Problem 2: Reitz Union Drive Intersection Improvement Strategies

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Continuing our analysis of the Reitz Union Drive intersection, we will now look at some additional options under signal control. We can first analyze operations of the improved intersection with no northbound approach and two exclusive right-turn lanes southbound, using an actuated, two-phase signal with a 120-second cycle. Then, we can investigate improvements to this design to quantify their effects as input to choosing alternatives to potentially minimize congestion.

Continuing to use future traffic projections and the improved geometric conditions suggested in Sub-Problem 1c, we can compute the delays and level of service at this signalized intersection, assuming that it is operating under fully-actuated control. As we work through these computations, we will be able to investigate several aspects of an actuated signal.

We can first look at some options to test phasing alternatives to accommodate pedestrians. Such analyses will allow us to quantify the effects of these signal modifications toward resolving these issues using:

Problem 2: Reitz Union Drive Intersection Improvement Strategies

Next, we will consider an option to improve the phasing at this intersection by investigating its operation with existing phasing to identify:

Finally, we can investigate semi-actuated control for potential coordination with adjacent signals and what effect it has on the overall operation in sub-problem 2c by looking at:

We will compare the overall operation of the signalized intersection operating with and without improved phasing and timing, as well as actuated versus semi-actuated control to better recommend alternative solutions.

In Problem 2, we are looking at strategies to improve the intersection, mostly as an isolated signal with the addition of the traffic generated by the new parking structure, but we are introducing the idea of semi-actuated control for potential coordination with the adjacent intersections.

Problem 2: Reitz Union Drive Intersection Improvement Strategies

We can set up this analysis by using the data from Problem 1 to analyze the actuated signal control of the improved intersection with projected traffic in a standard HCM application using the procedures in Chapter 16 for signalized intersections. The operational analysis of a signalized intersection under actuated control requires the following typical data:

The critical data for this analysis are shown in the following table:

Problem 2: Reitz Union Drive Intersection Improvement Strategies

Using this data to work through the HCM procedures will yield for each movement and approach:

Results of our analysis, including overall intersection delay and level of service, are shown in the following table:

Remember, since the HCM methodology assumes all lanes have adequate storage when computing delay, you must check the back-of-queue and queue storage ratio for each lane group to determine if there are queue storage problems.  (A queue storage ratio greater than 1.0 indicates estimated queues are greater than available storage).

With this analysis of the existing conditions at the Reitz Union Drive intersection, we will address intersection improvement strategies in the ensuing sub-problems.

Sub-problem 2a:Analyzing the effects of pedestrians

Sub-problem 2b:Analyzing the effects of signal phasing

Sub-problem 2c:Analyzing the effects of coordination
Discussion: Summary of Problem 2 results

Sub-problem 2a: Analyzing the Effects of Pedestrians

Step 1. Setup

Because of the large number of pedestrians at this intersection, we might want to consider an exclusive pedestrian phase. We have resolved part of the pedestrian conflicts with vehicles by closing the northbound approach to vehicular traffic in Problem 1. However, the northeast movement of pedestrians from parking to classes suggests a diagonal crossing might be worth some consideration.

Assuming we need to maintain the 120-second cycle, we need to compute the pedestrian green time for the diagonal movement crossing. Using a crossing distance of 50 feet, a crosswalk width of 10 feet and a 4-feet-per-second walk speed, the minimum pedestrian time is calculated to be about 20 seconds. This assumes 500 pedestrians in thirty 120-second cycles per hour, or 17 per interval in the following formula from HCM Equation 16-2:

Incorporating this value into an exclusive pedestrian phase yields the following timing for the intersection:

Discussion:
Take a few minutes to review the phasing plan with the exclusive pedestrian phase. By moving the pedestrians to their own phase, will they be assigned more or less time than they were before? What benefits are derived by assigning pedestrians with an exclusive phase? Click continue when you are ready to proceed.

Sub-problem 2a: Analyzing the Effects of Pedestrians

Step 2: Results

To evaluate the effects of this signal timing strategy, we will use the Chapter 16 HCM operational methodology. Running the analysis for a 120-second cycle results in the queues, delays, and LOS values shown in Exhibit 5-24:

While pedestrians are accommodated more efficiently and more safely, the intersection performance is worsened under this scenario, at least with respect to the LOS criterion. On the southbound approach, LOS drops from C to E/F. Since pedestrians are restricted to crossing only the northbound and westbound approaches (with the northbound approach closed to vehicles), the need for an exclusive pedestrian phase is probably minimal, especially with the deterioration of the intersection efficiency using this design.

Sub-problem 2b: Analyzing the Effects of Signal Phasing

Step 1. Setup

Remember from our initial analysis of the Reitz Union Drive improvement strategy (see Exhibit 5-23) that the most urgent deficiency is the eastbound left-turn movement, with an estimated delay in excess of 200 seconds per vehicle. The first mitigating option that would be the easiest to implement and the least costly would be to simply modify the signal phasing. We can test if providing a signal phase to alleviate the saturated movements might reduce the delay on those movements, being cognizant of the effects on the overall intersection.

Discussion:
Take a few minutes to determine a phasing plan that would reduce the delay of the critical movements. Click continue when you are ready to proceed.

Sub-problem 2b: Analyzing the Effects of Signal Phasing

Step 2: Results

Referring once again to the results of our initial analysis (see Exhibit 5-23), we see that there may be an opportunity to combine improvements through phasing design, since the two most deficient movements are the eastbound left and the southbound right. This allows us to introduce a phase that implements a leading protected left-turn phase eastbound that can run concurrently with a protected right-turn phase southbound as shown here:

This phasing provides more protected time to the most deficient movements to improve the overall efficiency of the intersection, as illustrated in the results below: 

Sub-problem 2b: Analyzing the Effects of Signal Phasing

Let's compare the results of this analysis (Exhibit 5-25) with our initial assessment (Exhibit 5-23). The eastbound left delay went from 219 sec/veh to 21 sec/veh while maintaining reasonable performance levels for other movements. The westbound movements were slightly improved (LOS C to B) with some deterioration for southbound movements (LOS C to D/E). The overall intersection improved from a delay of 53 sec/veh (LOS D) to a delay of under 22 sec/veh (LOS C). These results suggest that the added phase offers an overall improvement to the operation of the intersection at a level that warrants its implementation.

Sub-problem 2c: Analyzing the Effects of Coordination

Step 1. Setup

The previous analyses assumed fully actuated signal control. With adjacent signalized intersections located close by, coordination would be desirable. This Sub-problem provides an opportunity to compare the effects of operating the Museum Road/Reitz Union Drive signal as fully actuated with the effects operating it under semi-actuated control. A constant cycle length can be maintained if the intersection is operated under semi-actuated control, thereby allowing coordination with the other signals along Museum Road. In an HCM analysis, the factors affected in this comparison are arrival type, progression factor, unit extension, and the k-value.

Discussion:
Take a few minutes to identify why the factors mentioned above will be affected in this comparison. Click continue when you are ready to proceed.

Sub-problem 2c: Analyzing the Effects of Coordination

Step 2: Results

The arrival type is 3 for all movements under fully actuated control to model random arrivals. This results in a progression factor (HCM Equation 16-10) of 1.00 for all approaches, which means the first term of the delay equation (HCM Equation 16-9) for uniform delay (d₁) is not adjusted for coordination.

Under fully actuated control, the HCM procedures account for how responsive an actuated movement reacts to traffic by using the unit extension. This value represents how long (in seconds) a detector must be vacant before the controller will end the phase ("gap out"). In the HCM, the unit extension is used to determine the k-value for use in the delay equation (for incremental delay, d₂). So, while fully actuated control does not lower d₁, it does lower d₂.

Conversely, under semi-actuated control, the reverse is true. Since the major street through movements must be pretimed to accommodate coordination, the arrival type can vary, based on the degree of coordination provided. Under most situations, arrival type 4 is used for normal coordinated systems. (Arrival type 5 could be used in especially well coordinated systems like for one-way streets). Using arrival type 4 for both the eastbound through movement and westbound through and right-turn movements in this case results in progression factors from HCM Exhibit 16-12, based on the green (g/c) ratios but always values less than 1.00 to account for improvements in delay to these movements created by the coordination provided. The progression factor modifies the effects of uniform delay, d₁.

However, under semi-actuated control, the unit extension value for the eastbound through movement and the westbound through and right-turn movements will be ignored since these movements are under pretimed operation, resulting in a k-value of 0.50. This will not lower the d₂ value, so we have a trade-off between these two control strategies.

Sub-problem 2c: Analyzing the Effects of Coordination

Our task here will be to compare these results to see which combination provides the better overall efficiency. We can make two runs: one with all actuated movements, arrival types of 3 and unit extension values of 3.0 seconds, and the other with eastbound through movements and westbound through and right-turn movements coded as pretimed, using arrival type of 4, with the unit extension values to be ignored. The results of these two runs are presented in Exhibit 5-26 so that we can compare the affected approaches and the overall intersection operations:

As you can see, the unit extension creates k-values of less than 0.50, which lowers d₂. But, arrival type values of 3 result in progression factors of 1.00, with no effect on d₁. However, under semi-actuated control, the k-value stays at 0.50, not affecting d₂, but the progression factor is lower which reduces the d₁ term. Overall, we can see that the combined effects of semi-actuated control on the d₂ term outweigh those of fully-actuated control on the d₁ term.

Sub-problem 2c: Analyzing the Effects of Coordination

One last option we might consider, since most of these runs have assumed a  120-second cycle, is to investigate the possibility of a shorter cycle. However, in order to maintain the ability to coordinate with the other signals along the arterial (which we are assuming to be running the 120-second cycle), we must confine ourselves to a 60-second cycle. This is because the cycle length we use at Museum/Reitz Union must maintain an integer relationship with the 120 second cycle that controls other intersections along Museum Road. As examples, therefore, it could be 30, 40, or 60 seconds in duration. However, we must be cognizant of pedestrian crossing time requirements, and this is likely to eliminate anything less than 60 seconds. Operating the Museum Road/Reitz Union Drive intersection on a 60-second cycle while the remainder of the system operates on a 120-second cycle is commonly referred to as a  double-cycle option; it has the potential to lower delay at our intersection of interest (because of the lower cycle length) while still maintaining the benefits of coordination with upstream signals. The results of this comparison are shown in Exhibit 5-27.

Comparing the results between the 120-second and 60-second cycles illustrates a couple of points. Lower cycle lengths generally result in shorter delays overall, even though capacities go down. Also, the queues are reduced substantially because the red time per phase is less, reducing the time available for queues to lengthen. This is a good strategy where storage lengths are limited, as is the situation in this particular case study. The overall intersection delay was reduced and queues lowered for every movement by between 10% and 50%. Another by-product of this strategy is that the large number of pedestrians will not need to wait as long before receiving a WALK indication.

Discussion

In this problem we evaluated an implementation of an exclusive pedestrian phase, alteration of the existing signal phasing, and signal coordination with the adjacent intersections. Through this analysis we were able to identify the advantages and disadvantages associated with each of the mitigating alternatives at this location.