Problem 2: U.S. 95 Arterial

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The problem statement remains nearly the same as in Problem 1: should the intersection of U.S. 95/Styner-Lauder Avenue be signalized or left to operate as a TWSC intersection?

But we will now consider the operation of the intersection as part of the U.S. 95 arterial system. With this context in mind, we will complete three computations, using the HCM, to get a better picture of the operation of the intersection under both TWSC and signal control considering the effects of the adjacent intersections of Sweet Avenue and Palouse River Drive. We will also consider a fourth sub-problem in which we look at some of the issues involved in signal coordination.

In this problem, you will consider the following issues as you work through the computations for the three sub-problems:

This problem illustrates some of the important elements of performing an analysis of a signalized intersection operating in the vicinity of nearby intersections by addressing the following issues as they relate to the proposed signal at the U.S. 95/Styner-Lauder Avenue intersection:

Sub-problem 2a: Effect of upstream signals on TWSC intersection capacity
Sub-problem 2b: Arrival type at signalized intersections
Sub-problem 2c: Arterial (urban street) analysis
Sub-problem 2d: Signal progression and Time-space diagrams

Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

Step 1. Setup

In sub-problem 2a, we will evaluate the operational characteristics of the existing TWSC intersection at U.S. 95/Styner/Lauder. Here are some issues to consider as you proceed with the analysis of the existing intersection and its performance under two-way stop-control:

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

Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

Let's discuss each of these issues and how each affects the operational analysis that we are about to complete.

Why and how do upstream signalized intersections affect the operation of a TWSC intersection? Traffic departs a signalized intersection in well-structured platoons. These platoons begin to disperse as they travel downstream from the signalized intersection. When they arrive at a downstream TWSC intersection, the platoon may remain somewhat intact, depending on the distance from the signalized intersection. When a TWSC intersection is relatively close to the signalized intersection, the large gaps that are present between the arrivals of each platoon are available for use by minor street vehicles. These large gaps generally have a neutral or positive effect on the TWSC intersection's minor movements. Consideration must also be given to the arrival patterns on the major street from the opposite direction (where there might not be a signalized intersection), which may have the effect of negating the platoon effects from the signalized intersection.

You will recall that we are using the procedure from Chapter 17 of the HCM. However, this model assumes that vehicles arrive randomly or independently of each other. This random distribution of headways results in a lower capacity than we would observe from the platooned arrival condition described above. We should also note that the HCM procedure does include a means for accounting for the non-random effects of upstream signalized intersections, albeit a fairly rough approximation.

Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

What additional data are needed when we consider the effect of adjacent intersections? The additional data needed are for the upstream signalized intersection. We need to consider the following variables:

What is the potential effect of mid-block driveways on these arrival patterns? If there are mid-block driveways between the upstream signalized intersection and the subject TWSC intersection, vehicles other than those in structured platoons from the signalized intersection will arrive at the TWSC intersection. These vehicles may reduce the size of the large gaps that otherwise would be present at the TWSC intersection. While the HCM model does not account for this effect, you should keep this in mind if the intersection you are studying has mid-block driveways.

What computational tools are used to compute the capacity of a TWSC intersection when considering the effect of adjacent intersections? We will use the same procedure from Chapter 17 of the HCM for TWSC intersections. This time, however, we will consider the upstream signal module that is included in this procedure.

Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

Step 2. Results

The HCM procedure produces the following results for each minor stream movement:

These results are summarized in Exhibit 1-20, for the existing volumes.

Discussion:
If we review the results produced when we consider the effect of arrival patterns from the Sweet Avenue signal (above), we see that there is no difference from our previous analysis. Why?

Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

Exhibit 1-22 shows a plot of capacity for the EB through and right turn movement at the U.S. 95/Styner-Lauder intersection as a function of the distance away from an upstream signalized intersection. The last point on the right of Exhibit 1-22 shows a capacity of 329 veh/hr for a distance of 1,070 feet, which is the distance between Sweet Avenue and Styner-Lauder (Dataset). In fact, we can see that for any distance greater than about 600 feet, there is no capacity increase.

But when we get closer than 600 feet to the signalized intersection, the platooning begins to have an effect on the capacity of this movement. At 250 feet from the intersection, the capacity increases by 20 percent to 396 veh/hr. The delay is also reduced by 20 percent, from 25 sec/veh to 20 sec/veh.

So, while there may be some effect of upstream signals in some situations, this effect does not show up in this particular problem. It is important to point out that the method we've just used from the HCM to account for the effect of upstream signals is an approximation. Where the effect of the upstream signals is more important to the final result than in this example, other techniques are available for estimating this effect. These techniques are more microscopic, require more input data and effort to apply, but may not give any more accurate answer, unless more refined data is provided. They can, however, provide further insight and are warranted for use in some situations.

We will next consider, in sub-problem 2b, whether arrival patterns of Sweet Avenue vehicles have an effect on the Styner-Lauder intersection if the latter intersection were to be signalized.

Sub-problem 2b: Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections

Step 1. Setup

We will now look at the operation of U.S. 95/Styner-Lauder Avenue as a signalized intersection and consider the effects of the adjacent signalized intersection at Sweet Avenue. Sweet Avenue is located 1,070 feet to the north of Styner-Lauder Avenue and is the main southern entrance to the University of Idaho for university students, staff, and faculty. Vehicles traveling south on U.S. 95 from Sweet Avenue often arrive in platoons during the peak period. While we didn't see an effect of platooning on the operation of Styner-Lauder Avenue as a two-way, stop-controlled intersection, there may be an effect if the operation is controlled by a traffic signal.  What is the nature of this effect?

Discussion:
Take a few minutes to consider how the traffic signal at Sweet Avenue might affect the operation of Styner-Lauder Avenue, if the latter intersection were to be signalized.

Sub-problem 2b: Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections

The major effect that the traffic signal at Sweet Avenue has on the adjacent intersection at Styner-Lauder Avenue, if the latter intersection were to be signalized, is related to the pattern of vehicle arrivals. If the two signals are interconnected, with a fixed offset, the arrival pattern would be nearly the same during each cycle. The key issue is whether the platoons from Sweet Avenue arrive primarily during the green phase or primarily during the red phase, of if they arrive randomly during the cycle.

Preliminary studies indicate the offset of the north-south green phase on U.S. 95 at Styner-Lauder Avenue is 20 seconds. This would result in favorable progression for the southbound traffic, or arrival type 4. All other factors that we considered in sub-problem 1b would remain the same for this analysis.

Sub-problem 2b: Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections

Step 2. Results

The effect of the arrival pattern of vehicles from the Sweet Avenue intersection to the Styner-Lauder Avenue intersection depends on when platoons from Sweet Avenue arrive at Styner/Lauder. The HCM classifies the way in which platoons arrive at a signalized intersection according to arrival type. Exhibit 1-23 shows the results of the delay calculations for this problem, assuming three arrival types (2, 3, and 4). Again, for this problem, we've found from field studies that arrival type 4 best describes the conditions likely to be present once the intersection is signalized.

It should be evident even before we do any analysis that varying the arrival type from Sweet Avenue will have no effect on the eastbound approach, the westbound approach, or the northbound approach, since none of these approaches are affected by the Sweet Avenue signal.

However, we do see some effect for the southbound approach. For the case in which we don't consider the Sweet Avenue signal (see Exhibit 1-11), the average control delay is computed to be 6.6 seconds per vehicle for the through and right turning traffic. This is shown as arrival type 3 in the table. For our projected condition, with favorable progression and arrival type 4, the delay is reduced to about 4 seconds per vehicle for this movement. While this is not a significant decrease, it does represent some effect for traffic on this approach. For purposes of comparison, we have also shown a less favorable progression condition, arrival type 2. Here, the average control delay increases to nearly 9 seconds per vehicle for the through and right turning traffic.

Sub-problem 2c: Analysis of the U.S. 95 Arterial

Step 1. Setup

We now turn to a view of the U.S. 95 arterial, of which the Styner-Lauder Avenue intersection is one part. You can review the physical layout of the intersection in Exhibit 1-6. On the northern end is the intersection of U.S. 95 and State Highway 8.

The Sweet Avenue intersection is located 560 feet to the south of the State Highway 8 intersection. It handles about 1,600 vehicles during a typical afternoon peak hour.

Styner-Lauder Avenue is located 1,070 feet south of Sweet Avenue, while the Palouse River Drive intersection is located 2,410 feet further south. The average speed on U.S. 95 is 35 miles per hour.

Discussion:
How do we determine the operational performance of an urban street or arterial? Which tools should be used for the analysis of an urban street? What data are required for the analysis? Take a few minutes to consider these questions. When you are ready, proceed to the next page.

Sub-problem 2c:  Analysis of the U.S. 95 Arterial

Several tools are available to analyze the operations of urban streets or arterials. Microscopic simulation programs that go beyond the capabilities of HCM procedures can be used if we have oversaturated conditions, closely spaced intersections where queues from one or both intersections interact, or if we want to study specific actuated controller parameters. Macroscopic simulation/optimization programs are also available for use, either in conjunction with or in lieu of the HCM procedures, to assess the arterial operations for undersaturated operations. Here, we will use the HCM urban street procedure from Chapter 15 of the HCM.

What data are needed for application of the HCM methodology? In conducting an urban street analysis, you need first to identify the major cross streets (signalized intersections) along the arterial, the distances between these cross streets, and the free flow speed along the arterial. The free flow speed is the average travel speed that vehicles would operate without being affected by signalized intersections or other vehicles. You also must determine the urban street classification, using Exhibits 10-4 and 15-2 from the HCM. Finally, the traffic signal data for each signalized intersection is needed. In effect, what we must do is conduct an operational analysis for each signalized intersection and use the estimates of control delay that result from each intersection analysis.

Sub-problem 2c: Analysis of the U.S. 95 Arterial

Step 2. Results

The HCM urban streets methodology produces travel speed and level of service for each arterial segment, as well as for the entire urban street. But to get an idea of the effect of a signal at the Styner-Lauder Avenue intersection, we must also estimate the speed and level of service for the urban street, without the new signal at Styner-Lauder. Exhibit 1-24 summarizes these results.

Sub-problem 2d: Effects of a Signal on an Existing Coordinated System

In sub-problem 2c, we analyzed the operation of a portion of the arterial, of which the Styner-Lauder intersection is a part, using the HCM methodology. But we must also consider another factor in the decision to signalize this intersection. How will the new signal at U.S. 95/Styner-Lauder Avenue affect signal coordination along the U.S. 95 corridor? To answer this question, we must examine the three coordinated intersections as a system (see Exhibit 1-25).

The first task is to determine whether the existing system is coordinated (i.e. has a common cycle length). If the signals currently operate in an uncoordinated mode, we will have to establish coordination between them by choosing a common cycle length. There are three important considerations that should be taken into account when selecting an appropriate cycle length:

1. Individual intersection timing requirements. Intersection phasing, pedestrian timing, and other factors dictate the lower bound for any common cycle length. As an example, consider the U.S. 95/SH 8 intersection, which currently operates under split phasing to accommodate the existing lane configuration. Split phasing strategies typically require a higher cycle length than if the left turns were protected or permitted. The effect is to cause this intersection to have the highest cycle length requirement of the intersections on the arterial. Therefore, the SH8 cycle length establishes the lower bound of the common cycle length requirement.

2. Distance between intersections. Closely-spaced intersections such as Sweet and SH 8 often benefit from lower cycle lengths, which allow for better queue management characteristics. The distance between intersections and queue storage considerations are key in the development of signal timing plans. While lower cycle lengths may sacrifice progression efficiency, they can still perform better, because queue spillback and system delay (especially on the side street) will be minimized.

3. Potential for cycle failures. When the cycle length is too short, cycle failures will occur, and the responsive operation that is characteristic of low cycle lengths may be offset by increased delay on movements where demand exceeds capacity. In such cases, somewhat longer cycles may actually achieve better progression—for example, where arterial green phases must be displayed more or less simultaneously.

Sub-problem 2d: Effects of a Signal on an Existing Coordinated System

Finding the optimal common cycle length can be achieved by using any of several traffic models that take all of the previously-identified factors into consideration. The HCM, however, does not include a methodology that can be used to make this determination. For the purposes of this discussion we will choose a 90-second cycle; this choice will accommodate a progression band through the intersections that is no smaller than the shortest arterial green phase, which, in this case is found at SH 8.

Now, if we extend the time–space diagram (see Exhibit 1-26) to include the new intersection at Styner-Lauder, we can determine the effect of a signal at this intersection on the entire coordinated system. Note that the distance to Styner-Lauder dictates an alternating relationship between the green phases with the adjacent intersection. Similarly, the close spacing of the SH 8 and Sweet intersections dictates a more-or-less simultaneous green phase strategy for the through movements. The simple two-phase operation at Styner-Lauder provides a longer green phase on the arterial (NB and SB approaches), so the additional signal is able to fit into the progression scheme without encroaching into the bands of progression that currently exist between the two signals. 

The conclusion that we can draw from this is that it is possible to signalize the Styner-Lauder intersection without a significant adverse effect on the driver-perceived progression, even though the arterial street analysis conducted in subproblem 2c suggests that travel speed will be impacted by 13 percent.

Exhibit 1-26. Signal Coordination Time/Space Diagram

Problem 2: Analysis

It is now useful to bring together the results of the three sub-problems that we considered as part of problem 2. In this problem, we considered the question of whether or not to signalize U.S. 95/Styner-Lauder Avenue not as an isolated one, in which we look at only the conditions at the intersection, but rather in the context of the U.S. 95 arterial and the intersections adjacent to Styner-Lauder. 

We learned in sub-problem 2a that the flow patterns from the adjacent intersections (specifically Sweet Avenue) do not affect the capacity of the Styner-Lauder Avenue intersection when it is operating with stop-sign control. The distance between Sweet Avenue and Styner-Lauder is great enough that the platoons from Sweet Avenue have dispersed sufficiently so that the capacities of the Styner-Lauder approaches are not affected.

We learned in sub-problem 2b that if the intersection of Styner-Lauder were signalized, there would be some effect on the delay of the U.S. 95 traffic at the intersection. This reduction in delay over the conditions that we considered in problem 1 results from the degree of coordination that can be achieved between the Sweet Avenue and Styner-Lauder Avenue intersections.

Both results continue to leave open the opportunity to signalize the Styner-Lauder intersection.

We found in sub-problem 2c, however, that when we apply the HCM urban street methodology, the benefits to the side streets (Styner and Lauder approaches) must be traded off against a reduction in level of service along the U.S. 95 arterial. The level of service before Styner-Lauder is signalized is estimated to be B. After the signal is installed, even with the seemingly acceptable delay to U.S. 95 traffic, the level of service for this arterial segment would be reduced to C, because the travel speed on the arterial is reduced from 29 mi/hr to 25 mi/hr.

There are other ways to minimize the impacts of a new signal on arterial operations and the through-traffic travel times. One of these ways is to coordinate the system of signalized intersections within the arterial section, and the effect of this was demonstrated in sub-problem 2d. It is also possible to minimize these impacts by restricting the non-arterial green time at the new Styner-Lauder/U.S. 95 traffic signal. A methodology for accomplishing this latter approach is further discussed in Problem 4.

Problem 2: Discussion

What is next to consider in this problem, after looking at more of a system context for the operation of the intersection and the arterial?  Note that we have focused our analysis so far on just the afternoon peak period. It is common in traffic analysis to look at the weekday peak periods to ensure that the traffic system is operating in an acceptable manner during this time. However, it is also important to ask ourselves if there are other time periods that require consideration?

The answer here is yes. This is a university town; and in addition to normal weekday peak periods of travel, there are a number of other traffic patterns that we should consider. The university has a number of special events during the year, each attracting a large number of visitors and attendees at the events. In addition, the vehicle mix in the traffic stream varies during the year, a fact that should also be assessed.

In problem 3, we will consider these travel patterns and determine if they have an effect on our decision to signalize the intersection of U.S. 95/Styner-Lauder Avenue.