Problem 2: Planning Methodology or Service Volume Table Application

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The HCM conceptual chapters (Chapter 10 for arterials and Chapter 12 for highways) provide tables of service volumes that were developed from the full computational procedures using default values for the operating parameters. In this problem, we will apply the tables to determine a rough estimate of the LOS for the three facilities defined by the north, middle, and south sections of Krome Avenue, respectively. As an additional exercise, we will project the volumes using assumed growth rates to determine the year in which the volumes will cross the higher LOS thresholds.

Before beginning this problem it is important to realize that the tables we will be using derive from a number of specific assumptions. The tables were originally intended to serve as examples only, and so they are unlikely to give accurate results if real-world conditions vary significantly from the assumptions that are an inherent part of those tables. This problem offers an opportunity to demonstrate how the tables can be used in situations where actual conditions approximate the underlying assumptions.

The following sub-problems are included in Problem 2. Each sub-problem deals with a separate section of Krome Avenue.

Sub-problem 2a. North Section (Class I Two-lane Highway)

Sub-problem 2b. Center Section (Class I or II Two-lane Highway)

Sub-problem 2c. South Section (Class I Signalized Arterial)

Each of these sub-problems will be discussed separately.

Sub-problem 2a: Analysis of the North Section of Krome Avenue (Class I Two-Lane Highway Facility)

Step 1. Setup

In this problem we will perform a planning analysis on the north section of Krome Avenue, treating the facility as a two-lane highway.  There are several assumptions that must be made in order to determine the operational characteristics of the roadway, even on a planning level.  Because of these assumptions, and also in part due to the general categorization of the the roadways, this planning analysis will need to be further refined as the information becomes available.

Consider the following questions:

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

Sub-problem 2a: Analysis of the North Section of Krome Avenue (Class I Two-Lane Highway Facility)

Step 1. Setup

What information is gained by performing a planning analysis? The example service volume table (HCM Exhibit 12-15) will be applied to this roadway facility to estimate the LOS for planning purposes. Because of the assumptions and approximations that support this exhibit, the HCM suggests that the results should not be used for operational analysis or final design.

What assumptions are used in the service volume tables? The service volumes for LOS A-E are given in HCM Exhibit 12-15 as a function of the free-flow speed (45-65 mph) and the type of terrain (level, rolling or mountainous). The terrain is very flat as is typical for all coastal Florida highways. The free flow speed for this facility has been determined from field observations as 55 mph. The design hour volume has been determined from field observations as 1,110 vph.

What level of confidence do the results carry? The results of the analysis carried out in this sub-problem is a function of the changes that occur over the planning horizon for the analysis. Considering the potential for development previously cited, it is likely that the level of confidence for our analysis is low.

Sub-problem 2a: Analysis of the North Section of Krome Avenue (Class I Two-lane highway Facility)

Step 2. Results

Exhibit 3-13 shows the results of this analysis. The LOS thresholds are taken from the portion of the table that applies to level terrain with a free-flow speed of 55 mph. Note that the absence of a threshold entry for LOS A indicates that it is not possible to attain LOS A with a free flow speed of 55 mph.

The estimated LOS for this section is D, because the design-hour volume (1,110 vph) exceeds the LOS C threshold (870 vph) but falls below the LOS D threshold (1,460 vph).

HCM Exhibit 12-15 may also be used to project the year in which a given LOS would be exceeded. To illustrate this process, let’s determine how many years in the future we would expect the operation to pass from LOS D to LOS E. In other words, we must determine when the volume will increase from 1,110 to 1,460 vph.

The annual growth rate for this highway has been determined to be in the range of 2.4%. At this rate, applying the compound interest formula, we can determine that the volumes will cross the LOS D threshold approximately 12 years in the future.

How useful is a LOS projection based on service volume tables? The projections are based on solid mathematics, but they are also predicated on the assumption that none of the operating parameters (directional split, heavy vehicle percentage, etc.) will change in the future. Therefore, the validity of the projections will be somewhat sensitive to factors such as future roadside development that could shift the table thresholds by lowering the free flow speed. The accuracy of projections of this type is limited by the operating parameters which, in this case, are likely to change over the next ten years. In other words, the operating parameters have a significant impact on the assumptions that are used to create the service volume tables.

Based on our calculations, our estimated level of service is D.

In this sub-problem we have produced an estimate of the LOS for the facility defined by the north section of Krome Avenue, assuming that it operates with the characteristics of typical two-lane highways of the same class. In sub-problem 3a, we will examine the assumptions and substitute observed values to apply the more detailed operational procedures described in Chapter 20 of the HCM.

Sub-problem 2b: Analysis of the Center Section of Krome Avenue (Class I or II Two-lane Highway Facility)

Step 1. Setup

In this problem we will perform a planning analysis on the facility defined by the center section of Krome Avenue. In this section of Krome Avenue, the roadway has characteristics of both a Class I and a Class II facility. Since HCM Exhibit 12-15 provides an estimate only for Class I Facilities, we will have to rely on another method to analyze this facility. How do you think the speed of the roadway (5 mph lower than the northern section) and the volume (higher than the northern segment) will affect the operations along this facility?

Discussion:
Take a few minutes to consider how the designated class might affect the results. Click continue when you are ready to proceed.

Sub-problem 2b: Analysis of the Center Section of Krome Avenue (Class II Two-lane Highway Facility)

Step 2. Results

It was determined in Problem 1 of this case study that the facility defined by the center section of Krome Avenue has some of the characteristics of a Class I and Class II facility and should ideally be investigated using both classes. It must be noted, however, that HCM Exhibit 12-15 provides service volume estimates only for Class I facilities, so the option of a Class II facility cannot be determined using HCM Chapter 12.

This sub-problem will be carried out in the same manner as sub-problem 2a, using the service volumes for LOS A-E given in HCM Exhibit 12-15 as a function of the free-flow speed (45-65 mph) and the type of terrain (level, rolling, or mountainous). The terrain is very flat as is typical for all coastal Florida highways. The free flow speed for this facility has been determined from field observations as 50 mph, which is 5 mph lower than the value used in sub-problem 2a. The design hour volume has been determined from field observations as 1,190 vph, which is 90 vph higher than the sub-problem 2a volume. The combination of a lower free-flow speed (which produces lower service volume thresholds) and higher traffic volumes suggests that the performance of this facility will not be as good as the facility defined by the north section of Krome Avenue discussed in sub-problem 2a. The question at this point is whether or not the difference will be great enough to cross the LOS D threshold.

The following table shows the results of this analysis. The LOS thresholds are taken from the portion of the table that applies to level terrain with a free-flow speed of 50 mph. Note that the absence of a threshold entry for LOS A and B indicates that it is not possible to attain either LOS A or B with a free flow speed of 50 mph.

Sub-problem 2b: Analysis of the Center Section of Krome Avenue (Class I Two-lane Highway Facility)

The estimated LOS for this facility is E because the design-hour volume (1,190 vph) exceeds the LOS D threshold (1,000 vph) but falls below the LOS E threshold (2,770 vph).

It was pointed out in sub-problem 2a that HCM Exhibit 12-15 may also be used to project the year in which a given LOS would be exceeded. We will not repeat those computations here because the LOS D threshold has already been exceeded.

In this sub-problem, we have produced an estimate of the LOS for the facility defined by the center section of Krome Avenue, assuming that it operates with the characteristics of typical two-lane highways of the same class. In sub-problem 3b we will examine the assumptions and substitute observed values for this facility to apply the more detailed operational procedures.

Based on Exhibit 3-14, this section of Krome Avenue operates near LOS E.

Sub-problem 2c: Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

Step 1. Setup

In this problem we will perform a planning analysis on the facility defined by the south section of Krome Avenue, treating the facility as a Class I Signalized Arterial. What do you think the general assumptions used in the HCM are?

Consider:

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

Sub-problem 2c: Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

The facility defined by the south section of Krome Avenue was determined in Problem 1 to be a Class I signalized arterial. We must therefore switch chapters from the previous two sub-problems to analyze this facility as an arterial. The planning level procedures for signalized arterial facilities are presented in Chapter 10 of the HCM. Let's review each of these questions and how they are important to this analysis.

What information is gained by performing a planning analysis? Specifically, HCM Exhibit 10-7 gives the estimated service volumes as a function of arterial class and number of lanes in each direction. Because of the assumptions and approximations that support this exhibit, the HCM suggests that the results should not be used for operational analysis or final design.

What assumptions are made when performing an operations analysis on a signalized intersection? Exhibit 3-15 summarizes the assumptions and LOS parameters used in this analysis. The LOS thresholds are taken from the portion of the table that applies to Class I arterials with one lane in each direction. Note that the absence of a threshold entry for LOS A and B indicates that it is not possible to attain either LOS A or B with a free flow speed of 55 mph. Be sure to pay special attention to the turns-from-exclusive-lanes parameter, because this is something that is frequently overlooked or misunderstood.

What is the effect of cycle length on our analysis? For the purpose of our analysis, we have assumed a consistent cycle length of 110 seconds. The HCM suggests that the cycle length may be increased to provide additional capacity at the intersections. Local signal timing policies should be considered to determine whether an increased cycle length is practical. Another approach to consider is to increase the effective g/c ratio for the arterial through movement during the peak periods, allowing over-capacity conditions on the less critical intersecting streets. Queue storage concerns must be carefully weighed in these instances.

Sub-problem 2c: Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

Sensitivity Analysis
Using an established growth rate of 2.4%, we can now project the years in which the traffic volumes will cross the various LOS thresholds. Based on these volume projections, we can estimate that:

Note that these projections assume no changes in any of the operating parameters within the timeframe of this analysis. It must be recognized that some of the parameters could change as a result of increased traffic volumes. For example, the g/c ratios could be reduced if protected left-turn phases are required on approaches that currently have no protected left turns.

Summary Discussion
As pointed out at the beginning of this problem, it is important to realize that the planning tables we applied here were created on the basis of some very specific assumptions about traffic characteristics. The tables were originally intended to serve as examples only within the HCM, and so it is important for the user to remember that they are unlikely to give accurate results if real-world conditions vary significantly from the inherent underlying assumptions. Even so, this problem offered an opportunity to demonstrate how the tables can be used in situations where actual conditions approximate the underlying assumptions.