Problem 5: Signalization of Okeechobee Road

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The Manual of Uniform Traffic Control Devices (MUTCD) is the primary resource for determining whether a traffic signal is the appropriate application for a particular location. The MUTCD has a series of signal warrants that evaluate various aspects of an intersection to provide the engineer with guidelines for consideration in making this decision.

An HCM analysis of an unsignalized intersection alone is not enough to justify that a traffic signal is warranted at a particular location. In the case of this particular intersection, the engineering study considered the primary function of Okeechobee Road, adjacent facilities that could be considered, and the feasibility of a higher type interchange in concert with the operations of the Krome Avenue northbound movement.

That being said, we have estimated that a TWSC will result in significant delay to the northbound left-turn movement onto Okeechobee Road from Krome Avenue, and a traffic signal should be considered as a potential control alternative to accommodate the projected traffic volumes. Problem 5 will explore various aspects of the HCM treatment of signalization as it applies to this intersection.

Procedures are provided in HCM Chapter 16 for the analysis of signalized intersections. The procedures may be applied to each lane group to produce separate control delay estimates. The lane group values may then be combined in a volume-weighted manner to produce aggregate estimates for each approach and for the intersection as a whole.

Sub-problem 5a. Two-phase Traffic Signal Control

Sub-problem 5b. Three-phase Traffic Signal Control, adding a protected left turn

Sub-problem 5c. Pre-timed vs. Traffic-Actuated Operation

Problem 5: Signalization of Okeechobee Road

The HCM suggests in Chapter 16 that free-flowing right turns that are not under signal control should be removed entirely from the analysis. We have already established that the northbound and eastbound right turns are free flowing because of channelization. Therefore, neither of these movements will be considered as a part of the signalization.

The current TWSC operation at this intersection provides only one lane for the northbound left turn (see photo). Because of the available space and the capacity advantage of a second lane under signal control, two lanes will be assigned to this movement. In addition, because of the geometrics of this T intersection, the northbound left turn has more of the characteristics of a through movement than a left turn. Therefore, for purposes of signal analysis, the northbound left turn will be considered as a through movement.

The signal analysis sub-problems will be based on the following demand volumes:

Discussion:
Consider the following issue as you proceed through this problem: what criteria is necessary to define right turns as free-flowing right turns? Take a few minutes to consider this question. When you are ready to continue, click continue below to proceed.

Sub-problem 5a: Two-Phase Traffic Signal Control

Step 1. Setup

A two-phase control plan provides no protected phases for any of the left turns that are opposed by through traffic. The westbound approach has the only left-turn movement in this category. With a volume of 120 vph, it is conceivable that this movement could be accommodated without a protected phase.

Some agencies would decide to provide a protected left-turn phase for this movement, without further analysis, because of the high speed (50 mph) of the approaching traffic. This is a legitimate issue. The two-phase alternative is therefore presented in this sub-problem primarily as an illustration of the details of the HCM procedures. For some agencies, the outcome of this analysis will be a moot point with respect to the decision itself.

We will examine the two-phase alternative using both the Quick Estimation Method (QEM) presented in HCM Chapter 10 and the full operational analysis procedure presented in HCM Chapter 16.

Consider the following as you proceed through this problem:

Discussion:
Take a few minutes to consider these questions. Click continue when you are ready to proceed.

Sub-problem 5a: Two-Phase Traffic Signal Control

Step 2: Results

The QEM represents an extension of a technique known as Critical Movement Analysis, or CMA. This technique has appeared in the literature in several forms and is intended primarily for planning level analysis, though it is also very useful in both operational and design reviews. The QEM produces an estimate of the status of the overall intersection with respect to its capacity, based on the assumption that the signal timing plan will produce an equal degree of saturation among the critical movements on each phase. The intersection status is determined from the sum of the v/c ratios for the critical movements on each phase.

The QEM produces, as a by product, a synthesized signal operating plan consisting of:

The HCM offers the caveat that the synthesized plan may not be suitable for implementation because it does not include important considerations such as minimum green times. Nevertheless, it usually provides a good starting point for an operational analysis, which requires the signal timing plan to be specified along with several other items of geometric, operational, and traffic data that are not always available at the planning level.

Sub-problem 5a: Two-Phase Traffic Signal Control

Both the QEM and the operational analysis procedure have the same underlying logic. In many places, the QEM uses assumed or default values and the operational analysis procedure uses more precise site-specific data. Therefore, some differences can be expected in the results, and those differences are generally attributable to the approximate nature of the QEM.

The QEM provides two checks to evaluate the need for a protected left turn on each approach. The first involves computing the product of the volumes for the left turn and its opposing through movement. The cross product criterion has been described in the literature as a popular technique that generally preceded the availability of more complex computational models. Different cross product thresholds have been adopted by different agencies, and the threshold values are generally dependent on the number of available lanes. Thresholds of 50,000, 90,000 and 110,000 are typically recommended for 1,2 and 3, through lanes respectively.

Sub-problem 5a: Two-Phase Traffic Signal Control

The application of the QEM cross product check to this sub-problem is summarized as follows:

So, the QEM cross product check suggests the need for a protected left turn.

The second left-turn protection check in the QEM involves the determination of sneaker capacity, i.e., the number of left turns that can be accommodated assuming that two vehicles proceed at the end of each permitted phase. Sneakers are considered to create extra capacity for a left-turn movement. The HCM suggests that most of the capacity for permitted left turns will be generated by sneakers whenever the through vehicle equivalence of a left turn exceeds 3.5. HCM Exhibit C16-3 provides a table that expresses the left-turn equivatents as a function of the opposing flow and the type of left turn lane (shared or exclusive). The QEM prescribes a check for sneaker capacity when the left-turn equivatents suggest that most of the capacity will be associated with sneakers.

Sub-problem 5a: Two-Phase Traffic Signal Control

The application of the QEM sneaker check to this sub-problem is summarized as follows:

So, the QEM sneaker check also suggests the need for a protected left turn, because the left-turn demand (120 vph) is greater than the apparent sneaker capacity (60 vph).

The operational procedure presented in HCM Chapter 16 offers the perspective of delay and v/c ratio for the left-turn movement. These more detailed performance measures are beyond the scope of the QEM. Applying the operational procedure using the two-phase signal timing plan given by the QEM, we see the following results for the WB left turn:

These results confirm from three different perspectives that a signal operation is unlikely to accommodate the westbound left turn satisfactorily without a protected phase. The next sub-problem will examine a three-phase operation with protection for this movement.

Sub-problem 5b: Three-Phase Traffic Signal Control with a Protected Westbound Left Turn

Step 1. Setup

Having established the need for westbound left-turn protection in Sub-problem 5a, we will now examine the HCM treatment of protected left turns. Because of the isolated and high speed characteristics of this intersection, we would expect to implement a control scheme in which all movements are traffic-actuated. We will, however, limit the investigation to pre-timed control in this sub-problem, leaving traffic-actuated control for Sub-problem 5c. There are two reasons to separate the control treatment into different sub-problems. First, we can get a better idea of how these control modes differ if we examine both of them in detail. Second, the timing plan based on pre-timed control is often a useful input into the analysis of traffic-actuated operation.

Consider:

Discussion:
Take a few minutes to consider these questions.  Click continue when you are ready to proceed.

Sub-problem 5b: Three-Phase Traffic Signal Control with a Protected Westbound Left Turn

Step 2: Results

The signalized intersection operational procedure requires a full specification of the signal timing plan as input data. Appendix A to HCM Chapter 16 provides some guidance on timing plan development. Three timing plan strategies are mentioned in the HCM:

1. Equalizing the degree of saturation among the critical movements on each phase.
2. Equalizing the delay among the critical movements on each phase.
3. Minimizing the total delay to all vehicles using the intersection.

Each of these strategies was applied to the intersection in question. To facilitate comparison of the effect of the strategy, the cycle length was fixed at 120 sec. The results are summarized for comparison in Exhibit 3-39.

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

The Equal v/c Strategy
The equal v/c strategy was explored to some extent in the last sub-problem as the basis for the QEM timing plan synthesis. The QEM results are repeated in one column of Exhibit 3-39. The next column shows what happens when the QEM timing plan is transferred directly into the operational procedure. The following observations are offered:

The default yellow plus all red time for the QEM is 4 seconds per cycle per phase. The unusually wide intersection and high speed approaches dictates a longer inter-green period. For purposes of this discussion, values of 5 sec yellow and 1 sec all red will be used as an approximation of the local agency practice. While a 4-second inter-green is generally a reasonable default value for planning level analysis, this is a case in which the QEM assumptions do not apply to the intersection in question.

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

This analysis was repeated with two modifications. First, the inter-green times were increased from 4 sec to 6 sec. Second, the green times were redistributed by trial and error to produce a closer agreement between the v/c ratios for the critical movements. The results are shown in the next column of Exhibit 3-39. The following observations are offered:

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

The Equal Delay Strategy
The equal delay strategy was implemented by redistributing the green times, again by trial and error, to produce a closer agreement between the delays for the critical movements. The results are shown in the next column of Exhibit 3-39. The following observations are offered:

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

These observations reinforce the notion that, in the search for an equitable distribution of green times, there is a tradeoff between equalizing the v/c ratios and equalizing the delays. The question of which strategy is preferable raises an interesting philosophical question. Note that equalizing the delays has reduced the overall intersection LOS from C to D. So a reporting scheme that considers only the overall LOS would tend to favor the equal v/c strategy. On the other hand, the improvement in overall intersection LOS was achieved at the expense of the lower volume movements that were forced to operate at LOS E and F. So, a reporting scheme that is concerned with individual movements might look more favorably on equalizing the delay.

This debate might spawn a third strategy, namely that of equalizing the LOS among the competing movements without worrying too much about differences in delay. The results would be expected to fall somewhere between the two strategies that we have explored.

Now here is a question to ponder: why is the overall intersection delay of 42 sec per vehicle lower than the delays for any of the movements shown in Exhibit 3-39? The answer is that our analysis has focused on the critical movements and has neglected other movements such as the WB through traffic, which was not involved in any of the computations for the signal timing strategies we explored. The procedure prescribed by the HCM for estimating overall intersection LOS takes all movements into account, not just the critical movements.

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

The Minimum Delay Strategy
The final strategy mentioned in the HCM is that of minimizing the sum of the delays to all vehicles entering the intersection. This strategy seeks an optimal solution instead of an equitable solution to the design of the signal timing plan. The overall intersection delay shown in Exhibit 3-39 was 24.5 sec/vehicle for the equal v/c strategy, compared to 42 sec/vehicle for the equal delay strategy. It is this performance measure that we seek to minimize. 

The minimum delay strategy also requires an iterative process that can be implemented either with available software or by trial and error. It is common practice to start this process using the equal v/c strategy as an initial solution. Following this practice, it was found that the equal v/c solution could not be improved upon by redistributing the green times. In other words, it was not possible to lower the overall intersection delay below 24.5 sec/vehicle. So we must conclude that the equal v/c solution in this case was also the minimum delay solution. 

While this outcome should not be interpreted as a general signal timing principle, it will be found that the outcomes of these two strategies are frequently not far apart at isolated intersections, such as the one in question. Delay minimization is a more important strategy in coordinated systems where other design parameters related to the quality of progression between intersections must be optimized.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation 

Step 1. Setup

In Sub-problem 5b, we explored various strategies for allocating green time with pre-timed control. Because of the isolated high-speed nature of this intersection, it is important that traffic-actuated control be used. In this sub-problem, we will examine the HCM treatment of traffic-actuated control to see how it differs from pre-timed control. 

Consider:

Discussion:
Take a few minutes to consider these questions.  Click continue when you are ready to proceed.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation

The signal timing specification for traffic-actuated control has the same format on the HCM worksheets as pre-timed control. The phasing plan is required in both cases, along with the allocation of green times. In the case of pre-timed control, the green times are expected to be the actual green time settings for the controller. In the case of traffic-actuated control, the actual green times will vary from cycle to cycle. The values expected by the HCM procedure are the average green times that will be generated over the analysis period by the controller logic. Appendix B to HCM Chapter 16 gives guidelines for determining average green times.

What are the primary operation effects of using actuated control? The input data associated with traffic-actuated control (e.g., lower cycle lengths) will often produce lower delay estimates than those associated with pre-timed control. There is only one difference in the analytical treatment of pre-timed and traffic-actuated control at isolated intersections. That difference is found in the k parameter of the incremental delay equation. HCM Exhibit 16-3 gives a table of k values that account for traffic-actuated control as a function of the unit extension and the v/c ratio.

What additional information is needed beyond the data already used in Sub-problem 5b? Because the v/c ratio is already computed for pre-timed control, the only additional piece of information you need to apply the procedure to traffic-actuated control is the unit extension time setting. This setting represents the duration of the gap between vehicles on the approach being serviced that will cause the controller to terminate the phase. Smaller unit extensions will produce smaller k values, and smaller k values will produce lower delay estimates. For purposes of this discussion, we will adopt the default setting of 3.0 seconds given in HCM Chapter 10.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation

Step 2: Results

Three cases involving different timing plans will be examined in this sub-problem: The first will use the optimal timings from Sub-problem 5b. The difference in the results will be due solely to differences in the HCM treatment of traffic-actuated control. The results are summarized in Exhibit 3-40.

Note that there were no differences in the v/c ratio, g/c ratio, or uniform delay values in the comparison of pre-timed and traffic-actuated operation. The k values were all 0.5 for pre-timed control and 0.37 for traffic-actuated control. The reduction in the k values produced a slight decrease in the incremental delay for all movements, and this decrease was reflected in the control delay. The overall intersection delay decreased by 1.5 seconds per vehicle, or about 6 percent.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation

One of the purported benefits of traffic-actuated control is a more efficient timing plan. Appendix B of HCM Chapter 16 provides a method for estimating the average green times as a function of the traffic volumes, geometric configuration, left turn treatments, and detector parameters. The methodology of this appendix is iterative and somewhat complex analytically. It cannot be applied productively without software designed for the purpose.

While all of the controller and detector parameters can influence the cycle length and green times for actuated control, the most important setting is the maximum green time for each phase. The HCM suggests that it is clearly essential that some maximum green times must be imposed to control the apportionment of time between the competing phases. 

A number of techniques for assigning maximum green times have appeared in the literature. Most techniques involve some adjustment to the optimal green times for pre-timed operation. Three different ways of using the pre-timed optimal green times to determine the actuated average green times will be examined in this sub-problem:

1. Direct use of the optimal green times as average green times, as discussed in the first example in this sub-problem.
2. Application of the HCM appendix with maximum green times set equal to the optimal green times.
3. Application of the HCM appendix with maximum green times set equal to 150 percent of the optimal green times.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation

The results are presented on the next page in Exhibit 3-41. While there was a wide variation in the resulting cycle length, there was very little difference observed in the overall intersection delay. The shortest cycle, 109 sec, was obtained by using the pre-timed optimal greens as maximum greens. The longest cycle (140 sec) was obtained from 150 percent of the optimal greens. 

So, what have we learned? In this example, the only way that traffic-actuated control could beat optimized pre-timed control in this table is by assuming that the actuated controller would generate an optimal timing plan; consistent with the optimized pretimed average green times. Of course, this is all predicated on the completely unrealistic assumption that the approach traffic volumes will be identical for each day and for every cycle. On the other hand, there is nothing built into conventional actuated control logic that maintains average splits at an intersection without tight control of the maximum green times.

How can actuated control improve the safety of a signalized intersection? Someone might question whether traffic-actuated control at high-speed isolated intersections should be applied because of the operations analysis. The answer is an emphatic yes, because of the dilemma zone protection that is possible with the proper placement of detection zones on each approach and proper timing settings within the controller. Certainly, support for this answer will not always come from an HCM analysis, but it is always important to keep considerations such as this in mind when making implementation decisions.

Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation