagnosis and estimation of oversaturation and control strategies and scenarios. In diagnosis and estimation we focused the review on techniques for measuring queues and degree of saturation and surrogates for the degree of saturation. While there has been quite a bit of work in the past on estimation of delay during oversaturated conditions and approaches for modeling oversaturated conditions primarily for Highway Capacity Operation of traffic signal systems in oversaturated conditions Page 16

Manual-type analysis, these research efforts are not directly applicable to this project. Research on queue estimation is dominated by input-output modeling approaches. These methods are limited to estimating queues up to the point of the input detector, but not beyond. For arterial streets this requires installation of exit-side detection in order to measure a queue that is the length of the link. Such detector installation can be cost-prohibitive. Methods for measuring the degree of saturation can identify the saturation level up to the point of saturation, but estimates of saturation above 1.0 have not been shown to be reliable, except for those estimates used by SCOOT and SCATS which are not published. In this research, we developed a queue estimation methodology and quantitative measures of oversaturation that can be measured from advance detectors given high-resolution information on the phase timing. In the review of strategies and scenarios we looked at previous research on adaptive control systems, optimal control formulations, and various other approaches. Features of adaptive control systems are described in the literature in a qualitative manner. Concepts can be leveraged, but specific algorithms are not typically described quantitatively. Notably the features of SCOOT and SCATS that handle oversaturated conditions were found to be if…then type rules with thresholds that change some parameters or impose additional constraints on certain decision variables. Descriptions of these features are not accompanied by research indicating their ability to be effective in the real world or even in simulation. Optimal control formulations found in the literature all require information on traffic volumes, queue lengths, or both. Volume information is the most difficult to obtain during oversaturation using state-of-the-practice detection systems, which makes most of the optimal formulations difficult to apply directly. Consolidated Results of Interviews with Expert Practitioners The research team interviewed expert practitioners experienced with oversaturated conditions and who developed innovative solutions for addressing those conditions in the past. Each expert practitioner was interviewed for approximately one hour. The expert practitioner was provided a questionnaire that included the following general questions: • What would you consider to be a definition of oversaturated conditions? • What conditions cause you to take action to alleviate oversaturated conditions? How do you collect data/evidence on problems? • What are typical techniques you use to address oversaturated conditions? Examples? • Do we need better/different detection systems, algorithms, or approaches for detecting oversaturated conditions? What is the typical detection layout in your jurisdiction? Feedback was varied from on several issues. It was clear that all participants viewed the management of oversaturated intersections as one of the most important issues they face day to day. Almost all expert practitioners indicated that they spent a significant amount of time dealing with oversaturation and that existing tools such as signal timing optimization software are not Operation of traffic signal systems in oversaturated conditions Page 17

adequate for handling oversaturated conditions. They frequently relied on their personal experience and trial-and-error application than running models or performing extensive analytical analysis. In all cases, the expert practitioners brought up specific case studies as examples for where specific strategies were successful in addressing the problem. We have summarized detailed feedback from the expert practitioners on each of the topics above in the following sections. Expert Practitioners Thoughts on Definitions None of the expert practitioners were very concerned about identifying a precise definition of oversaturation. All agreed that demand greater than capacity was necessary, and that the condition needed to last at least a few cycles before it would be considered something they would spend any time to address with changes to signal timings. One expert practitioner stated that they considered at least one 15-minute period where the queue was persistent as a minimum level of congestion to call a facility oversaturated. To define the condition analytically as part of Task 2, we asked the expert practitioners if they considered an isolated intersection with a residual queue to be oversaturated even though there was no impact of the operation on other signal operations. Most agreed that an isolated condition would still be judged as oversaturated and worthy of signal timing revisions, but all considered that storing queues where there is adequate storage space (i.e. metering) was an effective strategy for many situations. Most also agreed that oversaturation is a condition that applies at the movement, approach, or phase level. One expert practitioner gave his definition at the intersection level, arguing that the changes necessary to address the oversaturated queue on one phase or movement will, by definition, degrade the performance on other movements and phases. So in his view, given the operating principles of modern signal controllers, the problem is at a minimum an intersection problem. Several of the expert practitioners stated the definition must relate to the storage area of the approach, link, or movement. Since it is not enough to simply say that volume is greater than capacity of the green time, the ability of the traffic facility to store the excess queue will affect what strategies they would try to apply. None of the expert practitioners focused on the growth rate of the residual queue as an important indicator of oversaturation. Expert Practitioners Thoughts on Diagnosis Almost all of the expert practitioners relied on personal observation of the conditions in the field more often than using any analytical tools or reports. Expert practitioners in large systems (e.g. Los Angeles) and systems with CCTV deployment also relied on performance reporting tools (system detector data) and remote monitoring to verify field reports. It was common that practitioners would be notified by municipal staff and citizens when abnormal (non-recurrent) congestion and oversaturated conditions were persistent in the field. Most expert practitioners noted certain locations were habitually oversaturated and other locations where oversaturation was recurrent but confined to peak travel periods. A third class of Operation of traffic signal systems in oversaturated conditions Page 18

conditions was identified by some expert practitioners. This type of condition was either situational or caused by incident conditions such as downstream crashes. One situation cited was an on-ramp backup that would frequently cause ancillary effects at the interchange that typical signal timing cannot address. Determining the cause of the oversaturation was frequently cited as important as formulating a strategy and significant field time was typically spent in identifying the cause, and/or verifying the reports from citizens or other agency personnel. Some expert practitioners indicated their agency uses a systematic approach, including both preventative maintenance of equipment and review of signal timing performance, to review operations throughout their jurisdiction. However, this was not common to all of the including both preventative maintenance of equipment and review of signal timing performance surveyed and the focus was on broad assessment of operational conditions rather than a specific focus on oversaturated conditions. Expert Practitioners Thoughts on Strategies The including both preventative maintenance of equipment and review of signal timing performance were not explicitly asked to identify differences in objective functions (maximizing throughput, managing queues) for different strategies, but common themes emerged that the expert practitioners acknowledged that the objective during oversaturation was indeed different. The focus of most, if not all, was to first make simple changes to splits or phase sequence to minimize delay before moving to more complex approaches unless the problem was identified immediately to be an arterial or grid problem. Most expert practitioners reported spending the most time on strategies at individual intersections or small groups of signals and could articulate approaches for intersection problems much more readily than describing arterial or grid methodologies. Many of the expert practitioners stated their approach to implement strategies (where possible) was to prevent oversaturation from occurring, rather than reacting to the issues after the fact. Local Strategies Many local strategies were referenced during the interviews. Most of the expert practitioners described local strategies in the context of one or more examples. A few described a systematic process to handling issues by attempting to address the problem with a sequence of actions. While most of the expert practitioners started their discussion of strategies at the local level, most indicated that in many situations there is always a need to consider the effects of the timings at an individual location and take those ancillary effects into account, if possible. Many of the expert practitioners addressed the use of lower cycle times and reducing the number of phases at oversaturated locations to improve performance. Following is a list of the strategies for individual intersections cited by the expert practitioners: • Re-allocate split time to the oversaturated phase. Operation of traffic signal systems in oversaturated conditions Page 19

• Decrease the cycle time when more than one phase is oversaturated. More opportunities to service the queue because there is a reduced efficiency of saturation flow rate in long queues. • Increase the cycle time at the oversaturated intersection and double-cycle other adjacent intersections. • Run closely spaced intersections on one controller, like an interchange, to avoid storage of vehicles on short links. One expert practitioner mentioned attempting to run three intersections on one controller. Another expert offered a rule-of-thumb that if two intersections were less than seven seconds travel time apart, they should be run on one controller. • Use lead-lag left turns to provide more green time for oversaturated through movements. • Use phase reservice. • Use adaptive control methods. This was primarily mentioned as an approach to delay the onset of congestion and reduce the total amount of time that an intersection is oversaturated by managing conditions beforehand. • Use queue detection loops to increase the max time (select an alternative MAX2 value via logic) for an oversaturated phase. • Use queue detection to decrease phase split time (select an alternative MAX2 value via logic) when downstream link is congested. • Use split phasing based on volume detection. • Run intersection free. • Do not run intersection free (this contradicts the bullet above) as this tends to exacerbate the problems for adjacent intersections. • Change the barrier structure for certain plans by time of day (omit phases, combine movements, remove protected phases when not needed). • Run fixed time, short cycles during construction. • Run fixed time at interchanges – early return to green at other locations can actually harm progression. • Use lane control signals or signs to provide flexible capacity increases by TOD / plan. • Time detector extension times appropriately to provide swift operation. • Allow pedestrian times to exceed cycle time for infrequent pedestrian operation on side streets. For queue detection strategies, several expert practitioners reported that this was implemented using detector delays from 7-30s (seconds) and special I/O logic. When the occupancy of the detector was 100% for the duration of the delay, the operational action was then taken. Several of the experts mentioned other uses of special I/O logic for certain conditions to improve efficiency of the controller operation. Some experts warned though that special configurations should be Operation of traffic signal systems in oversaturated conditions Page 20

avoided if at all possible since this tends to reduce the maintainability of the location. It was suggested that large systems should have standardized controllers, cabinets, and detection systems, erring towards convention over configuration. Arterial Strategies Strategies for managing oversaturated arterials and grids were less commonly mentioned than strategies for individual intersections. Most expert practitioners stated they applied both negative and simultaneous offsets for queue management on arterials. None of the experts mentioned using an optimization tool for designing negative offset strategies or a systematic method for determining the settings. At least two of the experts indicated that it is more reliable to “tweak” offset values in the field based on observed conditions than relying on any software tools, particularly when residual queues are prevalent. Double cycling and combinations of harmonics (3:2, 3:1, etc.) were mentioned as methods they previously used for handling atypical demand at one or more intersections. Two expert practitioners mentioned the use of coordination strategies in heavy flow conditions that do not attempt to progress flows for more than five or six intersections. One expert practitioner stated that this strategy seemed to match with driver expectations during peak hour operations. Another expert described an arterial strategy for selecting the loser intersection along the arterial and storing traffic on the approaches to this location, to the benefit of the other intersections in the system. Similarly another expert practitioner described the importance of maintaining throughput on major arterial routes with little regard to crossing route delays – particularly during peak periods. One expert indicated that their central system allowed operators to send override commands to pre-defined groups of intersections on arterials for flushing queues (note that this capability is not unique and is available in many central systems). They described that this type of operation was typically used only for handling special events. Network Strategies At the network level, most expert practitioners mentioned metering as the predominant approach for alleviating gridlock conditions in critical areas. Identifying metering locations was primarily suggested to be done on a case-by-case basis. The experts could not offer any systematic methodology for locating metering locations but most indicated that quite a bit of time was spent on designing ingress and egress strategies for special events. At least one agency expert reported the deployment of a “traffic action team” in the field to supplement CCTV coverage and help keep traffic moving during significant special events. When gridlock occurs, several agency experts reported that they relied upon traffic officers to direct traffic. Some practitioners did not have CBD experience to draw from, but in general the least amount of time during the interviews was spent discussing oversaturation issues related to grids. Those experts with CBD timing experience mentioned the use of simultaneous offsets and fixed timing to Operation of traffic signal systems in oversaturated conditions Page 21

handle grid issues, particularly due to the lack of detection in most CBD areas. Similarly to the strategies for arterials, two experts offered the strategies for simultaneous offsets in grids to be implemented for three to five intersections, and then apply another set of simultaneous offsets in an alternating fashion (e.g. zero offsets alternating with 50% offsets). Many of the experts stated having tried traffic-responsive methods in the past with mixed results, particularly due to waffling or difficulty in setting up the parameters so that actions taken were predictable. Automated Performance Measures and Central System Diagnostics Few expert practitioners reported having access to performance measures from central systems that were adequate for diagnosing problems with oversaturation. Most did indicate they had used system detector reports including volume and occupancy information when they were available. One expert practitioner’s system includes an arterial incident detection algorithm that helps to identify locations that are experiencing non-recurrent congestion. This method continuously compares current detector information with historical statistics in order to identify anomalous conditions. The expert practitioner noted that in a jurisdiction of appreciable size, automated methods such as this are necessary. The expert practitioners were split between a preference for automated handling of problems or central system features that would identify potential issues and bring them to the attention of staff. In general, all agreed that more diagnostics from either the local controller or the central system would be helpful to improve operations. One expert practitioner described a smart archiving capability that would allow queries to be processed such as “provide the top 10 heaviest left turns in the system over the last six months” or “tell me which non-coordinated phases maxed out most often during peak period.” Several expert practitioners stated that a dynamic max capability during free would be useful for a local controller. If the max for a phase was serviced several times in a row, the value would be increased (note that this feature is currently available in at least one controller firmware). Detection Systems and Needs All of the expert practitioners agreed that reliable detection systems are critical for adequate operation. Most indicated that in their jurisdictions a fully-actuated detection scheme was most common. Several experts had disapproving remarks about the limitations of video detection systems and indicated that a hybrid detector (inductive + radar, video + radar, infra-red, etc.) is probably the most likely candidate to overcome the limitations of a particular detection technology. Several of the expert practitioners mentioned the need for detection systems that can monitor turning counts and the importance of deploying enough appropriate detection systems. One expert practitioner noted that long cycles were more susceptible to detector failures on side streets (wasted green time) and that shorter cycles could mitigate the effect of detector failures. Operation of traffic signal systems in oversaturated conditions Page 22

Two expert practitioners noted that adaptive control algorithms need to be advanced to the point where they can explicitly handle oversaturation and accommodate the (inevitable) failures in detection systems when they occur. European representatives mentioned reasonably effective algorithms for measuring very long queues from standard placements (25m from the stop bar) could be found in adaptive systems such as MOTION. Summary of Expert Practitioner Interviews In general, the expert practitioner interviews confirmed there are a number of strategies that have been and can be applied to management of oversaturated conditions. The discussions focused more on local intersection and arterials issues than on network and grid issues. This is not surprising since network problems are especially difficult to conceptualize and “get one’s hands around” as stated by one expert practitioner. Expert practitioners tend to be more interested in solving problems than documenting effectiveness, so (like much of traditional traffic engineering) quantitative benefits of applying a particular strategy were not described by any expert. This lack of evidence of the approach effectiveness is exacerbated by the fact that automated data collection becomes problematic during congested condition. It was clear that the experts provided an adequate solution for a given situation and then moved on to other pressing field problems. The expert practitioners were united in the opinion that central systems and field controllers need more comprehensive and robust performance monitoring and diagnostics. When analyzing oversaturated conditions, most of their data collection is done in the field with personal observations.. The experts were split in the opinion that existing detection technology was adequate or that new technologies were necessary. They were united in the opinion that proper site identification, installation, and technology choice was critical to obtaining reliable information from existing detection systems. Summary of Motivation and Background From these considerations and the guiding principles provided by the experts, the research was focused on the following four areas of emphasis: • Development of quantitative measures of oversaturated conditions from traditional detection systems • Development of a multi-objective methodology to develop and evaluate mitigation strategies and combinations of strategies for complex scenarios • Development of an online tool to directly relate quantitative measures with selection of mitigation strategies • Documentation of application test results of mitigation strategies to specific real-world test cases Operation of traffic signal systems in oversaturated conditions Page 23

In addition to these four areas, the research also resulted in a rational guide for practitioners to identify oversaturated scenarios and apply appropriate strategies. The research focused on identifying traffic control strategies that can be implemented by traffic signal systems to handle certain types of oversaturated conditions on surface streets. The research did not address freeway operations, geometric reconfiguration, re-routing, traveler information, or other strategies that seek to influence travel demand, departure time choice, or route choice. Nor does the guide explicitly address strategies or oversaturated conditions for modes of travel other than private vehicles (including buses, pedestrians, bicycles, or trains). There were seven tasks in this research project: • Task 1: Literature Review • Task 2: Development of Diagnosis Methods and Definitions • Task 3: Development of Strategies and Objectives • Task 4: Synthesis of Interviews with Experts • Task 5: Interim Report • Task 6: Preparation of Practitioner Guidance • Task 7: Application of Guidance and Strategies on Test Scenarios The detailed literature review will be presented in Appendix A. Chapter 2 presents the summary of the research approach for Tasks 2 and 3. Chapter 3 summarizes the findings of Task 7. Task 6 (guidance documentation) is a stand-alone document provided as a supplement to this document. Chapter 4 summarizes the project and provides conclusions and recommendations for future research. Operation of traffic signal systems in oversaturated conditions Page 24

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Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 202: Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report documents the procedures and methodology used to develop quantitative metrics for oversaturated traffic conditions, identify operational objectives based on observed conditions, develop a methodology for generating timing plan strategies to address oversaturated scenarios, and develop an online tool to relate measurement of oversaturated conditions with pre-configured mitigation strategies.

Guidance to assist in the process of matching mitigation strategies with specific oversaturated condition scenarios is found in NCHRP Web-Only Document 202: Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 1 – Practitioner Guidance.

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