should be computed for each segment for the subject time period. These indices should be plotted on a map of the street system and used to identify logical groupings of signalized intersections. A separate signal system should be established for each group of intersections. Guidelines for implementing this step are
Essential data are identified in the two bullet lists in this section and should be collected during a site visit.
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provided in the next part of this chapter in the section titled Guidelines for Determining Coordination Potential.
Step 3. Assess Degree of Saturation
During this step, the presence of over-saturation should be assessed for each phase and intersection. As defined in the previous part of this chapter, saturation can be identified by the presence of an overflow queue at the end of the green indication. The objective of this evaluation is to determine the number of movements that are over-saturated (if any) and whether they result in bay overflow or spillback into an upstream intersection.
If only one phase is experiencing over-saturation, a timing plan that minimizes overall delay may provide a useful starting point. However, this plan should be “tuned” (i.e., phase splits adjusted slightly) such that the over-saturation is eliminated or reduced to the point that it does not cause overflow or spillback.
If many conflicting movements are experiencing over-saturation during a common time period, then a queue management timing plan that allocates cycle time in a manner that minimizes the disruption caused by spillback and overflow may be appropriate. This plan may be initially based on a minimum-delay timing plan, but it must be tuned such that queues are formed only in the least damaging locations and that traffic is progressed away from the over-saturated intersections. Step 4. Determine Controller Settings
During this step, the controller settings are determined based on consideration of the information obtained in the previous steps. The settings that are determined during this step can vary but are likely to include force mode, coordination mode, cycle length, phase splits, and offsets. These settings were defined in the previous part of this chapter. The tasks typically undertaken during this step include:
1. Determine force mode and coordination mode. 2. Determine cycle length.
3. Determine phase splits. 4. Determine offsets.
Guidelines are provided in the next part of this chapter to assist the analyst in making the determinations associated with each task. Guidelines for determining values for minimum green, yellow change interval, red clearance interval, walk interval, pedestrian change interval, and passage time are provided in Chapter 2. Guidelines for determining signal phase sequence are provided in Appendix A. There is also some discussion of phase sequence in the Guidelines part of this chapter. Guidelines for using advanced signal timing settings are described in Appendix B. Guidelines for designing the detection layout are provided in Appendix C.
There are several software products and spreadsheets available that automate many of the signal timing tasks. Most of these products can be obtained from the Center for Microcomputers in
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Transportation (McTrans) at the University of Florida (http://mctrans.ce.ufl.edu/). If traffic volume data are provided, then several of these products can also be used to evaluate the proposed controller settings in terms of their expected impact on intersection efficiency.
Step 5. Install, Evaluate, and Refine
The last step in signal timing plan development relates to the implementation and field verification of the proposed controller settings. This step consists of the following five tasks:
1. Install the settings in the signal controller.
2. Put the settings in operation during an off-peak period, and observe traffic behavior. 3. Refine the settings if so indicated.
4. Put the settings in operation during the intended period, and observe traffic behavior. 5. Refine the settings if so indicated.
The goal of the two refinement tasks is to make small changes in the settings, such that overall system safety or efficiency is improved. GUIDELINES
This part of the chapter provides guidelines for selecting key elements of the timing plan for a coordinated signal system. The information provided is based on established practices and techniques that have been shown to provide safe and efficient system operation. The guidelines address coordination potential, number of timing plans, system settings, and phase settings. Guidelines for Determining Coordination Potential
This section provides guidelines for identifying street segments that have some potential to benefit from coordination. The objective of these guidelines is to identify signals that will operate in an efficient manner if grouped together in a signal system. The evaluation is specific to a given traffic period (e.g., morning peak). It should be repeated for each traffic period of interest. Different signal groupings may be identified during different traffic periods and should be accommodated in the signal timing plans to the extent possible.
Coupling Index Calculation
As a first step, the coupling index should be computed for each street segment for the traffic period of interest. The coupling index for a segment can be estimated using Figure 3-1. The following guidelines are offered for interpreting the index values:
During Task 2, it may be useful to set all phases to maximum recall for a period of time sufficient to confirm that the intended minimum progression band is provided.
A coupling index of 0.5 or more indicates the potential for significant benefit from signal coordination.
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0 500 1000 1500 2000 2500 0 1000 2000 3000 4000 5000
Street Segment Length, ft
S e g m e n t V o lu m e (2 -wa y to ta l) , ve h /h CI = 0.9 0.7 0.5 0.3 0.1 CI = V/L where, CI = coupling index V = 2-way volume, veh/h L = segment length, ft
! 0.3 or less: unlikely benefit from coordination;
! 0.3 to 0.5: segment likely to benefit if mid-segment access point activity is low and turn bays are provided on the major-street at each signalized intersection; and
! 0.5 or more: likely benefit from coordination.
Experience in using this index indicates that segments shorter than 2600 ft will typically be coordinated, and those longer than 5300 ft will not typically be coordinated.
Figure 3-1. Coupling Index for Signalized Segments. Index Evaluation Procedure
To facilitate an examination of the computed indices, they should be plotted on a map of the street system being evaluated. Logical signal system groupings will be identified by collections of segments with high index values. The boundary between each system will be identified by a segment with a low index value. When an intersection is common to two systems, the coupling index can be used to prioritize the assignment of the signal to a particular system.
Figure 3-2 illustrates the mapping concept for a urban area with a downtown grid street network and an east-west arterial street that is bisected by a railroad line. The numbers shown represent the computed indices. The three signal systems formed include all signals with a coupling index of 0.5 or more. The railroad line forms a logical boundary segment for Systems 2 and 3.
Two other factors should be considered when forming signal systems. They are:
! Adjacent segments with similar directional traffic volume levels should be grouped.
! Crossing arterial streets with high index values on both streets and significant traffic volume should be used to define a system boundary.
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Figure 3-2. Application of Coupling Index to Form Separate Signal Systems. Considerations of Signal System Size
In general, small to moderate sized signal systems (i.e., 30 intersections or less) tend to be easier to manage and maintain. Large systems require considerable resources to develop, install, and maintain the signal timing plans. Also, they will often present a greater challenge in finding an optimal timing plan. Finally, with large systems, there is a significant increase in infrastructure associated with each signal added to a system and a corresponding increase in exposure to system performance degradation when any one element fails.
Guidelines for Determining Number of Timing Plans A timing plan needs to be defined
whenever a change in phase splits, offset, or cycle length is needed. Such changes are often justified by changes in traffic volume that occur during the course of the day and, sometimes, by
the day of week. As a minimum, one timing plan should be developed for each of the following periods:
! morning peak,
! evening peak, and
! off-peak.
Additional plans may be needed for noon peak or weekend traffic periods.
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Eighty percent of signal systems operated by TxDOT include at least three plans.
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Guidelines for Timing Plan Transition
A timing plan should be implemented before the start of the traffic demand period for which it was developed, especially if the subject traffic period is a peak period. If a plan is implemented after the start of a peak period, it may cause significant disruption to traffic flow.
There are some inefficiencies with the transition from one plan to another that suggest that a plan should be allowed to operate for a minimum time to achieve a break-even point between the delay caused by transition and the delay saved by the new plan. As a rule-of-thumb, the minimum time a plan should operate should be the larger of 15 minutes or 10 times the travel time through the signal system (6). This rule is most applicable to traffic-responsive timing plan implementation. Guidelines for System Settings
This section provides guidelines for selecting key controller settings that define the operation of a signal when it is part of a signal system. The settings addressed include:
! cycle length,
! offset,
! phase sequence,
! force mode,
! transition mode, and
! coordination mode.
Cycle Length
The cycle length used in the signal timing plan often represents a compromise value based on consideration of capacity, queue length, and progression quality. These considerations include:
! Longer cycle lengths increase capacity; however, the increase in capacity is only about 1 percent for a 10 s increase in cycle length.
! Shorter cycle lengths reduce delay, provided that the cycle length is not so short that there is inadequate capacity.
! Longer cycle lengths increase queue length.
! Longer cycle lengths tend to be more conducive to finding a two-way progression solution. Operational Considerations. If the
intersection is under-saturated, then preference is often given to having it operate at its minimum-delay cycle length. This cycle length is typically in the range of 60 to 150 s. If a phase or an intersection is over-saturated, then priority should be given to using a shorter cycle
for the purpose of minimizing queue spillback, turn bay overflow, or both.
For under-saturated intersections, use a minimum-delay cycle length.
For over-saturated intersections, use a shorter cycle length to minimize spillback.
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