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Problem 6: Planning Level Analysis 

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Thus far, we have taken an operational perspective in our analysis. Occasionally, when we are considering future conditions, we are better served by looking at a less detailed level of analysis, often called a planning level analysis. Let's take this planning perspective now, considering the twenty-year traffic projections. All of the intersections on U.S. 95 will be examined to assess their sufficiency at the end of the twenty-year period. The signalized intersection quick estimation procedures contained in Appendix A to Chapter 10 of the HCM 2000 will be applied for this purpose.

The question to consider now is whether or not the number of lanes at the U.S. 95/Styner-Lauder Avenue intersection will be sufficient to accommodate the projected traffic demand over the twenty-year planning horizon. A uniform increase of two-percent per year will be applied to the current volumes on all traffic movements. Compounded annually, the two-percent annual increase will produce a growth factor of 48.6% to apply to the current volumes.

The quick estimation method covers all aspects of signalized intersection analysis, including determination of left-turn treatments, lane-volume computations, estimation of signal timing plans, calculation of the critical v/c ratio, and calculation of average control delay. The delay calculations use the same mathematical procedures as the operational analysis method, except that certain details, such as pedestrian minimum times, are excluded from consideration. Liberal use is made of assumptions and approximations.

This method is well suited to analysis of conditions projected over the long term where the accuracy of the traffic volumes is questionable and immediate implementation of results is not an issue. The method will be applied to each of the intersections on U.S. 95 in separate sub-problems:

Sub-problem 6a: Planning Analysis at Palouse River Drive

Sub-problem 6b: Planning Analysis at Styner-Lauder Avenue

Sub-problem 6c: Planning Analysis at Sweet Avenue

Sub-problem 6d: Planning Analysis at SH-8

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Sub-problem 6a: Planning Analysis at Palouse River Drive

The Idaho Transportation Department has provided historic traffic volume data for U.S. 95 to help determine a historic growth rate. This data suggests a annual growth rate of 3.4 percent per year over the past 20 years, and 2.6 percent over the past 10 years.

The City of Moscow has provided planning model volumes that project 20-year traffic growth on the U.S. 95 corridor to be 50 percent over the next twenty years. Based on this information, what is the number of lanes required for the intersections along the U.S. 95 corridor in twenty years, assuming an annual traffic growth rate along the corridor of two percent?

In sub-problem 6a, we will use the planning analysis technique to evaluate conditions that occur when traffic is leaving a football game at the University of Idaho. Demand is high for about an hour after the conclusion of the game and the U.S. 95 corridor experiences a high level of congestion during this period.

Consider these questions:

• Will a planning analysis allow complex lane configurations?

• Should right turns be considered a separate movement or combined with through movements?

• What is the difference between volume and demand, and why is it important to distinguish between these two terms?

• Can the intersection operate at level of service F even when demand is less than capacity?

• What is the appropriate value of the duration of analysis parameter when demand exceeds capacity?

• When should multiple time periods be considered in a capacity and level of service analysis?

• What is the accuracy of the data used at the planning level?  What does this suggest about the results?

Discussion:
Take a few minutes to consider these questions.  When you are ready, continue to the next page.

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Sub-problem 6a: Planning Analysis at Palouse River Drive

The twenty-year projected volumes for this intersection, adjusted for the peak hour factor are shown in Exhibit 1-47.

Each of the steps in the quick estimation method will be presented in detail in this sub-problem. The method involves a series of five detailed worksheets on which the data are entered and computations are performed.

The quick estimation steps are as follows (click on each step to see a more detailed discussion):

1. Determination of left-turn treatments

2. Determination of lane volumes

3. Phasing plan synthesis

4. Cycle Length determination

5. Determination of intersection status

6. Phase time determination

7. Performance estimation

For purposes of this discussion, the detailed computations are skipped and the results are summarized in Exhibit 1-48:

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Sub-problem 6b: Planning Analysis at Styner-Lauder Avenue

A similar evaluation will be performed for the Styner-Lauder intersection, but the detailed discussion found in sub-problem 6a will not be repeated here; projected traffic volumes at Styner-Lauder are much lighter than at Palouse River Drive.

Similar to the discussion included in sub-problem 6a, there are several issues to consider for a planning analysis with light volumes as found at the Styner-Lauder Avenue intersection:

• What site-specific factors may reduce the accuracy of the planning level analysis?

• How should the type of signal phasing be decided?

• What is the reliability of a planning level analysis?

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

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Sub-problem 6b: Planning Analysis at Styner-Lauder Avenue

The twenty-year projected volumes for the Styner-Lauder Avenue intersection, adjusted for the peak hour factor are shown in Exhibit 1-49.

Click here to see the cross-product calculations. None of the left-turn cross products suggests any need for left-turn protection, so a two-phase operation will be used.

Exhibit 1-50 shows the results from this exercise. Even with the 20-year traffic volume projections, the critical v/c ratio is only 0.52. These results let us say with great confidence that the existing intersection configuration will be able to accommodate the anticipated traffic 20 years from now with no operational problems or congestion.

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Sub-problem 6c: Planning Analysis at Sweet Avenue

The U.S. 95/Sweet Avenue intersection has only three approaches. The planning level analysis of a T-intersection leads to some questions to consider:

• What implications does the lack of a fourth leg have on the analysis?

• What type of phasing would you expect at a 3-legged intersection?

• What impact does a protected left-turn have on the intersection vs. a permitted left-turn movement?

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

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Sub-problem 6c: Planning Analysis at Sweet Avenue 

The twenty-year projected volumes for this intersection, adjusted for the peak hour factor, are shown in Exhibit 1-51.

This is a T-intersection with no approach for westbound traffic. The northbound movement has the only left turn opposed by through traffic. That movement now has a protected left turn phase. Therefore, it is not necessary to apply the cross product computational step to this sub-problem, because the left-turn treatments have already been determined for all approaches. Instead we must specify the type of protection for each approach.

The choices are:

• Permitted: The left turn moves on a solid green but no protected left-turn phase is displayed at any point in the cycle.

• Protected: The left turn moves only on a green arrow.

• Protected plus permitted: The left turn moves at one point in the sequence on a green arrow and at another point on a solid green, yielding to oncoming traffic.

• Not opposed: There is no protected left-turn phase, but the left turn is never opposed at any point in the cycle. This choice applies at T intersections, one-way streets, and intersections with full directional separation (split-phase operation) between opposing movements.

Discussion:

Which treatment will apply to each of the approaches at the U.S. 95/Sweet Avenue intersection? When you are ready, click continue below to proceed.

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Sub-problem 6c: Planning Analysis at Sweet Avenue

For this example, the Northbound left turn is protected, the eastbound left turn is not opposed, and the other two left turns do not exist.

The analysis results are presented in Exhibit 1-52. Note that the minimum cycle length of 60 seconds will accommodate the projected 20-year traffic volumes with no apparent operational problems.

Notice also that, even though this is a T-intersection with no westbound approach, the phasing designation for east west traffic is EWT, indicating the presence of westbound vehicles. This is an important principle that describes the way that traffic movements are represented in the planning level model. The principle is that, for the quick estimation method, all movements must exist in the model. Those that don’t exist on the street are simply assigned zero volume and capacity. This is an important distinction between the quick estimation method and more detailed traffic models.

As indicated above, the HCM quick estimation method distinguishes between protected and protected-plus-permitted left-turn treatments. It is important to note, however, that the mathematical treatment of the permitted phase differs from the full operational procedure. The full operational procedure computes a capacity for vehicles turning left on the permitted phase, based on advance knowledge of the signal timing plan. The quick-estimation procedure does not require this information, and cannot, therefore, perform the computations at the same level of detail. This is an essential feature of the tradeoff between the reduced data requirement for planning level applications and the amount of detail that can be expected in the results.

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Sub-problem 6c: Planning Analysis at Sweet Avenue

The quick estimation method takes protected plus permitted phasing into consideration by subtracting two sneakers per cycle from the left-turn volume. The volume is not reduced at all for protected only operation, and it is not reduced below four vehicles per cycle in either case. The four vehicles per cycle lower limit was imposed to prevent unreasonably short left turn phases from occurring in the sequence. If the cycle length is not known, the HCM suggests using the maximum allowable cycle length to provide a conservative estimate.

For planning level analysis with projected volumes, many analysts prefer not to count on the additional permitted phase to provide capacity for left turns well into the future.  There are several reasons, including safety problems, that a protected left turn supplemented by a permitted phase today may not be able to retain that option twenty years from now.

Consider what would happen if the phasing for this example were modified to allow the left turn to proceed on the solid green for north-south traffic. The full operational procedure would assign some additional capacity to the left turn, thereby reducing the delay, and possibly improving the level of service. 

The quick estimation method, on the other hand, would see a much smaller difference in the operation. Based on the maximum cycle length of 120 seconds, four vehicles per cycle would create a volume of 120 vph.  So the left-turn volume entry would be reduced from 152 vph to 120 vph, but no other benefit of the permitted phase would be applied.

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Sub-problem 6d: Planning Analysis at SH-8 Intersection

The same quick estimation will be done for the U.S. 95/SH-8 intersection with the following issue to consider:

• If a protected left turn and an exclusive right turn share a phase, what type of reduction should be applied to the exclusive right turn volume?

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

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Sub-problem 6d: Planning Analysis at SH-8 Intersection

The twenty-year projected volumes for this intersection, adjusted for the peak period factor are shown in Exhibit 1-53.

This is an unusual intersection that will require careful thought before applying the quick estimation method. As indicated in Exhibit 1-54, The eastbound approach is one-way towards the intersection. The departure roadway to the north is one-way away from the intersection. Therefore, there is no westbound through movement, no northbound left turn, and no southbound traffic at all. The phasing is as shown in the diagram here.

Exhibit 1-54. SH-8 Phase Diagram

So, the first thing to consider is the type of treatment (permitted, protected, protected plus permitted or not opposed) to specify for each of the left turns. Since the eastbound and westbound movements are fully separated, the not opposed treatment will apply. While the northbound and southbound left turns do not exist, they must be specified as permitted because of the way that the quick estimation method represents traffic movements. For more detail on this subject, see the discussion presented under sub-problem 6c.

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