Problem 4: Okeechobee Road Stop-Controlled Analysis

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Exhibit 3-24. Okeechobee Road Configuration

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This intersection is located at the north end of the Krome Avenue study area. It is a T intersection currently stop-controlled, with the northbound movement stopped in favor of all east-west traffic. The intersection geometrics are shown in Exhibit 3-24.

This is the only movement on the entire Krome Avenue route that is stop controlled. It is currently operating beyond capacity. By 2010, volumes will exceed the capacity by a substantial amount.

The eastbound right turn is unopposed, because the T intersection configuration and the geometrics allow it to flow freely. The northbound right turn merges with the eastbound through movement at a point approximately 400 ft east of the intersection. Both the northbound and westbound left turns conflict with the eastbound through traffic. The critical movement is the northbound left turn.

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Problem 4: Okeechobee Road Stop-Controlled Analysis

Stop-controlled intersection analyses will be treated in four separate sub-problems:

Sub-problem 4a will examine the capacity of the critical minor street movement (the northbound left turn), using the graphical solution presented in the HCM, without going through the full procedure.
Sub-problem 4b will invoke the full HCM procedure, treating the operation as a conventional stop-controlled intersection and ignoring the unusual separation between the conflict points.
Sub-problem 4c will separate the conflict points for stop-control and treat each conflict point individually.
Sub-problem 4d will consider the question of how best to analyze the capacity of the northbound right turn, which is well removed from the intersection operation by channelization.

Questions to consider as you proceed through this problem:

• How do you think the analysis methods compare to the actual performance of the intersections?

• Do you feel that this should be modeled as a single intersection?
• How does the merge play a role in the intersection capacity?

Discussion:
Take a few minutes to consider these questions. When you are ready to continue, click continue below to proceed.

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Sub-problem 4a: Potential Left-Turn Capacity

Step 1. Setup

Exhibit 3-26 shows the northbound left-turn queue at the Okeechobee Road Intersection. Observe the number of heavy vehicles in the traffic stream.

The heavy northbound congestion evident in the current operation (see Exhibit 3-26) suggests that the capacity of the northbound left turn should first be examined by the basic principles set forth in the HCM before the full procedure is invoked by software. This step will give us a better understanding of the basic relationships that apply to TWSC control.

Consider:

• What volume-related factors affect the northbound left-turn movement capacity?

• What geometry-related factors affect the northbound left-turn movement capacity?

• How could we take into account the separation of the roadways?

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

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Sub-problem 4a: Northbound Left-Turn Capacity

Step 2: Results

What volume-related factors affect the northbound left-turn movement capacity? The basic relationship between movement capacity is defined by the conflicting flow rate and the driver characteristics (critical gap and follow-up time). Using what are widely considered default values for critical gap and follow up time as described in HCM Exhibit 17-5, one can graphically represent the relationship as shown below (which is similar to HCM Exhibit 17-7). This exhibit shows how the capacity for the stopped movement decreases as the conflicting volume (flow ratio on the x axis) increases. At very high levels of conflicting traffic, the capacity for the stopped movement becomes effectively zero because the availability of acceptable gaps is eliminated.

What geometry-related factors affect the northbound left-turn movement capacity? How could we take into account the separation of the roadways? A review of the aerial shows that the median space provides a potential refuge for vehicles that use two-stage gap acceptance. The right-turning traffic is removed from the intersection. Thus, consideration of the northbound left-turn movement becomes the eastbound through traffic as the only opposing movement in consideration of the first part of the two stage movement. The analysis that follows considers the first stage in consideration of the capacity of the northbound Krome Avenue left-turn movement. It should be noted that this is a simplification and may not consider the operation of vehicles in the median blocking northbound left-turn traffic from initiating this first stage of the two-stage gap acceptance. In this example, it is clear that the primary conflict under these traffic conditions is the eastbound through movement which is significantly higher than the westbound through movement.

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Sub-problem 4a: Northbound Left-Turn Capacity

Exhibit 3-27 shows several lines plotted on this exhibit which represent different types of stopped movements (through, left, etc.). The dashed red line has been added to identify the relationship for the minor street northbound left-turn movement. The westbound left-turn movement (solid black line) represents the potential capacity for the major street left turn, which must yield to the oncoming eastbound traffic and has a higher capacity than the movements entering from the minor street.

The dotted blue lines near the bottom of the graph represent the demand volume (Volume) and the estimated potential capacity (Capacity) based on the conflicting volume for the northbound left-turn movement.

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Sub-problem 4a: Northbound Left-Turn Capacity

The logical conclusion to draw from the graph on the preceding page is that the minor street conflicting movement volume is too heavy to permit a viable TWSC operation at this intersection. Without going into the numbers, the graphical presentation indicates that the demand volume is considerably higher than the capacity. Keeping in mind that the potential capacity for a movement does not consider the competition from other movements at the same priority level, it will generally represent an optimistic assessment of the capacity. When even this optimistic assessment fails, you would conclude that there is no point in proceeding any further with the investigation of TWSC.

Normally you would stop at this point and look at some other control alternatives. We will, however, carry the TWSC concept into a couple of other sub-problems to illustrate some features of the HCM analysis procedure and to set the stage for the consideration of other alternatives.

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Sub-problem 4b: Two-way Stop Controlled Analysis

Step 1. Setup

The unusual geometrics, especially the physical distance separating the conflicting movements at this intersection, will require some thought about how to represent the intersection for analysis by the HCM procedures. The conventional intersection conflict points are shown in Exhibit 3-28. Because of the wide separation of conflicts at this intersection, it should occur to us that we probably shouldn’t treat this situation as a typical urban intersection.

In this sub-problem, we will carry out a conventional intersection analysis. Then we will examine the results to determine if our treatment was appropriate.

Consider:

• What movements are considered in the HCM procedures?

• What is the basis for determining LOS in the unsignalized intersections methodology?

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

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Sub-problem 4b: Two-way Stop Controlled Analysis

What movements are considered in the HCM procedures? The HCM procedures compute the capacity, control delay, and level of service for all movements that must yield to other movements, including the left turns from the major street. Through and right-turn movements on the major street are excluded from the analysis and are assumed to have no delay.

This simplifying assumption raises a point of interest. Heavy vehicles making right turns will sometimes cause significant delays to traffic on the major street. This phenomenon is overlooked by the HCM procedure. If such delays are of concern to a particular analysis, it will be necessary to apply microscopic simulation modeling tools to supplement the HCM analysis. For purposes of this discussion, we will assume that traffic delay to the through movements on the major street is not an issue.

What is the basis for determining LOS in the unsignalized intersections methodology? The level of service is based on the control delay according to Exhibit 3-29. HCM Chapter 17 prescribes the full procedure for computing control delay.

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Sub-problem 4b: Two-way Stop Controlled Analysis

Step 2. Results

The results of this analysis are presented in Exhibit 3-30. These results reaffirm the conclusions drawn from sub-problem 4a, specifically that TWSC is not a viable control alternative. The v/c ratio for the NBL movement was 3.72, i.e., the volume was 372% of the capacity. The NBR movement v/c ratio was 1.92.

The WBL movement, on the other hand, appears to be operating within its capacity, with a v/c ratio of 0.71. This presents an interesting contrast with the NBL movement, since both movements have to contend with the same conflicting volume (i.e., 2,010 vph from the WBT). The difference may be seen in both the graphical representation of Exhibit 3-27 and the numerical presentation on Exhibit 3-. In the graph, the line representing the WBL shows a much higher capacity for a given level of opposing volume than the line representing the NBL movement.

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Sub-problem 4b: Two-way Stop Control with a Normal Urban Intersection Treatment

Exhibit 3-30 explains why these lines are in different places. The formula for computing the capacity of a movement that must yield to an opposing movement is given in HCM equation 17-3. This equation contains two parameters:

1. The critical gap, which indicates the length of a gap in the opposing flow required to accommodate the first queued vehicle trying to cross the opposing flow.

2. The follow-up time, which indicates the additional gap length required to accommodate each subsequent vehicle entering the same gap in the opposing traffic.

Larger values for each of these parameters will lower the capacity for the entering movement. The values shown in Exhibit 3-30 indicate lower values for the left turn crossing the opposing traffic than for the minor street entry movements. This indicates that drivers making left turns from the major street are willing to accept shorter gaps in the opposing traffic than drivers that are entering the major street from a minor street approach. The result is a higher capacity for the WBL movement compared to the NBL movement.

One of the objectives of this exercise was to judge whether it is appropriate to consider the intersection in the context of a normal urban intersection with TWSC control. This task can be accomplished best by comparing the results in Exhibit 3-30 with the corresponding results obtained by treating each of the conflict points separately. We will examine the separation of conflict points in the next sub-problem.

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Sub-problem 4c: Separating the Conflict Points for TWSC Control

Step 1. Setup

Because of the wide median and the high speed rural type channelization of the right turns, it could be argued that the Okeechobee road intersection is likely to operate not as the single urban intersection considered in sub-problem 4a but as four separate intersections, with each intersection representing one of the conflict points, as shown in the diagram at the right. The separation of conflict points will usually give a more optimistic assessment of the operation than will the aggregation of conflict points into a single intersection. 

Consider:

• Why would separating conflicts produce a more optimistic assessment of the intersections?

• How are the conflict points inter-related?

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

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Sub-problem 4c: Separating the Conflict Points for TWSC Control

Let's consider the questions from the previous page.

Why would separating conflicts produce a more optimistic assessment of the intersections?  Separating conflicts may produce a more optimistic assessment, because as conflicting streams of traffic are removed from consideration, it results in more opportunities for acceptable gaps. The relationship is exponential and depending on conditions that may result in significant overestimation of capacity. For this reason, caution must be used when separating the conflict points for an unsignalized intersection.

How are the conflict points inter-related?  The most obvious relationship between the conflict points is how the paths of vehicles overlap multiple conflict points. For example, the northbound left-turn movement must pass through two points. Thus, if the second conflict path (northbound left turn and westbound through) is currently blocked by a queue of vehicles waiting for access, the analysis may be invalid. To determine whether the conflict points at an intersection may be separated, it is necessary to estimate the queue length for the each of the entering movements. If the estimated queue lengths are greater than the available storage space, then the separation of conflict points may overestimate or produce an unrealistic assessment of the operation.

Step 2. Results

Exhibit 3-31 shows the results of this analysis. In all cases, the movement capacities were improved in comparison with Sub-problem 4b, which considered all of the intersection conflicts simultaneously. This would be expected, but the important question is whether or not the queue backup would exceed the available storage space, thereby invalidating the analysis. Inspection of Exhibit 3-31 indicates that the 95^th-percentile queue lengths remained well within the storage boundaries. So, it could be concluded that it is appropriate to separate the conflict points for this intersection. While the separation of conflict points improved the operation slightly, some of the movements remain badly oversaturated—and the earlier conclusion that TWSC will result in a peak hour deficiency is confirmed.

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Sub-problem 4c: Separating the Conflict Points for TWSC Control

One more observation may be made from Exhibit 3-31. The NB right turn, even with the conflict points separated, indicates an oversaturated condition when analyzed as a TWSC operation. Because of the geometrics, the right-turn entry has more of the characteristics of a merge than a stop-controlled approach. This should raise some question as to whether another analysis procedure might be more appropriate. The treatment as a freeway entrance ramp will be considered in the next sub-problem.

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Sub-problem 4d: Further Consideration of the Northbound Right Turn

Step 1. Setup

The analyses conducted at Krome Avenue and Okeechobee Road to this point have treated the northbound right turn as a stop-controlled movement. As shown in the previous problem, this results in an estimated capacity that anticipates a failing condition. Closer inspection of the assumption reveals that this may be too conservative and that it should be considered as a merge. As shown in the aerial photograph of Exhibit 3-24 and in Exhibit 3-32, below, the right-turn channelization is designed more as a merge than a conventional intersection.

Intersection

Merge Area

Sub-problem 4d: Further Consideration of the Northbound Right Turn

What HCM procedure might be employed to consider the operations of this location? The ramps and ramp junctions procedure (Chapter 25) will be considered in order to explore an alternative approach to assessing the performance of the northbound right-turn movement. In order to analyze the intersection as a merge area, we must find a better way to analyze the capacity for this movement. Ideally, we would like to eliminate the movement from consideration. Before we do that, we must satisfy ourselves that it will not experience capacity problems.

The HCM does not prescribe an explicit procedure for at-grade intersections with merge area characteristics. Therefore, we must perform an approximate estimation of performance using one of the available procedures. We have already tried the HCM procedure for TWSC analysis and have not been able to justify the elimination of the movement based on the v/c ratio. On the other hand, we must view the TWSC procedure as pessimistic because of the design of the merge area.

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Sub-problem 4d: Further Consideration of the Northbound Right Turn

What is the basis for determining LOS in the unsignalized intersections methodology? HCM Chapter 25 provides a procedure for estimating freeway merge area performance in terms of the traffic density. Density is used in all HCM freeway-related chapters as an indicator of congestion level. The density thresholds for each LOS are given in the table at the right.

Chapter 25 of the HCM suggests that these procedures may be applied, in an approximate manner, to completely uncontrolled ramp terminals on other types of facilities such as multilane and two-lane highways. Since an approximation is what we are seeking, we can apply the procedures with some confidence.

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Sub-problem 4d: Further Consideration of the Northbound Right Turn

Step 2. Results

Exhibit 3-34 shows the assumptions and results for the merging analysis. The estimated merge area density is 17.7 vehicles per mile per lane, indicating LOS B. Based on this analysis, we would be fairly safe in concluding that the NB right turn will operate well within its capacity. We can therefore feel comfortable about eliminating this movement from the TWSC analysis.

We have performed an exhaustive analysis of TWSC operation in this problem. Several interesting situations have been explored with respect to the HCM; however, our basic conclusion about the feasibility of TWSC as a control mode has not changed. We will therefore explore the idea of signalization in the next problem of this case study.

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