The Highway Capacity Manual - 6 Edition: Overview and What's New

A SunCam online continuing education course
The Highway Capacity Manual -

6th Edition: Overview and What’s New
by
Scott S. Washburn, Ph.D., P.E.

Leslie D. Washburn, P.E.
Highway Capacity Manual – 6th Edition: Overview and What’s New
COURSE DESCRIPTION
The 6th Edition of the Highway Capacity Manual (HCM) was released in 2016 and is now the
standard for both the Fundamentals of Engineering (FE) exam and Principles and Practice of
Engineering (PE) exam. The title of this new HCM is “HCM 6th Edition: A Guide for Multimodal
Mobility Analysis”. This edition of the HCM provides methods for evaluating multimodal
operations of freeways, highways, and arterial streets. The focus of this course is on providing a
general overview of the content and organization of the HCM 6th edition and highlighting
revisions/updates from the previous edition of the HCM (HCM 2010). The course does not provide
detailed coverage of analysis methodology elements. Due to the volume of material covered in
the HCM, some general knowledge of the document(s) may be helpful to completing this course,
but it is not required.
Figure 1. Highway Capacity Manual 6th Edition Cover

Source: Transportation Research Board
www.SunCam.com Copyright 2019 Page 2 of 43

Scott S. Washburn and Leslie D. Washburn
In this course, you will learn about:

• HCM edition history
• Why the HCM title was changed
• Why the need for a new HCM edition
• HCM 6th edition structure – revised chapter layout design to help practitioners use the
manual
• New HCM analysis methodology capabilities
• Future directions/updates to the HCM
Source of photo is Scott or Leslie Washburn, unless noted otherwise.

Contents
Course Description.......................................................................................................................... 2
Introduction and History ................................................................................................................. 6
Need for a New Edition .................................................................................................................. 8
HCM 6th Edition organization....................................................................................................... 10
Overview .................................................................................................................................. 10
Structure ................................................................................................................................... 11
Volume 1 - Concepts ........................................................................................................... 12
Volume 2 – Uninterrupted Flow.......................................................................................... 13
Volume 3 – Interrupted Flow .............................................................................................. 13
Volume 4 (Online) – Concepts Supplemental ..................................................................... 14
New Methodology Capabilities: Uninterrupted Flow................................................................... 16
Freeway Facilities (Chapters 10/25)......................................................................................... 16
Freeway Reliability Analysis (Chapter 11) .............................................................................. 17
Freeway and Multilane Highway Segments (Chapters 12/26)................................................. 18
Unified speed-flow equation ............................................................................................... 19
Analysis Flow Rate.............................................................................................................. 23
Passenger Car Equivalents (PCE) for Heavy Vehicles (ET) ................................................ 24
Other Freeway Segments.......................................................................................................... 25
Weaving (Chapter 13/27) .................................................................................................... 25
Merge/Diverge (Chapter 14/28) .......................................................................................... 26
Two-Lane Highways (Chapter 15/26) ...................................................................................... 28
New Methodology Capabilities: Interrupted Flow ....................................................................... 29
Urban Street Facilities (Chapters 16/29) .................................................................................. 29
Urban Street Reliability and ATDM (Chapter 17/37) .............................................................. 29
Urban Street Segments (Chapters 18/30) ................................................................................. 29
Signalized Intersections (Chapters 19/31) ................................................................................ 32
Two-Way Stop-Controlled Intersections (Chapters 20/32)...................................................... 33
All-Way Stop-Controlled Intersections (Chapters 21/32) ........................................................ 33
Roundabouts (Chapters 22/33) ................................................................................................. 34
Ramp Terminals and Alternative Intersections (Chapter 23/34).............................................. 35
Off-Street Pedestrian and Bicycle Facilities (Chapter 24) ....................................................... 38
Advanced Traffic and Demand Managment (ATDM) (Chapter 37) ....................................... 39
Future Updates .............................................................................................................................. 39
Resources ...................................................................................................................................... 40
Software ........................................................................................................................................ 41

List of Figures
Figure 1. Highway Capacity Manual 6th Edition Cover ................................................................. 2
Figure 2. HCM Covers for 1950 and 1965 editions........................................................................ 7
Figure 3. HCM Covers for 1985 and 2000 editions........................................................................ 7
Figure 4. HCM Cover for 2010 edition .......................................................................................... 8
Figure 5. HCM 6th Edition, Volume 4: Application Guide .......................................................... 15
Figure 6. Freeway managed lanes and general-purpose lanes interaction .................................... 16
Figure 7. Freeway ......................................................................................................................... 18
Figure 8. Unified speed-flow equation ......................................................................................... 20
Figure 9. Basic Freeway Segment Speed-Flow Curves ................................................................ 22
Figure 10. Multilane Highway Segment Speed-Flow Curves ...................................................... 23
Figure 11. Weaving in a Managed Lane Access Segment............................................................ 26
Figure 12. Managed Lane Weaving Segment ............................................................................... 26
Figure 13. Managed Lane Merge Example................................................................................... 27
Figure 14. Managed Lane Diverge Example ................................................................................ 27
Figure 15. Two-Lane Highway Segment ...................................................................................... 28
Figure 16. Midsegment Lane Blockage Dipiction ........................................................................ 30
Figure 17. Roundabout in both directions..................................................................................... 30
Figure 18. Right-turn-on-red vehicles incorporated into volume-balancing method ................... 31
Figure 19. Signalized Intersection ................................................................................................ 32
Figure 20. Work zone presence at approach to signalized intersection ........................................ 33
Figure 21. Roundabout Example .................................................................................................. 35
Figure 22. Ramp Terminal Forms ................................................................................................. 36
Figure 23. Intersection Forms ....................................................................................................... 37
Figure 24. Example of experienced travel time at an RCUT intersection .................................... 38
List of Tables
Table 1. Travel Speed Threshold by Base Free-Flow Speed (mi/h) ............................................. 29

INTRODUCTION AND HISTORY
“The Highway Capacity Manual, Sixth Edition: A Guide for Multimodal Mobility Analysis (HCM)
continues the manual’s evolution from its original objective— providing methods for quantifying
highway capacity—to its current form as a fundamental reference on concepts, performance
measures, and analysis techniques for evaluating the multimodal operation of streets, highways,
freeways, and off-street pathways. The Sixth Edition incorporates the latest research on highway
capacity, quality of service, and travel time reliability, and improves the HCM’s chapter outlines
to help practitioners better understand basic concepts, computational steps, and outputs when
applying HCM methods. These changes are designed to keep the manual in step with its users’
needs and present times.”
The Transportation Research Board’s Highway Capacity and Quality of Service (HCQS)
committee is responsible for development and maintenance of the HCM. The HCQS committee
is comprised of the following subcommittees:
• freeways/multilane highways,
• two-lane highways,
• signalized intersections,
• unsignalized intersections,
• simulation applications,
• performance measures,
• user liaison,
• planning and preliminary engineering, and
• active transportation and demand management.
The first HCM was published in 1950. Since that time, new editions of the HCM have been
published every 10-20 years. Previous HCM editions have had a year in the title, although they
have also been referred to with an “edition” number. The previous HCM editions are listed in
Figures 2-4.

1950: Only 160 pages long, and focused on 1965: Introduction of the level of service
capacity related to the U.S. roadway network (LOS) concept and bus transit chapter.
expansion after World War II.
Figure 2. HCM Covers for 1950 and 1965 editions
Starting with the 1985 edition, results of new research on highway capacity and quality of service
were presented for measuring roadway performance.
1985: Significant research incorporated 2000: Added new research with increase in
adding pedestrians and bicycles chapters. volume and breadth.
Updates to this edition were made in 1992,

1994, and 1997
Figure 3. HCM Covers for 1985 and 2000 editions

In order to make the material in the HCM 2000 easier to understand and follow, the HCM 2010
was reorganized with new material added from research projects. A broader range of performance
measures were introduced for assessing the performance of transportation facilities and all
roadway users.
2010: New research added, additional focus on non-

automobile modes, four volumes.
Reorganized to make it more clear and understandable.
Figure 4. HCM Cover for 2010 edition
The four volumes of the HCM 2010 include:

• Volume 1: Concepts,
• Volume 2: Uninterrupted Flow,
• Volume 3: Interrupted Flow, and
• Volume 4, Applications Guide.
Volume 4 was introduced online to facilitate issuing new and updated chapters.
NEED FOR A NEW EDITION
The interim chapters and soon-to-be-completed research needed to be integrated into the HCM in
an organized manner for users to follow. New methodolologies require complicated and rigorous
calculations, where computer software is needed in order for practionars to realistically
impletment the new methods. The new methods needed to be outlined and explained in detail in
order for software developers to replicate the methods in software programs and so practioners
understand the calculations.
The content of the HCM has changed and evolved from simply highway capacity to include many
facility type performance measures and multimodal analysis. Transportation professionals and
decision-makers have not always been aware of these additional features. The analysis needs of

transportation professionals changed, but the HCM title remained the same since 1950. Providing
mobility for people and goods is transportation’s most essential function. Thus, “A Guide for
Multimodal Mobility Analysis” captures the HCM’s ability to quantify roadway performance
across multiple dimensions and travel modes.
The new HCM consists of four dimensions:
Also, in order for research to become
implemented, it needs to be usable by  Quantity of travel, the magnitude of use of
practitioners. This now often means that a transportation facility or service;
methods need to be implemented in software.  Quality of travel, users’ perceptions of
In order for software to be developed, the new travel on a transportation facility or
methods need to be integrated with existing service with respect to their expectations;
 Accessibility, the ease with which
material and explained in sufficient detail that
travelers can engage in desired activities;
software developers can accurately replicate and
the method(s). At the same time, the need exists  Capacity, the ability of a transportation
to explain the methods clearly to end users. As facility or service to meet the quantity of
a result, a new edition of the HCM was travel demanded of it.
necessary.
HCM 6th Edition revisions includes:
 New title, to reflect how HCM content has expanded

considerably beyond simply “highway capacity”
 Multiple performance measures
 Many facility types
 Multimodal analysis (autos, trucks, peds, bikes,
buses)
 Added travel time reliability, and managed lane, work
zone, and alternative intersection operations.

Several million dollars in new research has been performed since the HCM 2010 was published.
Some of these projects introduce new capabilities to the HCM:
• analyzing managed lane and work zone operations,
• measuring and predicting travel time reliability,
• analyzing alternative intersection forms, and
• evaluating roadway corridors incorporating a series of roundabouts.
Some projects update previous research to

account for driver behavior and vehicle New Research since HCM 2010
technology changes:  NCFRP 41: Truck analysis
• truck analysis,  NCHRP 03-96: Managed lanes
• roundabouts, and  NCHRP 03-100: Roundabouts in corridors
 NCHRP 03-107: Work zone capacity
• Active Traffic and Demand
 NCHRP 03-115: HCM production
Management (ATDM).
 NCHRP 07-22: Planning guide to HCM
 SHRP2-L08: Travel time reliability
Finally, some projects have focused on
 FHWA: ATDM, roundabouts, alternative
enhancing how all of this new and existing
intersections
information can be effectively communicated to
HCM users.
HCM 6TH EDITION ORGANIZATION
Overview
The HCM 2010 reorganized the manual to better serve its users. Basic information about each
chapter’s methods was provided in the printed chapters. Depending on the complexity of the
method, this information could range from full step-by-step instructions to a higher-level overview
of a method’s inputs and flow of calculations. Supplemental online chapters, provid calculation
details for the more complex methods and other information primarily of interest to a subset of
HCM users. For some methods requiring iterative calculations, computational engine code listings
provided in the online technical reference library offered the greatest level of detail. The online
library also provides access to many of the research reports that document the development of
HCM methods.

The HCM 6th Edition

Additional changes made for the HCM 6th Edition
continues the
 Standardized chapter outlines
evolution of how
 Presentation of the core chapter methodology, followed by
HCM information is
extensions
presented to better
 Summary tables listing data requirements, potential data sources,
serve the needs of
suggested default values, and sensitivity of results to inputs
today’s users. As will
 Example results in many chapters
be described in detail,
 Example problems moved to Volume 4
methodological
chapter outlines have been standardized. Summary tables are provided that list data requirements,
potential data sources, suggested default values, and the sensitivity of results to particular inputs.
Many Volume 2 and 3 chapters now provide example results to help analysts determine the
reasonableness of the results they obtain from software, to highlight a method’s sensitivity to
particular inputs, and to highlight possible methodological quirks (e.g., step-function behavior).
They are not intended to substitute for an actual analysis, as the results depend on the assumptions
used for other inputs, and are therefore deliberately printed at a size large enough to indicate trends
but too small to pick out specific results.
Example problems have been moved to the supplemental chapters in online Volume 4 to free up
space for new material in the printed HCM and allow access to all, including students who do not
yet own an HCM. The creation of online Volume 4 allows the HCM to be updated more quickly
after research is completed, compared to the typical multi-year wait for a new edition. Four such
chapters; Active Traffic and Demand Management (ATDM) update (Chapter 35), Travel time
reliability (Chapters 36, 37), and Managed lanes (Chapter 38) were released online in 2014. Each
chapter was stand-alone and could not be used in conjunction with other new material (e.g.,
reliability of managed lanes).
Structure
The HCM is divided into four volumes: three printed and one online, just as for the HCM2010.
Volume 1 houses concepts material, Volume 2 describes the methodologies for freeways and
highways, while Volume 3 describes the methodologies for urban streets and off-street pedestrian
and bicycle facilities. Volume 4 provides additional resources that support the material in the
printed HCM.

General highway capacity and quality of service concepts were addressed in Volume 1. The typical
Volumes 2 and 3 section layout consists of:
• Detailed chapter-specific concepts, such as service volume tables, have been added.
• Methodological chapters begin with a short introduction describing the purpose of the
chapter, the information found within it, and cross-references to related material in other
chapters.
• The core motorized vehicle methodology (e.g., two-lane highway analysis for motor
vehicles) is presented next, including the method’s strengths and limitations, circumstances
when alternative tools might be considered, the method’s required input data (and potential
sources), and a description of the flow of calculations.
• Mode-specific methodologies (e.g., bicycle travel on two-lane highways), to better
highlight the existence of these methods, are provided after the motorized vehicle
methodology
• An applications section provides information about applying the chapter’s method to
planning and preliminary engineering, operations, and design analyses; often provides
example results; and discusses the use of the method in conjunction with alternative tools,
when available.
• A reference section for each chapter is provided.
Volume 1 - Concepts
Volume 1 introduces concepts that all HCM users should be familiar with before applying the
manual. The chapter structure of Volume 1 is the same as the HCM 2010. Chapter 36 in online
Volume 4 provides supplemental information that supports Volume 1; new information in this
chapter includes guidance on measuring travel time reliability in the field and listings of reliability
values for selected U.S. facilities.
Printed Chapters Online Chapters (Volume 4)

1. HCM User’s Guide 36. Concepts: Supplemental
2. Applications
3. Modal Characteristics
4. Traffic Operations and Capacity Concepts
5. Quality-of-Service Concepts
6. HCM and Alternative Analysis Tools
7. Interpreting HCM and Alternative Tool Results
8. HCM Primer
9. Glossary and Symbols

Volume 2 – Uninterrupted Flow

HCM Volume 2 provides concepts, performance measures, and analysis methodologies specific
to uninterrupted-flow facilities such as freeways. Chapter 11 on freeway reliability analysis is new.
In addition, the HCM 2010 chapters on basic freeway segments and multilane highways have been
merged into a single chapter (Chapter 12), as these roadways share many similar characteristics.
Chapters 25-28 in online Volume 4 provide material that supplements the Volume 2 chapters. In
particular, the example problems for uninterrupted-flow facilities have been moved into the
Volume 4 chapters.

10. Freeway Facilities Core Methodology 25. Freeway Facilities: Supplemental
11. Freeway Reliability Analysis (New) 26. Freeway and Highway Segments:
Supplemental
12. Basic Freeway and Multilane Highway 27. Freeway Weaving: Supplemental
Segments
13. Freeway Weaving Segments 28.Freeway Merges and Diverges:
Supplemental
14. Freeway Merge and Diverge Segments
15. Two-Lane Highways
Volume 3 – Interrupted Flow

HCM Volume 3 provides concepts, performance measures, and analysis methodologies specific
to interrupted-flow facilities such as urban streets and the intersections found along them.
Chapter 17, presenting methods for evaluating travel time reliability on urban streets and
analyzing the effects of ATDM strategies, is new in the sixth edition. In addition, Chapter 23 has
been expanded to cover alternative intersection and interchange types, such as diverging
diamond interchanges and median U-turn intersections. Chapters 29-35 in online Volume 4
provide supplemental information, including example problems.


16. Urban Street Facilities 29. Urban Street Facilities: Supplemental
17. Urban Street Reliability and ATDM 30. Urban Street Segments: Supplemental
18. Urban Street Segments 31. Signalized Int.: Supplemental
19. Signalized Intersections 32. Stop-Controlled Intersections: Supplemental
20. Two-Way Stop-Controlled Int. 33. Roundabouts: Supplemental
21. All-Way Stop-Controlled Int. 34. Interchange Ramp Terminals: Supplemental
22. Roundabouts 35. Pedestrian & Bikes: Supplemental
23. Ramp Terminals and Alternative 37. ATDM: Supplemental
Intersections
24. Ramp Terminals and Alternative
Intersections
Volume 4 (Online) – Concepts Supplemental

Volume 4 provides more than just access to supplemental chapters. Unlike the 2010 version,
Volume 4 is open to all, including those who do not have a personal copy of the HCM. A free user
account is required for access. It is available online at http://www.hcmvolume4.org/.

Figure 5. HCM 6th Edition, Volume 4: Application Guide

Source: Transportation Research Board
TRB’s HCQS Committee periodically issues interpretations and errata for the HCM that result
from user questions and feedback. These errata are posted in Volume 4. The technical reference
library provides access to many of the source research reports that form the basis for the HCM
analysis methods, including:
• The HCM Applications Guidebook: This is an online tool that demonstrates how the
HCM can be applied to a project as it evolves from concept to design to implementation.
• The Planning and Preliminary Engineering Applications Guide: This demonstrates how
HCM concepts and methods can be incorporated into planning-level evaluations.

• Discussion Forum: This allows HCM users to ask questions of the broader HCM
community on proper application of the HCM methodologies, technical interpretations,
provide feedback about the HCM, etc. Committee members monitor and respond to the
forum.
NEW METHODOLOGY CAPABILITIES: UNINTERRUPTED FLOW
Freeway Facilities (Chapters 10/25)
A number of new capabilities have been added to Chapter 10 as a result of new research, including
core methods to analyze freeway work zones and managed lanes, including interactions between
managed lanes and general-purpose lanes. Truck effects are discussed in Chapter 12 in the context
of freeway segments, but carry over into a facilities analysis. Similarly, ATDM strategy evaluation
is discussed in Chapter 11 in the context of a reliability analysis, but Chapter 10 provides guidance
on evaluating the effects of these strategies on typical-day (average) freeway operations.
Finally, in light of the increasing availability of freeway travel time data from commercial
providers, Chapter 10 provides guidance on matching those databases’ sections to HCM segments.
Figure 6. Freeway managed lanes and general-purpose lanes interaction
Chapter 25, Freeway Facilities: Supplemental, provides additional details related to the freeway
facilities method. It describes how the method can be calibrated to match existing conditions
through the application of capacity and speed adjustment factors. It also provides a method for
evaluating truck performance on composite grades—that is, a series of consecutive grades with
differing gradients. A new planning-level method, compatible with the Chapter 10 operational

method, is presented. Finally, new example problems have been developed that demonstrate the
new capabilities.
Freeway Reliability Analysis (Chapter 11)
A new Chapter 11, Freeway Reliability Analysis, integrates material from the former interim
Chapters 36 and 37 with the other enhancements to the freeway facility methodology. Rather than
evaluating only an ”average” or ”typical” day with good weather and no incidents, a reliability
analysis looks at the effects of various things (for
example, severe weather and incidents) that temporarily  Significant extension of
reduce a freeway’s performance over the course of year material on travel time
or other long-term timeframe. reliability and ATDM
 Applies HCM Chapter 10
Each combination of demand level, weather condition, method repeatedly with
and incident and work zone effects during a given day is adjusted demands, capacities,
called a ”scenario”. A set of scenarios is randomly lanes, and free-flow speeds to
generated in software based on the probability of a develop a travel time
particular condition occurring; one scenario is generated distribution
for each day within the ”reliability reporting period” (for  Incorporates demand variation,
example, each non-holiday weekday throughout the weather, incident, work zone,
year). The facility travel time is evaluated for each 15- and special event effects
minute analysis period within each scenario by repeatedly  Produces a variety of
applying the Chapter 10 method, resulting in a reliability-related performance
distribution of travel times that can be used to generate a measures
variety of useful reliability-related performance
measures.
Chapter 11 also describes how the effects of ATDM strategies on freeway reliability can be
evaluated, including the process for developing reliability “scenarios” and for accounting for
weather and incident effects, which has been improved. Supplemental Chapter 25 provides
computational details and describes how the reliability method can be calibrated to existing
conditions.

Freeway and Multilane Highway Segments (Chapters 12/26)
Figure 7. Freeway
Because the basic methods for evaluating freeway and multilane highway segments are very
similar, they have been merged into one chapter for ease of presentation. Factors that distinguish
multilane highway operation from freeway operation are still accounted for. To facilitate the
integration of the new reliability, managed lanes, and work zone methods, a unified speed-flow
equation has been developed applicable to both freeway and multilane highway segments, but the
forms of the curves are different.
Other notable changes in Chapter 12 include updated truck passenger car equivalency tables, based
on new research. Capacity and free-flow speed adjustment factors (CAFs and SAFs) are now the
primary way of making adjustments to the speed-flow relationship. Changes to the multilane
highway methodology include making the density at capacity value no longer dependent on free-
flow speed and setting this value consistent with that for basic freeway segments (i.e., 45 pc/mi/ln)
and providing speed-flow curves for 65 and 70 mi/h free-flow speeds.

The performance measure for level of service (i.e., service measure) remains unchanged (i.e.,
density). Thus, the equation for basic freeway and multilane highway segment remains.
vp
D=
S
where:
D = Density, in pc/mi/ln
vp = Analysis flow rate in pc/h/ln, and
S = Average speed, in mi/h.
Unified speed-flow equation

Previous editions of the HCM defined separate equations for basic freeway segments and multilane
highway segments, although they were largely similar. In the 6th edition of the HCM, a single
equation has been defined for both basic freeway and multilane highway segments, with
parameters whose values can vary by segment type. This new equation is referred to as the unified
speed-flow equation, and is as follows:
S = FFSadj vp ≤ BP
where:
FFSadj = Adjusted free-flow speed, in mi/h, and is calculated as follows:
FFSadj = FFS × SAF for basic freeway segments

FFSadj = FFS for multilane highway segments
 cadj 
 ( v p − BP )
a
 FFS adj −
S FFS adj − 
Dc 
= BP < vp ≤ cadj
( cadj − BP )
a
where:
cadj = Adjusted capacity, in pc/h/ln, and is calculated as follows:
cadj = c × CAF for basic freeway segments

cadj = c for multilane highway segments
vp = 15-min passenger car equivalent flow rate, in pc/h/ln,

BP = Linear to curvilinear flow rate breakpoint value in pc/h/ln,
Dc = Density at capacity (45 pc/mi/ln), and
a = Exponent value.

The unified speed–flow equation defines a region where the speeds are equal to the free-flow
speed and a region where speeds drop until capacity is reached, which is defined as occurring at
a density of 45 pc/mi/ln for both freeways and multilane highways. A breakpoint flow rate marks
the boundary between the two regions. The exponent a, which defines the shape of the curve
beyond the breakpoint flow rate, is different for freeways and multilane highways. The capacity
for a given free-flow speed is also different for freeways and multilane highways. Both free-flow
speeds and capacities can be adjusted to reflect the effects of severe weather, incidents, work
zones, and special events, or to calibrate the HCM speed-flow curves to local conditions. An
example illustration of the unified speed-flow equation is shown in Figure 8.
Figure 8. Unified speed-flow equation

The calculation or assignment of the parameter values, specific to each segment type are as
follows:
Capacity (c)
For freeway segments:

c= Min  2200 + 10 ( FFS − 50 ) , 2400  55 ≤ FFS ≤ 75
For multilane highway segments:

c= Min 1900 + 20 ( FFS − 45 ) , 2300  45 ≤ FFS ≤ 70
Breakpoint (BP)
For freeway segments:

BP = 1000 + 40 ( 75 − FFS adj )  × CAF 2 55 ≤ FFS ≤ 75
For multilane highway segments:

1400 pc/h/ln
Exponent a
For freeway segments: 2.0

For multilane highway segments: 1.31
Roadway base conditions for a basic freeway segment are defined by the HCM as 12-ft minimum
lane widths, 6-ft minimum right-shoulder clearance, only passenger cars, 6 miles or greater
interchange ramp spacing and level terrain (2% grade or less). The following figure shows some
example speed-flow relationships based on the unified speed-flow equation and the parameter
values for basic freeway segments.

Figure 9. Basic Freeway Segment Speed-Flow Curves
Roadway base conditions for a multilane highway are defined by the HCM as 12-ft minimum lane
widths, 12-ft minimum total lateral clearance, only passenger cars, no access points, divided
highway, free-flow speed of 60 mi/h or more and level terrain (2% grade or less). The LOS E–F
density threshold has been revised to use 45 pc/mi/ln for all free-flow speeds (the same value for
basic freeway segments), rather than varying by free-flow speed as in the HCM 2010. The unified
equation parameter calculations also allow speed-flow curves to be developed for free-flow speeds
up to 70 mi/h, versus a maximum free-flow speed of 60 mi/h for the HCM 2010. The following
figure shows some example speed-flow relationships based on the new unified speed-flow
equation and the parameter values for multilane highway segments.

Figure 10. Multilane Highway Segment Speed-Flow Curves
The bicycle analysis methodology for multilane highways is identical to the method for two-lane
highways, and users are referred to Chapter 15 for the details of that method.
Analysis Flow Rate

The analysis flow rate equation no longer includes the driver population factor (fp). The impact to
the traffic stream density due to driver population is now handled by revising the speed-flow
relationship, by applying adjustments to the free flow speed (through the SAF) and the capacity
(through the CAF). More information on this topic is provided in Chapter 26, exhibit 26-9.
V
vp =
PHF × N × f HV

where:
vp = 15-min passenger car equivalent flow rate in pc/h/ln,
V = hourly volume in veh/h,
PHF = peak-hour factor,
N = number of lanes, and
fHV = heavy-vehicle adjustment factor.
Passenger Car Equivalents (PCE) for Heavy Vehicles (ET)

The categorization of heavy vehicles changed. In the HCM 2010, heavy vehicles were categorized
as either large trucks or buses (the “Trucks” category) or as recreational vehicles (the “RV’s”
category). Likewise, there were separate passenger car equivalent (PCE) values for each category,
ET and ERV, for the Trucks and RV’s categories respectively. In the HCM 6th edition, the categories
for heavy vehicles now consist of single-unit trucks (SUTs) and tractor-trailers (TTs). Buses and
RV’s are considered to be in the SUT category (generally, FHWA classes 4 and 5). Single-unit
trucks with three or more axles and tractor+semi-trailer combinations are considered to be in the
TT category (generally, FHWA classes 6 and above). There is no longer separate PCE values for
the different heavy vehicle categories; thus, the new equation for calculating the heavy vehicle
adjustment factor only includes an ET parameter, rather than ET and ERV parameters, as follows:
1
f HV =
1 + PT ( ET − 1)
fHV = heavy-vehicle adjustment factor,

PT = proportion of heavy vehicles in the traffic stream, and
ET = passenger car equivalent for heavy vehicles.
Considering the heavy vehicle category, either SUT or TT, in determining a single PCE value (ET)
depends on the terrain classification used for analysis. If a general terrain classification is used
(i.e., level or rolling terrain), only the overall percentage of heavy vehicles in the traffic stream is
considered and not the specific SUT/TT split. The PCE values for a general terrain classification
(HCM Exhibit 12-25) analysis are as follows:

ET for level terrain is 2.0 (formerly 1.5 in HCM 2010)  Basic freeway segments and
ET for rolling terrain is 3.0 (formerly 2.5 in HCM 2010) multilane highways merged
into one HCM Chapter, 12
It should also be noted that the “mountainous” general  One unified speed–flow
terrain classification in the HCM 2010 has been removed. equation applicable to both
For this situation, where trucks are likely to operate at freeway and multilane highway
crawl speed for significant distances or frequent intervals, segments, but the forms of the
a new analysis approach is included, referred to as the curves are different
“mixed-flow model”. This approach, detailed in Chapters  Revised truck PCE values
25 and 26, is the recommended approach for analyzing  Increased emphasis on
such conditions. However, such terrain conditions can be calibration through capacity
analyzed using the specific-terrain component of the and speed adjustment factors
methodology, albeit with likely less accuracy than the (CAFs and SAFs)
mixed-flow model. For terrain that is not considered level  Driver population effects now
or rolling, but trucks will operate at speeds higher than handled by CAFs and SAFs
crawl speed, the specific terrain methodology, described
as follows, should provide reasonably accurate results.
When the analysis segment consists of terrain that cannot be classified as general terrain (i.e., level
or rolling), it should be analyzed with its specific terrain characteristics, which consider the slope
and length of segment. Additionally, the percentage split between the SUT and TT categories of
heavy vehicles in the traffic stream must be considered. PCE tables are provided in the HCM 6th
edition for three different truck category splits: Exhibit 12.6 for 30% SUTs/70% TTs, Exhibit 12.7
for 50% SUTs/50 TTs, and Exhibit 12.8 for 70%SUTs/30% TTs. For splits less than 30/70 (e.g.,
20/80) the values in Exhibit 12.6 should be used. For splits greater than 70/30 (e.g., 75/25), the
values in Exhibit 12.8 should be used. For other splits (and not 50/50), PCE values should be
interpolated linearly between the relevant tables.
Additionally, a new method for estimating capacity from field measurements and updated example
problems can be found in supplemental Chapter 26.
Other Freeway Segments
Weaving (Chapter 13/27)

The core methodology from the HCM 2010 and HCM 6th Edition (Chapter 13) is unchanged. The
new edition includes guidance on how to apply the weaving analysis procedure for segments that
provide access to/from managed lanes adjacent to general purpose lanes, such as illustrated in

Figure 10. It also includes guidance on how to apply the weaving analysis procedure to a managed
lane weaving segment, such as illustrated in Figure 11.
Figure 11. Weaving in a Managed Lane Access Segment
Figure 12. Managed Lane Weaving Segment
There is an emphasis on the use of CAFs and SAFs for calibration. Chapter 27 provides new
example problems demonstrating the new capabilities.
Merge/Diverge (Chapter 14/28)

The core methodology from the HCM 2010 and HCM 6th Edition (Chapter 14) is unchanged. The
chapter provides new formalized guidance for aggregating merge and diverge segment densities
for segments with three or more lanes The new edition includes guidance on how to apply the
merge/diverge analysis procedures to merges and diverges on managed lanes, such as illustrated
in Figures 12 and 13.

Figure 13. Managed Lane Merge Example
Figure 14. Managed Lane Diverge Example
Again, emphasis on the use of CAFs and SAFs for calibration is provided. Chapter 28 provides
new example problems demonstrating the new capabilities.

Two-Lane Highways (Chapter 15/26)
Figure 15. Two-Lane Highway Segment
No significant changes have been made to the method. Some calculation steps that previously were
always skipped (because they were not needed to calculate LOS for a particular two-lane highway
class) have been made optional, to clarify that they can be applied if the user is interested in
determining the performance measure calculated by that step. This chapter’s example problems
now appear in Chapter 26.
It should be pointed out that NCHRP Project 17-65 (Washburn et al., 2018) developed a new two-
lane highway analysis methodology. This project was completed in March 2018 and is currently
going through the review process with the Highway Capacity and Quality of Service Committee
for potential adoption and inclusion in the HCM.

NEW METHODOLOGY CAPABILITIES: INTERRUPTED FLOW
Urban Street Facilities (Chapters 16/29)
The service measure has been changed from ’average travel speed as a percent of free-flow speed’
to just ’average travel speed’. This was done to make the service measure more intuitive to
practitioners and because the previous service measure resulted in LOS results that were more
sensitive to rounding effects in the calculations. LOS F, in addition to the average speed thresholds,
also applies anytime the d/c ratio exceeds 1.0. Table 1 provides the new service measure
thresholds. Example problems have been moved to Chapter 29.
Table 1. Travel Speed Threshold by Base Free-Flow Speed (mi/h)

Travel Speed Threshold by Base Free-Flow Speed (mi/h)
LOS 55 50 45 40 35 30 25
A > 44 > 40 > 36 > 32 > 28 > 24 > 20
B > 37 > 34 > 30 > 27 > 23 > 20 > 17
C > 28 > 25 > 23 > 20 > 18 > 15 > 13
D > 22 > 20 > 18 > 16 > 14 > 12 > 10
E > 17 > 15 > 14 > 12 > 11 >9 >8
F ≤ 17 ≤ 15 ≤ 14 ≤ 12 ≤ 11 ≤9 ≤8
Source: HCM
Urban Street Reliability and ATDM (Chapter 17/37)
Chapter 17 is a new chapter that integrates the ATDM material previously found in Chapter 35
and the urban street travel time reliability material previously found in interim HCM Chapters 36
and 37. Similar to freeway reliability analysis, the urban street reliability method repeatedly applies
the core Chapter 16 facility method with varying demands, capacities, free-flow speeds, and
number of lanes that reflect the variations in demand and the effects of severe weather, incidents,
work zones, and special events over a long timeframe (up to a year). Due to complexity in the
analysis for the urban street reliability method can only practically be implemented through
software. New conceptual information for ATDM and techniques for evaluating ATDM strategies
can be found in Chapter 17.
Urban Street Segments (Chapters 18/30)
The urban street segment service measure has been changed to average travel speed, and the LOS
A/B threshold has been reduced to the equivalent of 80% of free-flow speed, same as urban street

facilities. New and improved methods include the capability to evaluate segments with
midsegment lane blockage.
Figure 16. Midsegment Lane Blockage Dipiction
Urban street segments also includes an improved procedure for predicting segment queue spillback
time, and a new adjustment factor for parking activity that affects free-flow speed estimation as
well as procedures for evaluating segements with roundabouts at one or both directions.
Figure 17. Roundabout in both directions
Supplemental Chapter 30 provides calculation details for estimating segment travel time when
roundabouts are located at one or both ends of the segment.
Pedestrian and bicycle LOS scores are now weighted by link travel time instead of link length, to
reflect the amount of time these modes experience conditions on the link. There are also some
changes to the bicycle and bus default values.

The unsignalized conflicts factor term for the bicycle mode has been revised to consider 20 conflict
points per mile as the base (no-effect) condition, rather than 0 conflict points per mile. Default
transit vehicle acceleration and deceleration rates have been added.
Right-turn-on-red vehicles are incorporated into the volume-balancing method for flows into and
out of a segment.
Figure 18. Right-turn-on-red vehicles incorporated into volume-balancing method
Supplemental Chapter 30 provides calculation details, a planning method for urban street
segments, and updated example problems.

Signalized Intersections (Chapters 19/31)
Figure 19. Signalized Intersection
The saturation flow adjustment factors for heavy vehicles and grade are now combined into a
single factor. The delay of unsignalized movements at a signalized intersection are now considered
when calculating approach and intersection delay where the user must supply these delay values.
New saturation flow adjustment factors are provided for intersection work zone presence,
midsegment lane blockage, and downstream spillback.

N Work area length
Drivew ay
Bay length
Median width
Width of each lane
Drivew ay
Entry taper length

Subject direction of travel
Speed limit
Figure 20. Work zone presence at approach to signalized intersection
Source: HCM
Chapter 31 provides an improved planning method with reduced input data requirements outlined
in a worksheet. The simplified calculations are demonstrated in example problems.
Two-Way Stop-Controlled Intersections (Chapters 20/32)
There are no significant changes in the two-way stop control methods, but
how the peak hour factor (PHF) should be applied is clarified. Example
problems have been moved to Chapter 32.
All-Way Stop-Controlled Intersections (Chapters 21/32)
There are no significant changes in the all-way stop control methods, but
how the peak hour factor (PHF) should be applied is clarified. Example
problems have been moved to Chapter 32.

Roundabouts (Chapters 22/33)
Roundabout capacity models have been updated as a result of new research in NCHRP 03-100:
Roundabouts in corridors as well as a new calibration procedure is provided. Additionally,
clarification of how the peak hour factor (PHF) should be applied. Example problems have been
moved to Chapter 33.

Figure 21. Roundabout Example
Ramp Terminals and Alternative Intersections (Chapter 23/34)
The former Interchange Ramp Terminals chapter has been expanded to include a greater variety
of distributed intersections. Distributed intersections include those with two or more intersections
with close spacing and displaced or distributed traffic movements that are operationally
interdependent and are best analyzed as a single unit.
Chapter 23 now covers both interchange ramp terminals and alternative intersection forms. It is
divided into three parts: distributed intersection concepts, interchange ramp terminal evaluation,
and alternative intersection evaluation. The interchange ramp terminal forms addressed in Chapter
23 are the different varieties of partial cloverleaf, diamond, single point urban, and diverging
diamond interchanges.

Partial Cloverleaf Diamond
Single Point Urban Interchange (SPUI) Diverging Diamond Interchange (DDI)
Figure 22. Ramp Terminal Forms

The distributed intersection forms addressed in Chapter 23 are displaced left-turn, restricted-
crossing U-turn, and median U-turn intersections.
Continuous Flow Intersection/ Restricted Crossing U-Turn (RCUT)

Displaced Left Turn
Median U-Turn (MUT)
Figure 23. Intersection Forms

Both ramp terminals and alternative intersections now use a new service measure, experienced
travel time (ETT), that is intended to allow these intersections’ performance to be compared on an
equivalent basis with other intersection forms. ETT includes the sum of average control delays
experienced by a given movement, plus any extra distance travel time the movement may
experience as a result of being rerouted.
Figure 24. Example of experienced travel time at an RCUT intersection

Source: HCM
New and updated example problems are located in supplemental Chapter 34.
Off-Street Pedestrian and Bicycle Facilities (Chapter 24)
Some variable names and equations have been changed to improve their understandability without
affecting results to the off-street pedestrian and bicycle facility calculations found in Chapter 24.
Additional guidance has been provided on applying the methods and interpreting results such as
graphs that demonstrates how the bicycle LOS score for a shared off-street path is influenced by
path width and two-directional path volume. The new Chapter 35 provides example problems.

Advanced Traffic and Demand Managment (ATDM) (Chapter 37)
Chapter 37 includes the emerging topics chapter in HCM 2010 that was later updated as a result
of FHWA research. Most of the material has been integrated into Chapters 11 (freeways) and 17
(urban streets) but Chapter 37, ATDM: Supplemental provides descriptions of common ATDM
strategies, guidance on analyzing shoulder lane, median lane, and ramp metering strategies and
guidance on designing an ATDM program.
FUTURE UPDATES
Looking ahead, there are several areas the HCQS committee will be focused on for potential
additions/revisions to the HCM, such as:
• Impacts on capacity and operations as a function of advances in connected and
autonomous vehicles. For example, see SunCam course 208-Future Highways –
Automated Vehicles (https://www.suncam.com/courses/100250-01.html.)
• Capacity measurement: Many default values for capacity, and methods for estimating
capacity, have been included in the HCM over the years. However, little guidance was
provided for methods/techniques for measuring capacity from the field. Some
improvement in this area was made for the HCM 6th edition, but this will continue to be a
major focus for the committee, so that transportation agencies can determine capacity
values most appropriate for their region.
• ATDM adjustment values: Many of the adjustment factor values provided for the ATDM
methodology are based on minimal research and/or expert opinion. The FHWA is putting
considerable resources into quantifying the impacts of various ATDM strategies, and the
HCQS committee will update the HCM with these results as they become available.
• Planning methods: Many of the operational analysis methods in the HCM have become
quite complicated and difficult, if not impossible, to solve without software. Many
transportation agencies, particularly those that include large rural areas, still desire simple-
to-apply analysis methods that will give reasonably accurate results for planning purposes.
The HCQS committee is focused on the continued development of planning methods,
derived from the full operations methods.
• Simulation: While the HCM methods are not intended to compete, per se, with simulation
methods, the increased complexity of the HCM methods over the last 15 years, combined
with the near ubiquity of simulation, has blurred the line on which approach is most
applicable for a given analysis situation. Some guidance to help practitioners determine
when simulation might be a more appropriate approach than the HCM was provided in the

HCM 2010 and 6th editions, but there is still much more room for improvement in this
area. This will be a continued area of focus for the HCQS committee.
• Two-lane highway analysis: If the new two-lane highway analysis methodology,
developed from the aforementioned NCHRP Project 17-65, is approved by the HCQS
committee, it should appear in the HCM by the end of 2019.
While previous HCM edition titles included the

publication year, the new HCM omits this element, Examples of future updates include:
with the intent that chapters will have their own  Connected and autonomous vehicles
version number, starting with 6.0. Version numbers  Capacity measurement methods
will be incremented as errata are released and as new  Planning methods
research is incorporated. This approach will allow the  Advances in ATDM
HCM to continue to present state-of-the-art methods,  Simulation guidance
while allowing analysts to tie their analysis to a  Two-lane highway analysis update
specific version of a methodology.
RESOURCES
The HCM 6th Edition, A Guide for Multimodal Mobility Analysis can be purchased from the
Transportation Research Board at https://www.mytrb.org/Store/Product.aspx?ID=8313.
Free Volume 4 Online http://www.hcmvolume4.org/
Transportation Research Board Highway Capacity and Quality of Service Committee ID:
AHB40
https://www.mytrb.org/CommitteeDetails.aspx?CMTID=1526
https://sites.google.com/site/ahb40hcqs/
https://www.linkedin.com/groups/8493127/
Free HCM software is available at http://swashware.com/HCMCalc.

SOFTWARE
Improvements in computing power over time have resulted in increased use of technology to
control the road system (e.g., more advanced signal systems), the ability to create more
sophisticated models of roadway capacity and quality of service, and the development of software
packages to perform HCM analyses. Today, most HCM users apply the HCM methods via
software.
Even though the majority of HCM users

apply HCM methods through software, there Originally, HCM methods could be applied
is still a need for analysts to understand how with pencil and paper techniques.
a method works, even though they may not be Today, the HCM is typically applied through
performing step-by-step calculations. They software
 Faster, reduces computational mistakes
need to know what inputs are required, how
 Larger study areas
variations in those inputs may impact the  More complex phenomena being modeled
results, the research foundation for the  Iterative calculations increasingly
method, the basic flow of calculations, the required
range of likely results, and resources to turn
to in case of questions. In addition, a subset of users (including analysts who want to explore the
reason behind a particular result) still require in-depth information about each method.
A software program, HCM-CALC, which implements the calculations for the uninterrupted flow
analysis methodologies described in this document, is available free of charge. The software is not
required for this course. To the knowledge of the authors, the software provided at the link below
is the only free software available for the full range of uninterrupted flow analysis methodologies
of the Highway Capacity Manual. The software runs both the 5th edition (2010) and 6th (2016)
edition manual calculations. The Highway Capacity and Quality of Service Committee does not
officially endorse any software implementation of the calculation procedures contained in the
HCM.

For information on how to obtain the HCM-CALC software, please visit

http://swashware.com/HCMCalc/.

REFERENCES
Mannering, Fred L. and Washburn, Scott S. Principles of Highway Engineering and Traffic
Analysis, 6th Edition. John Wiley and Sons, Hoboken, NJ, 2016.
Transportation Research Board. Highway Capacity Manual: A Guide for Multimodal Mobility
Analysis. Washington, DC: National Research Council, 2016.
www.trb.org/main/blurbs/175169.aspx
Washburn, Scott S., Al-Kaisy, Ahmed, Luttinen, Tapio, and Dowling, Richard, Watson, Donald,
Jafari, Amirhossein, Bian, Zilin, and Elias, Aaron. Improved Analysis of Two-Lane Highway
Capacity and Operational Performance. NCHRP Project 17-65. Transportation Research
Board. March 2018. 670 pages. DOI: 10.17226/25179


The Highway Capacity Manual - 6 Edition: Overview and What's New

Uploaded by

Copyright:

Available Formats

The Highway Capacity Manual - 6 Edition: Overview and What's New

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

The Highway Capacity Manual - 6 Edition: Overview and What's New

Uploaded by

Copyright:

Available Formats

A SunCam online continuing education course

The Highway Capacity Manual -

Scott S. Washburn, Ph.D., P.E.

Figure 1. Highway Capacity Manual 6th Edition Cover

www.SunCam.com Copyright 2019 Page 2 of 43

In this course, you will learn about:

Source of photo is Scott or Leslie Washburn, unless noted otherwise.

www.SunCam.com Copyright 2019 Page 3 of 43

www.SunCam.com Copyright 2019 Page 4 of 43

www.SunCam.com Copyright 2019 Page 5 of 43

INTRODUCTION AND HISTORY

www.SunCam.com Copyright 2019 Page 6 of 43

Updates to this edition were made in 1992,

www.SunCam.com Copyright 2019 Page 7 of 43

2010: New research added, additional focus on non-

Reorganized to make it more clear and understandable.

Figure 4. HCM Cover for 2010 edition

The four volumes of the HCM 2010 include:

NEED FOR A NEW EDITION

www.SunCam.com Copyright 2019 Page 8 of 43

HCM 6th Edition revisions includes:

 New title, to reflect how HCM content has expanded

www.SunCam.com Copyright 2019 Page 9 of 43

Some projects update previous research to

HCM 6TH EDITION ORGANIZATION

www.SunCam.com Copyright 2019 Page 10 of 43

The HCM 6th Edition

www.SunCam.com Copyright 2019 Page 11 of 43

Printed Chapters Online Chapters (Volume 4)

www.SunCam.com Copyright 2019 Page 12 of 43

Volume 2 – Uninterrupted Flow

Printed Chapters Online Chapters (Volume 4)

Volume 3 – Interrupted Flow

www.SunCam.com Copyright 2019 Page 13 of 43

Printed Chapters Online Chapters (Volume 4)

Volume 4 (Online) – Concepts Supplemental

www.SunCam.com Copyright 2019 Page 14 of 43

Figure 5. HCM 6th Edition, Volume 4: Application Guide

www.SunCam.com Copyright 2019 Page 15 of 43

NEW METHODOLOGY CAPABILITIES: UNINTERRUPTED FLOW

Freeway Facilities (Chapters 10/25)

Figure 6. Freeway managed lanes and general-purpose lanes interaction

www.SunCam.com Copyright 2019 Page 16 of 43

Freeway Reliability Analysis (Chapter 11)

www.SunCam.com Copyright 2019 Page 17 of 43

Freeway and Multilane Highway Segments (Chapters 12/26)

www.SunCam.com Copyright 2019 Page 18 of 43

Unified speed-flow equation

FFSadj = FFS × SAF for basic freeway segments

cadj = c × CAF for basic freeway segments

vp = 15-min passenger car equivalent flow rate, in pc/h/ln,

www.SunCam.com Copyright 2019 Page 19 of 43

Figure 8. Unified speed-flow equation

www.SunCam.com Copyright 2019 Page 20 of 43

For freeway segments:

For multilane highway segments:

For freeway segments:

For multilane highway segments:

For freeway segments: 2.0