Problem 1: Rietz Union Drive Intersection

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We can start to study the corridor by looking at each intersection and analyzing the current and planned conditions toward developing alternatives to mitigate any deficiencies, initially at the intersection level. To begin, we will focus on the Reitz Union intersection with Museum Road, first analyzing existing operations, then proceeding to add improvements to quantify their effects to help select the best set for implementation.

Using the existing traffic levels and geometric conditions, we can compute the delays and level of service at this intersection, currently unsignalized, operating under two-way stop-control (TWSC). As we work through these computations, we will be able to learn about several aspects of a TWSC intersection in Sub-problem 1a, including the effects of pedestrians blocking the minor approaches.

We will then compare the results from the analysis of existing conditions with those using projected traffic volumes generated from the new parking structure. We will then investigate some alternatives using information from queuing levels of different movements, as well as delays and LOS for movements and approaches.

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Problem 1: Rietz Union Drive Intersection

Next, we will consider whether signal control offers a viable solution to the future conditions by analyzing the intersection controlled by a traffic signal in Sub-problem 1b to:

• Analyze actuated control of the existing four-leg intersection.

• Compare the actuated signal with the TWSC scenario.

In Sub-problem 1c, we will consider the effects of pedestrians and bicycles on the signalized intersection, then look at geometric improvements to mitigate any negative effects. So, we will analyze:

• Actuated signal control with and without pedestrians and bicycles.

• Actuated signal control of a T-intersection with additional turn lanes.

We can then compare the overall operation of the TWSC versus the signalized intersection to better understand and recommend which implementation offers the better solution.

Our goal in Problem 1 is to investigate a variety of possible small-scale solutions to help alleviate the congestion created by the traffic generated by the new parking structure. Subsequent problems will look at the other alternatives for this intersection (in Problem 2) then analyze the urban street for cumulative and interacting effects and scenarios (in Problems 3 and 4).

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Sub-problem 1a: Analysis of the Existing TWSC Intersection

Step 1. Setup

In consideration of this analysis, we must first recall the importance of this facility as a multimodal corridor. The presence of pedestrians on this corridor will present some challenges for our analysis. In this problem, we will also interpret results from the HCM methodology, translating results that are mere mathematical equations to more meaningful results to present to the stakeholders involved in the project. Here are some issues to consider as you proceed with the analysis of the existing intersection and its performance.

• What is the data that is needed for this analysis?

• What is the level of service of this unsignalized intersection today? In the future?

• How might you interpret the estimates of delay at an unsignalized intersection to acknowledge the limitation of the methodology?

• What is the effect of pedestrians on the intersection?

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

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Sub-problem 1a: Analysis of the Existing TWSC Intersection

Step 2. Results

What is the data that is needed for this analysis? We can set up this analysis by compiling data on the existing conditions for use in a standard HCM application using the procedures in Chapter 17 for Unsignalized Intersections. The Two-Way Stop-Control (TWSC) methodology requires typical data, such as:

• peak-hour turning movement volumes

• peak-hour factors

• pedestrian volumes and walking speeds

• lane numbers, widths, and usage

• right-turn flares or channelization

• approach grades

• heavy vehicle data

The volume data for this analysis are shown below in Table 1.

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Sub-problem 1a: Analysis of the Existing TWSC Intersection

Step 2: Results

What is the level of service of this unsignalized intersection today? In the future? Results of our TWSC analysis of the Museum Road/Reitz Union intersection are shown in Table 3 below.

As you can see, the most pertinent information gained from this exercise might be the delay and level of service experienced by the SB movements, which we expect to be intensified by the addition of traffic for the new parking structure. Another item to note is that the northbound approach shows LOS F; but with so few vehicles, a field visit would show no queues or apparent problems. This is because LOS is based on delay in seconds per vehicle without regard to the number of vehicles being affected.

How might you interpret the estimates of delay at an unsignalized intersection to acknowledge the limitation of the methodology? While the queue and delay estimates are what the HCM methodologies provide, one might consider whether 999 seconds of delay is realistic. Essentially, this value is calculated from the equation in the HCM, and one might question whether a vehicle would actually experience over 18 minutes of delay during the peak 15 minutes. For this reason, reporting that the delay is greater than the upper threshold of LOS (>50 sec) may be more appropriate. For further detail, see page 17-26 in the HCM.

What is the effect of pedestrians on the intersection? One area worth a closer look would be the effects of high pedestrian volumes. Comparing the results with the high pedestrian volumes to a similar run without pedestrian interference will help illustrate the effects this aspect of the intersection has on the results. This comparison of queue, delay, and LOS is shown in Table 4.

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Sub-problem 1a: Analysis of the Existing TWSC Intersection

The effects of pedestrians on queue and delay, even LOS, can be dramatic. In this case, delays for the northbound and southbound approaches were increased by hundreds of seconds per vehicle, due to the time blockage (21% on three approaches) created by pedestrians at the stop-controlled approaches.

Assuming the geometry and pedestrian volumes remain unchanged for now, we can perform an analysis on the TWSC intersection with projected traffic levels reflecting the additional volumes generated by the new parking structure. The table below provides details on queue, delay, and LOS, comparing the current condition with projected traffic and no improvements to the intersection:

These results suggest that the deficiencies have worsened on the southbound approach due to the increased traffic levels for these movements. Delays listed as 999 seconds per vehicle represent values where parameters, such as the v/c ratios, drive the delay calculation to very high (>999 sec/veh) values. Also notice that the queue estimates for this approach increased from 7 or 8, to 29 and 50.

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Sub-problem 1b: Analysis of the Signalized Intersection with Projected Traffic

Step 1. Setup

With projected traffic levels, the TWSC intersection has substantial delays and queues. In this sub-problem, we will investigate the possibility of signalizing the intersection to determine if that would improve the situation. As noted previously, this is an operational look at signalizing the intersection. A signal warrant analysis using the procedures defined in the MUTCD would normally be necessary to pursue this option.

Here are some issues to consider as you proceed with the analysis of the existing intersection and its performance.

• What data is needed for this analysis?

• How can you compare the delay at an unsignalized intersection and a signal?

Discussion:
Consider the factors listed above. Each of these factors has been discussed in previous Case Studies within the HCMAG. How would you obtain the required data for each of these factors to analyze the operations of a signalized intersection? Take a few minutes to consider these factors. Click continue when you are ready to proceed.

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Sub-problem 1b: Analysis of the Signalized Intersection with Projected Traffic

What data is needed for this analysis? To analyze this intersection under signal control, the following data is required:

• signal phasing

• signal timing

• unit extension

• pedestrian crosswalk data

• bicycle volumes

• lost time data

• parking and maneuvers

• bus stop data

• lane utilization

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Sub-problem 1b: Analysis of the Signalized Intersection with Projected Traffic

Step 2: Results

A comparison of operations between a TWSC and signal controlled intersection with existing traffic and geometry would be informational and provide some data for further comparisons later. The results, including queue, delay, and LOS (assuming an actuated signal running a 120-second cycle) are shown below:

We can also perform a signalized intersection analysis using projected traffic under the same actuated control parameters. A similar comparison between TWSC and signal control is listed in the following table:

The results are very much the same as in the first comparison but intensified because of the increased volumes being analyzed.

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Sub-problem 1b: Analysis of the Signalized Intersection with Projected Traffic

How can you compare the delay at an unsignalized intersection and a signal? In these comparisons, you can see that a signal imposes delay (zero in TWSC) on the major street through and right-turn movements, which are also the movements with the highest volumes. On the other hand, delays are reduced substantially for all minor street movements. So, which case really represents the better control strategy, in terms of minimizing delay, for the overall intersection?

To better understand whether this is really an improvement to the operation of the intersection, at least as defined by levels of delay, we must realize that these delay values are in seconds per vehicle. So, for comparison purposes, it is important to apply these estimates to the number of vehicles affected for each movement. To accomplish this, we can compare total delay in seconds applied to all vehicles. This requires that we apply the computed delay in seconds per vehicle to the number of vehicles traveling through the intersection for both the signalized and TWSC intersection scenarios. This comparison is illustrated below:

As you can see, for existing geometry and projected traffic, the signalized intersection actually performs better (less delay) than under TWSC. Since the delays were very large on the minor street movements under TWSC, values of 999 seconds per vehicle were assumed, when the delay equation actually predicts even larger values. Even with this assumption, the delays on the minor street are reduced significantly with signal control. The delays do increase for the major street movements under signal control but are far outweighed by the improvements on the minor street approaches.

It should be noted that normally for undersaturated conditions, the benefit of zero delay for the major street throughs and rights would outweigh the benefit to side street traffic in this comparison. However, the very long delays computed for the side street movements make this case somewhat unique.

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Sub-problem 1c: Analysis of the Signal with Pedestrians and Bicycles

As we noted in the TWSC portion of our analysis, pedestrians can have a significant effect on delay. We can see how pedestrians (and bicycles assumed at 10% of pedestrians) affect our signalized intersection by making an additional run without the pedestrians and bikes present to facilitate the comparison below:

It is obvious that the effects of pedestrians and bicycles on this intersection are significant, almost doubling queues for the southbound right turns and increasing the delay by about 35% (106 sec/veh to 162 sec/veh) across the overall intersection.

This occurs due to adjustments to the saturation flow rates to account for the effects of pedestrians and bicycles on the conflicting vehicular movements, which is new in HCM2000 and described in Chapter 16, Appendix D. In this analysis, adjustments are significant, especially for the southbound right turns where the flow rate was reduced by 40%; but all movements had adjustment factors lower than 1.00. With this level of effect on flow rate, and subsequently capacity and delay, strategies to reduce this effect would be reasonable to pursue.

Testing an alternative that requires pedestrians to cross on only two of the four approaches might make sense here. Strategically, this can be orchestrated to benefit the right turns that need it most. From a practical viewpoint, we know the predominant pedestrian flow is from the southwest (student parking) to the northeast (classes) and the reverse. Because the right turns are heaviest (by far, which makes this alternative worth considering) in the southbound and westbound vehicular movements, we can consider closing the crosswalks on the western and northern legs of the intersection. The pedestrian and bicycle flow rates for the closed crosswalks have to be added to those left open.

Discussion:
Consider the potential treatment described above and consider how the various stakeholders identified in the overview may react to this proposed treatment.

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Sub-problem 1c: Analysis of the Signal with Pedestrians and Bicycles

Step 2: Results

A comparison of these results is shown in the following table:

Closing two crosswalks improves the right-turn movement operations for two approaches and increases the delay for those where the pedestrians and bicycles are consolidated. These benefits come from the adjustments to the right-turn movement saturation flow rates. It appears that the benefit to the eastbound and southbound approaches outweighs delay added to the westbound and northbound approaches. This was expected (and by design), since the major right-turn traffic is eastbound and southbound.

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Sub-problem 1c: Analysis of the Signal with Pedestrians and Bicycles

Finally, we will analyze the intersection as a T-intersection, closing the northbound approach. This will allow two exclusive right-turn lanes on the southbound approach within the proposed right of way, since no through movement is now needed. Closing the northbound approach (being considered by the university) also facilitates the consolidating of pedestrians and bicycles to crossing this approach. Results with this new lane configuration are shown below:

The improved geometry provided to mitigate the pedestrian and bicycle issues, especially as they relate to the heavy right-turning vehicular traffic, shows significant improvement in the operation of the intersection. Overall delays were reduced from 110 sec/veh (LOS F) to 53 sec/veh (LOS D).

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Sub-problem: Analysis of the Signal with Pedestrians and Bicycles

A table to illustrate the total delay comparison table with the improved geometry and additional considerations is presented below.

We have successfully shown that in this case, the signal control out-performs the TWSC even more (86% reduction in total delay) with the improved geometry.

Note that both the signalized and TWSC procedures assume adequate left-turn and through movement storage when computing delay. This means that these procedures do not account for left-turn queues that might exceed the provided storage and interfere with the through movements. The procedure is also treating the intersection as isolated and will not account for through movement queues that could spillback into adjacent intersection queues. We will look at how to compute and compare interacting queues in Problem 5.

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