Problem 5: U.S. 95 South of Moscow

Printable Version

U.S. 95 continues south out of Moscow bound for Lewiston, ID, 32 miles to the south. Exhibit 1-41 shows an aerial photograph of about a four-mile stretch of the highway just south of Moscow. A short length of suburban two-lane arterial leads quickly to a rural two-lane highway.

The state highway agency would like to evaluate the performance characteristics of U.S. 95 as a single facility. To help address this issue, we will explore issues related to the short section of U.S. 95 that acts as a main street for the somewhat developed area you can see toward the bottom of the aerial photograph. Currently, the development in this area does not generate much activity, but trip making is expected to increase over the next 10 years as the area grows. 

Current estimates are that 10 years from now U.S. 95 south of Moscow will carry about 1,100 vehicles per hour during the PM peak and about 700 of these trips will be generated by the development area (400 originating and 300 destined for this area). Another 300 will be bound toward Moscow from this area, which we will hereafter refer to as a hamlet. In addition, the hamlet will generate a total of about 2,000 vehicle trips during the PM peak (the 700 trips mentioned above are included in this figure), 100 of these trips will go to and from points further south, and the remaining 1,200 trips will be local within the hamlet.

How will U.S. 95 operate in the future based on these forecast traffic conditions? Continue to the next page for additional discussion.

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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. 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.

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Problem 5: U.S. 95 south of Moscow

It is also recognized that, in real terms, the range of potential solution alternatives to any deficiencies that might be identified is fairly limited:

• a bypass could be constructed, although this is typically very expensive;

• U.S. 95 could be widened through the hamlet to provide a 3-, 4-, or even 5-lane cross-section; and/or

• a raised median could be installed to limit or even prohibit turn movements.

Exhibit 1-42 shows a schematic of the area being studied. First, there is a 4.5 mile section of two-lane rural highway, then the 1-mile section through the hamlet, then another 4.5-mile section of two-lane highway.

The figure also provides you a broad-brush sense of the traffic volumes. Whether a bypass is built or not, the figure shows that during the PM peak hour, there are 600 vehicles traveling south and 500 traveling north. Of the 600 coming south, 400 are destined for the hamlet, while the remaining 200 are going further south. Another 50 vehicles originate in the hamlet for trips further south so that the southbound volume below the hamlet is 250 vehicles per hour. In the northbound direction, the flow rate is 250 vehicles per hour just south of the hamlet. Fifty of these vehicles are destined for the hamlet and the remainder are traveling further north toward Moscow. An additional 300 trips originate in the hamlet bound for Moscow and further north. This means the northbound volume just above the hamlet is 500 vehicles per hour.

Analysis Plan
To provide the performance evaluation that is desired for this highway section, it will be necessary to conduct a number of separate analyses. The remainder of this discussion is presented in the context of the following three sub-problems:

Sub-problem 5a: Existing analysis of 10-mile segment of U.S. 95

Sub-problem 5b: Future analysis of 10-mile segment of U.S. 95 with recognition of the hamlet

Sub-problem 5c: Analysis of 10-mile segment with a bypass

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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.

Many 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. The two-phase alternative is therefore presented in this sub-problem primarily as an illustration of the details of the HCM procedures. For many agencies, it will be a redundant step 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:

• The Quick Estimation Method (QEM) uses the critical movement technique (also known as the critical movement analysis) to estimate the intersection capacity. At a conceptual level, how would you use the critical movement technique to estimate the capacity of an intersection?

• The QEM is similar to the critical movement analysis in that they are both planning level analyses. What factors will cause variation between the QEM and the full operational analysis?

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

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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. 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:

• A phasing plan determined by the left turn treatments for each approach. The left-turn treatments may be specified, or they may be synthesized, based on the volumes of the left-turn movement and its opposing through movement.

• A cycle length that will produce a target v/c ratio of 90%.

• An allocation of phase times that will equalize the degree of saturation among the critical movements on each phase.

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.

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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.

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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:

• During this sub-problem, signal timing strategies are explored. There are three different schools of thought on the issue: one thought is to equalize the v/c ratio for each approach, another is to equalize the delay, and the last is to minimize delay. What pros and cons do you think are associated with each of these strategies? Considering roadway volumes, lane groupings, and other issues that may affect each of the parameters, do you think one solution will always be desirable?

• After you have read through this sub-problem, reflect on the first question and see if your initial thoughts still hold true.

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

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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.

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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 critical phase v/c ratios computed by the operational procedure are nearly, but not quite, equal (0.75 vs. 0.79). In other words, the more detailed treatment of saturation flow rate, lost time, etc., by the operational procedure has produced minor differences in the results.

• Additional performance measures are provided by the operational procedure, including v/c ratios delays and level of service for each lane group. The QEM does not carry its computations to this level.

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.

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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:

• The critical v/c ratio for the whole intersection was increased from 0.79 to 0.83 as a result of increasing the inter-green times and thereby reducing the effective green times.

• The increase in v/c ratios was, predictably, accompanied by an increase in delay; but the delay increase was not sufficient to change the level of service for any of the movements.

• The critical v/c ratios for each phase are in closer agreement (0.83, 0.83 and 0.84). This was the closest possible agreement that could be produced by trial and error with 0.1 sec resolution in the green times.

• The control delays and levels of service differ widely among the various movements. This observation makes it clear that equalizing the v/c ratios does not necessarily equalize the delays among competing movements. Note that the delays varied from 18 sec per vehicle to 96 sec per vehicle and the LOS varied from B to F.

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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:

• The signal timing for this strategy is noticeably different than the corresponding timing for equalizing the v/c ratios.

• The previously equal v/c ratios for the competing movements now vary from 0.49 to 1.0 as a result of redistributing the green times.

• The critical v/c ratio for the whole intersection has not changed from its previous value of 0.83. The critical v/c ratio is not affected by the distribution of green times. The computations for this performance measure are always based on the assumption of equal v/c ratios. This is an important point. The critical v/c ratio is a measure of overall intersection sufficiency and does not reflect the actual distribution of green times.

• The control delays are now in much closer agreement among the competing movements. The EB through movement still has a slightly lower delay (47 sec per vehicle vs. 52 sec per vehicle for the other two movements). Note that the v/c for this movement is 1.0. To fully equalize the delays for all movements, the EB through movement would have to be forced to operate beyond its capacity. It is common signal timing practice to halt the iterative distribution of green times when further redistribution would create an oversaturated movement.

• In spite of the slight difference in the control delay values, LOS D now applies to all of the competing movements and to the overall intersection.

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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.

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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:

• What are the primary operation effects of using actuated control?

• What additional information is needed beyond the data already used in Sub-problem 5b?

• How can actuated control improve the efficiency of a signalized intersection?

• How can actuated control improve the safety of a signalized intersection?

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

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