ID# C303001

Problem 3: Application of HCM Chapters to Arterial and Highway Segments

Printable Version Printable Version

The HCM procedural chapters (Chapter 15 for arterials and Chapter 20 for two-lane highways) provide a detailed computational methodology for estimating the level of service on these two types of facilities. In this problem, we will apply those procedures to compute the performance measures that determine the LOS for thefacilities defined by the three sections. We will also compare the results with those of the planning level analyses performed in Problem 2.

The following sub-problems are included in Problem 3.  Each sub-problem deals with a separate section of Krome Avenue.

Sub-problem 3a. North Section (Class I Two-lane Highway facility)
Sub-problem 3b. Center Section (Class I or II Two-lane Highway facility)
Sub-problem 3c. South Section (Class I Signalized Arterial facility)

Each of these sub-problems will be discussed separately.

Before we begin to compute the operational parameters of the various facilities along Krome Avenue, consider the following key issues:

bullet

What (if any) potential issues were identified during the planning level analysis that may require mitigation?

bullet

What benefit was derived through the planning level analysis in this case study?

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

 [ Back ] to Sub-Problem 2c [ Continue ] to Sub-problem 3a

Page Break

ID# C303A01

Sub-problem 3a: Analysis of the North Section of Krome Avenue (Class I Two-lane Highway Facility)

Step 1. Setup

In this sub-problem, we will replace the assumptions used in our planning analysis with field data. We will then be able to compare the HCM planning analysis from Problem 2 with the operations analysis presented in this problem.

Consider:

bullet

What is the difference between the planning and operations analysis?

bulletWhat additional data will be applied in the operations procedure?

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

[ Back ] [ Continue ] with Sub-Problem 3a

Page Break

ID# C303A02

Sub-problem 3a: Analysis of the North Section of Krome Avenue (Class I Two-lane Highway Facility)

In sub-problem 2a, we 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 this sub-problem, we will examine the assumptions and substitute observed values for this section to apply the more detailed operational procedures.

What is the difference between the planning and operations level analyses? It is important to recognize the difference between the planning and operational level procedures. The operational procedure estimates the level of service from computed performance measures that are compared against established LOS thresholds for those measures. The two performance measures are percent time spent following (PTSF) and average travel speed (ATS). The LOS thresholds for these measures are shown in Exhibit 3-17 for a Class I two-lane highway. For this highway class, the more critical of the two measures will determine the LOS.

Exhibit 3-17. LOS Thresholds for Class I Two-Lane Highways

LOS

Percent Time-Spent-Following

Average Travel Speed (mph)

A

<35

>55

B

>35-50

>50-55

C

>50-65

>45-50

D

>65-80

>40-45

E

>80

<40

The planning level procedure presented in HCM Chapter 12 was derived from the operational procedure, assuming typical values for all operating parameters. The service volume table in HCM Exhibit 12-15 was produced by applying the operational procedure repetitively with different volumes and noting the volume levels at which the LOS changed from one value to the next. As such, the service volume table results should be identical to the operational level results, but only when the same operating parameters are applied to both procedures.

For example, the service volume tables presented in the HCM and used within the planning analysis assumes 14% trucks and buses. Data collected for Krome Avenue indicates the corresponding value for Krome Avenue is 27%. Similarly, the default peak hour factor for rural conditions assumed in the HCM is 0.88, whereas the actual measured PHF is 0.94. The differences between these values will cause the results of the two methods to depart from each other; and the operational level results must be considered more accurate, because they are based on actual field data instead of assumptions that do not apply to the facility under study.

Planning-level analyses are appropriate when operational parameters do not exist (for example, in a future-year analysis) or cannot be feasibly obtained. In such circumstances, detailed analysis such as queue length estimates are not appropriate because the quality of the available data does not support this level of analysis.

[ Back ] [ Continue ] with Sub-Problem 3a

Page Break

ID# C303A03

Sub-problem 3a: Analysis of the North Section of Krome Avenue (Class I Two-lane Highway Facility)

The procedures given in HCM Chapter 20 will be applied to the facility defined by this section of Krome Avenue.

What is the additional data that will be applied in the operations procedure? The additional data (i.e., beyond the sub-problem 2a requirements) include:

bullet

percent trucks

bullet

directional split

bullet

percent no passing zones

bullet

shoulder width

bullet

lane width

bullet

PHF

bullet

access points per mile

bullet

segment length

The last item, segment length, is not actually required for estimation of LOS, but it is used for calculation of travel time and vehicle-miles of travel. All of these data items with their associated sources and assumptions were discussed in the Getting Started section of this case study.

[ Back ] [ Continue ] with Sub-Problem 3a

Page Break

ID# C303A04

Sub-problem 3a: Analysis of the North Section of Krome Avenue (Class I Two-Lane Highway Facility)

Step 2. Results

The table below compares the results from the planning level analysis in sub-problem 2a with the operational level analysis. This table shows the values assumed by the service volume tables for all parameters, as compared with the values that apply to this facility. It shows the computations and results for both methods. The average travel speed (ATS) was computed as 45.2 mph, which suggests LOS C. The percent time-spent-following (PTSF) was computed as 66.9%, which suggests LOS D.

So, the resulting LOS was based on the PTSF and was found to be LOS D. This result was identical to the LOS estimate given by the service volume tables. The interpretation of the agreement between the two procedures is that the sum total of all of the differences between the assumed parameters and the site-specific parameters for this facility was not sufficient to produce a difference in the estimated level of service. 

Exhibit 3-18. Sub-Problem 2a Planning Analysis vs. Sub-Problem 3a  Operational Analysis
Input Data Sub-problem 2a Sub-problem 3a
Terrain Level Level
Base Free Flow Speed N/A 56.5
Access point / mile N/A 1
Shoulder Width N/A 3
Free Flow Speed 55 55
Directional Split 60/40 62/38
Percent Trucks 14  27
Percent RV’s Included in Truck %
Percent No Passing 20 18
PHF N/A 0.94
Design Hour Volume 1,110  1,110

Comparison of Results

Table Thresholds

  Computations
LOS A   ---   ATS = 45.2 mph
LOS B 330 PTSF = 66.9
LOS C 870 v/c = 0.38
LOS D  1,460  
LOS E   2,770
Estimated  LOS: D D

[ Back ] [ Continue ] with Sub-Problem 3a

Page Break

ID# C303A05

Sub-problem 3a: Analysis of the North Section of Krome Avenue (Class I Two-lane Highway Facility)

The question of base free flow speed deserves further discussion. The service volume tables deal in 5 mph increments of free flow speed. The operational method requires a specified base free flow speed, which is adjusted to reflect the effects of the specified shoulder width and the number of access points per mile in computing the actual free flow speed. In the course of the computations, the actual free flow speed was adjusted downwards in this case by 1.5 mph. So, to promote a fair comparison, the base free flow speed was specified as 56.5 mph, to produce the same free flow speed of 55 mph that was used by the service volume tables. This modification to the base free would not normally be recommended as a sound analytical practice. It was applied in this sub-problem to facilitate comparison between the planning and operational level procedures.

The HCM Chapter 20 procedure has given an overall level of service for this facility based on the performance measures for two-lane roadways. This procedure does not recognize any intersection-related problems. Therefore, a complete assessment of the facility requires a check of all intersections to ensure that problems are not being overlooked. The proper procedures to apply to intersections are found in HCM Chapter 16 (signalized) and HCM Chapter 17 (unsignalized). The details of the intersection analyses will not be presented here; however, it was found that two intersections experienced problems that will require further attention.

bullet

The intersection with Okeechobee Rd operates under two-way stop control (TWSC). This is a T intersection in which Krome Avenue is stopped at its northern terminus. The northbound approach here is oversaturated, and mitigation measures will be required. The alternative measures will be discussed in Problem 4.

bullet

The southern boundary of this section includes conditions that warrant further consideration. In this case, the cross street movements at Kendall Road are oversaturated. Mitigation measures will be discussed in Problem 5.

[ Back ] [ Continue ] to sub-problem 3b

Page Break

ID# C303B01

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

Step 1. Setup

In this sub-problem, we will replace the assumptions used in our planning analysis with field data for the facility defined by the center section of Krome Avenue. We will then be able to compare the HCM planning analysis to the operations analysis for this example, based on what is known about the assumptions made in the planning analysis verses the actual conditions along Krome Avenue.

Consider:

bullet

What are the differences in analyses for Class I and II facilities?

bulletWhat LOS do you expect the results of the analysis to show?

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

[ Back ] to Sub-Problem 3a [ Continue ] with Sub-Problem 3b

Page Break

ID# C303B02

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

The procedures given in HCM Chapter 20 will be applied to the facility defined by this section. It was determined in sub-problem 1b that this facility has characteristics that could normally be associated with both Class I and Class II two-lane highways. Therefore, the analysis will be repeated for both Class I and Class II facilities, and the results will be compared. The results will also be compared with those of the planning level analysis performed in sub-problem 2b.

Exhibit 3-19. LOS Criteria for Two-Lane Highways

  Class I Class II
LOS Percent Time- Spent-Following Average Travel Speed (mph) Percent Time-Spent-Following
A <35 >55 <40
B 35.1-50 50.1-55 40.1-55
C 50.1-65 45.1-50 55.1-70
D 65.1-80 41.1-45 70.1-85
E >80 <41 >85

What are the differences in analyses for Class I and II facilities? We will begin with a discussion of the differences in the LOS estimation procedures for Class I and II facilities.

As we pointed out in sub-problem 3a, the key performance measures for two-lane highways are the percent time spent following (PTSF) and the average travel speed (ATS). The LOS thresholds for these measures are shown in Exhibit 3-19 for both facility Classes.

On a Class I facility, the more critical of the two measures will determine the LOS. On a Class II facility, only the PTSF is considered, and the LOS thresholds for Class II are shifted upwards by 5 percentage points from Class I to reflect the lower driver expectation on a Class II highway. For example, the threshold for LOS E for Class I facilities is 80% and for Class II facilities it is 85%. One important point is that the performance measures will be computed in exactly the same way for both classes. In other words, the Class that you specify will not affect either the PTSF or the ATS. Only the thresholds will be applied differently.

[ Back ] [ Continue ] with Sub-Problem 3b

Page Break

ID# C303B03

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

What LOS do you expect the results of the analysis to show? This is difficult to determine because of the differences in the criteria used between the two class types. As we will see in the next few pages, the various criteria lead to different estimates for LOS.

Which factor do you expect to determine the level of service on a Class I Facility? In a vast majority of cases involving Class I two-lane roadways, the LOS will be determined by the PTSF. On the rare occasion that the ATS emerges as the determining factor, it is a good idea to revisit the question of whether this really should be considered as a Class I facility. The ATS will generally be the critical determinant of LOS only when the free flow speed is low. A low free flow speed is frequently the result of the same factors that would reduce the driver’s expectation of a high speed. As a Class I facility, the operation of this section of Krome Avenue would probably be considered unacceptable at LOS E. As a Class II facility it would fall into LOS D, only 1/10 of a PTSF percentage point away from LOS C (70.0% vs 70.1%). This raises a separate but frequently stated point about the value of judgment in dealing with thresholds.

[ Back ] [ Continue ] with Sub-Problem 3b

Page Break

ID# C303B04

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

Step 2. Results

Exhibit 3-20 compares the results from the planning level analysis in sub-problem 2b with the operational level analysis. It shows the values assumed by the service volume tables for all parameters, as compared with the values that apply to this facility. It also shows the computations and results for both methods. The ATS was computed as 39.3 mph. The PTSF was computed as 70.1%.

If this facility were designated as a Class I highway, the ATS would be the critical determinant of LOS, producing a value of E. The PTSF would have suggested LOS D. If the designation were changed to Class II, the ATS would be eliminated as a determinant of LOS, and the PTSF would establish LOS D.

Exhibit 3-20. Center Section: Comparison of Planning and Operational Level Analysis Results

 

Input Data

Table Assumptions

HCM Chapter 10

Sub-problem 2b

Field Observations

HCM Chapter 20

Sub-problem 3b

Terrain Level Level
Base Free Flow Speed N/A 53.1 mph
Access Points Per Mile N/A 2
Shoulder Width N/A 3
Free Flow Speed 50 mph 50
Directional Split 60/40 57/43
Percent Trucks 14 26
Percent RV's 4 Included in Percent Trucks
Percent No Passing 20 8
PHF N/A 0.91
Design Hour Volume 1,190 1,190
  Comparison of Results
Table Thresholds Computations
LOS A: ---

ATS=39.3

PTSF=70.1

v/c=0.42

LOS B: ---
LOS C: 330
LOS D: 1,000
LOS E: 2,770
Estimated LOS
Class I Facility E E
Class II Facility N/A D

[ Back ] [ Continue ] to Sub-Problem 3c

Page Break

ID# C303B05

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

The decision on class designation rests solely with the operating agency. The purpose of this Guide is to point out the factors that should be taken into consideration and their relationship to the HCM procedures. Therefore, the question of whether this portion of Krome Avenue should be a Class I or II facility will remain open.

Before we leave this sub-problem, we should take a look at how the operational analysis results compared with the service volume table results from sub-problem 2b. The comparison is evident in Exhibit 3-20. The bottom line is that, for a Class I facility the same estimation of LOS was produced by both procedures. While the LOS was improved for a Class II facility for the reasons just stated, it is not possible to compare this result with the service volume tables, because those tables are limited in scope to Class I facilities.

One last point: the base free flow speed was adjusted upwards to produce the same actual free flow speed used in the service volume tables to facilitate the comparison of the planning and operational level analyses. This topic was explained in detail in sub-problem 3a. The amount of the adjustment in this case was 3.1 mph, resulting in a base free flow speed of 53.1 mph in the table on the previous page.

[ Back ] [ Continue ] with Sub-Problem 3b

Page Break

 
Exhibit 3-20. Center Section: Comparison of Planning and Operational Level Analysis Results

 

Input Data

Table Assumptions

HCM Chapter 10

Sub-problem 2b

Field Observations

HCM Chapter 20

Sub-problem 3b

Terrain Level Level
Base Free Flow Speed N/A 53.1 mph
Access Points Per Mile N/A 2
Shoulder Width N/A 3
Free Flow Speed 50 mph 50
Directional Split 60/40 57/43
Percent Trucks 14 26
Percent RV's 4 Included in Percent Trucks
Percent No Passing 20 8
PHF N/A 0.91
Design Hour Volume 1,190 1,190
  Comparison of Results
Table Thresholds Computations
LOS A: ---

ATS=39.3

PTSF=70.1

v/c=0.42

LOS B: ---
LOS C: 330
LOS D: 1,000
LOS E: 2,770
Estimated LOS
Class I Facility E E
Class II Facility N/A D
 
 

 

 

 

Page Break

ID# C303B06

Sub-problem 3b: Operational Analysis of the Center Section of Krome Avenue (Class I or II Two-Lane Highway Facility)

The HCM Chapter 20 procedure has given an overall level of service for this facility, based on the performance measures for two-lane roadways. This procedure does not recognize any intersection-related problems. Therefore, a complete assessment of the facility requires a check of all intersections to ensure that problems are not being overlooked. The proper procedures to apply to intersections are found in HCM Chapter 16 (signalized) and 17 (unsignalized). The details of the intersection analyses will not be presented here.

Apart from the northern boundary at Kendall, which was mentioned in sub-problem 3a, the only operational problem was found at Biscayne. This is an unsignalized intersection with the cross street operating under stop control. Signalization will be required to overcome this deficiency. The signalized intersection analysis will not be covered in this case study.

[ Back ] [ Continue ] to Sub-Problem 3c

Page Break

ID# C303C01

Sub-problem 3c: Operational Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

Step 1. Setup

In this sub-problem, we will replace the assumptions used in our planning analysis with field data. We will then compare the results of the HCM planning analysis to the results of the operations analysis. Based on assumptions made in the planning analysis of problem 2 versus the actual conditions of Krome Avenue described in this case study, do you think the results of the two analyses will be similar? Better or worse?

Consider:

bullet

What additional data will be applied in the operations procedure?

bulletWhat are the factors that are used to determine urban street LOS?

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

[ Back ] to Sub-Problem 3b [ Continue ] with Sub-Problem 3c

Page Break

ID# C303C02

Sub-problem 3c: Operational Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

In sub-problem 2c, we have produced an estimate of the LOS for the facility defined by the facility defined by the south section of Krome Avenue, using the service volume tables in HCM Exhibit 10-7. These tables assume that the facility operates with the characteristics of typical signalized arterials of the same class. In this sub-problem, we will examine the assumptions and substitute observed values from the field to apply the more detailed operational procedures.

What additional data will be applied in the operations procedure? The procedures given in HCM Chapter 15 will be applied to this facility. The additional data (i.e., beyond the sub-problem 2c requirements) include:

bullet

percent trucks

bullet

directional split

bullet

percent turns from exclusive lanes

bullet

cycle length

bullet

effective green ratio

bullet

degree of saturation

bullet

lane group capacity

bullet

arrival type

The application of the HCM Chapter 15 procedure is considerably more complicated than the service volume tables in HCM Chapter 10. Each signalized intersection must be analyzed in detail using the signalized intersection analysis procedures prescribed by HCM Chapter 16. Knowledge of the signal timing plan, as well as the operating parameters for each approach, is required. Be sure to pay special attention to the percent-turns-from-exclusive-lanes parameter, which is frequently overlooked or misunderstood.

[ Back ] [ Continue ] with Sub-Problem 3c

Page Break

ID# C303C03

Sub-problem 3c: Operational Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

What factors are used to determine urban street LOS? The LOS for a signalized arterial is based on the average speed over its entire length. The average speed is computed by dividing the facility length by the total travel time for each vehicle. The total travel time includes the running time between signals and the control delay for through movements. The LOS thresholds for a Class I arterial are shown in Exhibit 3-21.

Exhibit 3-21. LOS Thresholds for Class I Arterials
LOS A >42 mph
LOS B >34-42 mph
LOS C >27-34 mph
LOS D >21-27 mph
LOS E >16-21 mph
LOS F <16 mph

The running time is determined by an Exhibit within the HCM that provides a running time based on the number of signals per unit length, the free flow speed, and the Class Type.

[ Back ] [ Continue ] with Sub-Problem 3c

Page Break

ID# C303C04

Sub-problem 3c: Operational Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

Step 2. Results.

Exhibit 3-22 compares the results from the planning level analysis in sub-problem 2c with the operational level analysis. It shows the values assumed by the service volume tables for all parameters, as compared with the values that apply to this facility. It also shows the computations and results for both methods. The overall average speed was 37.3 mph, which suggests LOS B. This result is consistent with the planning level analysis from sub-problem 2c.

Exhibit 3-22. South Section: Comparison of Planning and Operational Level Analysis Results

 

Table Assumptions

HCM Chapter 10

Sub-problem 2c

Field Observations

HCM Chapter 15

Sub-problem 3b

Notes

Classification I I Signal Density≈1 signal/mile
Free Flow Speed 50 mph 50 mph Posted speed +5 mph
Signal Density 0.8 per mile 1.0 per mile Scaled from map
Cycle Length 110 seconds 88 seconds varies from 76 to 106 sec
Effective g/c 0.45 0.53 varies from 0.48 to 0.57
Adj. Sat. Flow Rate 1,850 vphgpl 1,744 vphgpl varies from 1,675 to 1,810
Arrival Type 3 3 No coordination
Unit Extension 3 3 Average value
Initial Queue 0 0 None observed
Other Delay 0 0 None observed
PHF 0.92 0.92 None observed
Turns from Exclusive Lanes 10% 11% Varies from 5% to 22%
Left Turn Bays Yes Yes Except for Avocado
Lane Utilization Factor 1 1
Design Hour Volume 812 812 Average value
Chapter 10 Results
LOS Thresholds
LOS A

---

>42 mph
LOS B <860 vph >34-42 mph
LOS C >860-930 vph >27-34 mph
LOS D >930-1,020 vph >21-27 mph Travel Time=681 sec
LOS E >1,020-1,140 vph >16-21 mph Section Length=7.08 miles
LOS F >1,140 vph <16 mph Average Speed=37.4 mph

LOS = B

LOS = B

[ Back ] [ Continue ] with Sub-Problem 3c

Page Break

ID# C303C05

Sub-problem 3c: Operational Analysis of the South Section of Krome Avenue (Class I Signalized Arterial Facility)

To fully assess the operation of an arterial route, each signalized intersection must be examined to ensure that it is free of operating problems. The individual segment results are presented in the table below. The volume to capacity (v/c) ratios are all well below 1.00, indicating that none of the approaches was oversaturated. The average speeds are all within the 30-40 mph range, and the segment LOS values are all within the B-C range. 

Exhibit 3-23. Summary of HCM Arterial Analysis Results (Northbound)
  Segment Length (mi) Travel Time (sec) Cycle Length (sec) g/c ratio v/c ratio Delay (sec) Average Speed (mph) LOS
Biscayne Dr 0.51 45 76 0.54 0.66 14 31.1 C
Bauer Dr 1.52 122 79 0.52 0.71 16 39.7 B
Coconut Palm Dr 1.01 81 81 0.57 0.58 12 39.3 B
Silver Palm Dr 1.02 82 91 0.56 0.64 15 38.1 B
Hainlin Mill Dr 1.01 81 81 0.57 0.63 13 38.8 B
Quail Roost Dr 1.00 80 81 0.57 0.61 12 38.9 B
Eureka Dr 1.01 81 106 0.48 0.81 28 33.4 C

Exhibit 3-23 covers the peak direction on the arterial route. As an additional check on the operation, each intersection should be examined to determine whether or not any of the other movements experienced congestion. A check of all movements indicated that two movements within this facility were operating with oversaturated conditions:

bullet

Eastbound at Quail Roost Drive: The single lane on this approach is not able to accommodate the left, through, and right turning movements. The problem may be solved easily by the addition of one lane to this approach. The HCM analysis is very straightforward and does not merit further discussion as a separate problem in this case study.

bullet

Westbound at Biscayne Drive: Again, the problem is caused by a single lane that must accommodate all movements. In this case, there are some alternatives that arise with respect to lane utilization. These alternatives will be considered in Problem 6.

Apart from these two trouble spots, the arterial operation is very simple. Most of the intersections are operating satisfactorily without protected left turns on the cross street. There are much more complex arterial facilities covered in other case studies in this guide.

[ Back ] [ Continue ] to Problem 4