| Capacity Manual.
This document provides a brief overview of the changes in
analysis methodologies that occurred between the 1997 and year 2000 editions of the
Highway Capacity Manual (HCM). The purpose of this document is to help Highway Capacity
Manual users to identify where and understand why reported level of service results might
differ between the 1997 and Year 2000 editions of the Highway Capacity Manual. Editorial
changes (which are extensive) are not covered in this summary. This summary only
identifies those analytical changes that would affect the numerical results produced when
following the capacity analysis and level of service procedures in either edition of the
HCM. The summary is organized by chapter number of the 1997 edition. This summary compares
the "customary unit" version of the HCM 2000 to the 1997 HCM. The metric unit
version of the HCM 2000 may have some additional slight numerical variations from the 1997
HCM due to rounding in the conversion between metric and customary units.
Chapters
1 and 2 – Principles of Capacity
These chapters of the 1997 HCM do not contain analytical procedures. The material in
these two chapters has been expanded to several chapters and placed in Parts I and II of
the HCM 2000.
Chapter
3 – Basic Freeway Sections
This 1997 HCM chapter becomes Chapter 23 in the year 2000 HCM. The speed-flow curves
are unchanged. However the level of service thresholds have been increased slightly since
1997 (compare table 3-1 in the 1997 HCM to Exhibit 23-2 in the HCM 2000). The maximum
allowable v/c ratios have gone up 3 to 5 percentage points for LOS "A" through
"D". LOS "E" isunchanged at v/c =1.00. The maximum service flow rates
for LOS "A" through "D" have increased about 10% over the 1997 HCM.
There is no change in capacities for each free-flow speed curve. The passenger car
equivalents (ET) or trucks and buses have been reduced for specific grades and for rolling
and mountainous terrain between the 1997 and 2000 HCM’s (compare Exhibits 3-2 and 3-3
in the 1997 HCM to Exhibits 23-8 and 23-9 in the HCM 2000).
Chapter 4
– Weaving Sections
This 1998 HCM chapter becomes Chapter 24 in the HCM 2000. The parameters in the
equation used to predict the weaving intensity factor (W) which in turn is used to compute
weaving and non-weaving speeds have been changed (compare Table 4-3 HCM 1997 to Exhibit
24-6 HCM 2000). A new table of weaving segment capacities as a function of the Volume
Ratio and the weaving length is included in the HCM 2000 (see Exhibit 24-8 in HCM 2000).
The weaving length limit for Type "A" weaves has been increased to 2,500 feet.
The level of service thresholds for weaving sections are unchanged.
Chapter
5 – Ramps and Ramp Junctions
This 1998 HCM chapter becomes chapter 25 in the HCM 2000. New data is provided on ramp
roadway capacities (see Exhibit 25-3 of HCM 2000). The secondary level of service criteria
of minimum speed has been dropped (compare Table 5-2 HCM 1997 to Exhibit 25-4 of HCM
2000). Capacity values for merge and diverge areas are unchanged (compare Table 5-1 HCM
1997 to Exhibit 25-7 and 25-14 of HCM 2000).
Chapter 6 - Freeway
Systems
This 1997 HCM chapter is replaced with an entirely new procedure in Chapter 22 of the
HCM 2000. This new chapter provides adjustment procedures for poor weather conditions and
construction, a method for identifying freeway segment types, and procedures for computing
mean speed, density, and delay for under saturated and over saturated flow conditions. No
level of service thresholds are provided for freeway systems.
Chapter 7
– Multilane Highways
This 1997 HCM chapter becomes Chapter 21 in the HCM 2000. The level of service (LOS)
thresholds for LOS "A" through "D" have been modified slightly
(compare Table 7-1 HCM 1997 to Exhibit 21-2 HCM 2000) (LOS "A" is reduced from
12 to 11 passenger cars per lane. LOS "B" is reduced from 20 to 18. LOS
"C" is reduced from 28 to 26. LOS "D" is increased from 34 to 35.).
The LOS E maximum density is unchanged. These changed thresholds affect the maximum
service flow rates for each LOS. There is a new driver population adjustment factor
for the computation of equivalent passenger car flow rate (compare equation 7-3 HCM 1997
to equation 21-3 HCM 2000). The passenger car equivalents (ET) for trucks and buses in
mountainous and rolling terrain, and on specific grades have been reduced.
Chapter
8 – Two Lane Rural Highways
This 1997 HCM chapter has been completely replaced by a new methodology in Chapter 20
of the HCM 2000. Two lane rural highways are divided into two classes, with differing
level of service criteria. The new analytical procedures permit evaluation of both
directions of travel combined, as in the 1997 HCM, but also permit separate analyses for
each direction of travel. The base capacity of the two lane highway is increased from 2800
to 3200 passenger cars per hour (total of both directions). The analytical procedure is
now sensitive to the free-flow speed of traffic on a two-lane highways. The factors for
heavy vehicle and grade effects have been thoroughly revised. New analytical procedures
have been added for passing lanes in level and rolling terrain, climbing lanes on steep
upgrades, and operation of steep downgrades on which trucks must use crawl speeds.
Chapter
9 – Signalized Intersections
This 1997 HCM chapter becomes Chapter 16 in the HCM 2000. Two new
adjustment factors (one for pedestrians, the other for bicyclists) have been added to the
saturation flow computation (compare equation 9-10 HCM 1997 to equation 16-4 HCM 2000)
(see Appendix E of HCM 2000). There is also a new treatment for protected plus permitted
left turns from a shared lane. Computation of the saturation flow left turn adjustment
factor for lefts from a shared lane with protected –permitted phasing requires that
the analyst divide the left turn volumes between the protected and permitted portions of
the phases (see Appendix "C", HCM 2000) There is a new model for predicting
queue lengths (see Appendix G, HCM 2000).
Chapter 9 -
Planning Analysis Method Signalized Intersections
The planning analysis method (described on page 9-50 of the HCM 1997) has been revised
and moved to an appendix of Chapter 10 of the HCM 2000. The maximum cycle length that was
previously used to compute the intersection critical volume/capacity ratio has been
replaced with the computed cycle length. The critical v/c ratio is computed using 1710
saturation flow rather than 1900 vehicles per hour of green. A caution has been added
about using the planning method when left turn protection is recommended by the planning
method, but only permitted left turn phasing is in place. The appendix providing
suggestions for estimating design elements (Appendix I in 1997 HCM) has been revised and
moved to Chapter 10 of the HCM 2000.
Chapter
10 – Unsignalized Intersections
This 1997 HCM chapter becomes Chapter 17 in the HCM 2000. There are no methodological
changes to this chapter.
Chapter
11 – Urban and Suburban Arterials
This 1997 HCM chapter becomes Chapter 15 in the HCM 2000. The concept of aggregating
analysis segments into analysis sections has been dropped from the chapter. There are no
other methodological changes.
Chapter 12
– Transit Capacity
This 1997 HCM chapter is replaced with an entirely new chapter 27 in the HCM 2000. This
chapter provides new quality of service measures, new level of service thresholds, and new
methodologies for computing capacity for various transit modes and facility types.
Chapter 13 –
Pedestrians
This 1997 HCM chapter is replaced with an entirely new chapter 18 in the HCM 2000. This
chapter provides new quality of service measures, new level of service thresholds, and new
methodologies for computing capacity for several exclusive and mixed flow pedestrian
facility types.
Chapter 14 –
Bicycles
This 1997 HCM chapter becomes chapter 19 in the HCM 2000. This chapter provides new
quality of service measures, new level of service thresholds, and new methodologies
for computing capacity for several bicycle facility types.
Appendix
A – Glossary and Symbols
The appendix does not contain any analytical procedures. The material in the Appendix
of HCM 1997 has been expanded and moved to two new chapters in Part I of the HCM 2000.
Corridor and
Area wide Analysis
The 1997 HCM did not provide analytical procedures for the analysis of corridors and
large areas. This new material, which consists of additional analytical procedures but no
new level of service measures, is contained in Chapters 29 and 30 of the HCM 2000. |
