26 TR News, March–April 1987
RESEARCH PAYS OFF
The Highway Capacity Manual
Development and Application
Problem
The major problems facing motorists on
today's highways are congestion, acci-
dents, and costs—too many automo-
biles traveling to the same place at the
same time; too many automobiles,
trucks, buses, bicycles, and pedestrians
attempting to use the same space; too
many motorists losing time unneces-
sarily; too many accidents occurring;
and too many decisions being made on
highway improvements without ade-
quate analytical tools.
From these problems comes the need
to quantify congestion and highway
operations in such a way that rational
solutions can be determined. For more
than 35 years, the Highway Capacity
Manual has been used extensively for
this purpose. However, much has
changed in the years since the earlier
editions were published, regarding both
travel characteristics and improved pro-
cedures for capacity analyses. Travel
characteristics have changed because of
a shifting mix in both the driver and
vehicle populations, and also because
operational changes have occurred,
such as the 55-mph speed limit and the
right-turn-on-red regulations. Research
by many individuals, private organiza-
tions, and public agencies has led to
new understandings and insights,
which, in turn, have resulted in proce-
dural revisions and new techniques in
highway capacity analysis. The com-
bined effect of these changes created a
pressing need for a revised manual.
Solution
In the early 1970s the National Coop-
erative Highway Research Program
(NCHRP) and the Federal Highway
Administration (FHWA) initiated ma-
jor research efforts to study various
aspects of highway capacity analysis.
Final preparation of the 1985 Highway
Capacity Manual was accomplished
by the Transportation Training and
Research Center of the Polytechnic
Institute of New York with the assis-
tance of the Texas Transportation In-
stitute of Texas A&M University. The
research team performed massive ed-
iting of source materials in order to
provide a uniform approach, philoso-
phy, and style. Some procedures un-
derwent additional development, while
others were only slightly modified. All
of this work was done under the guid-
ance of the Transportation Research
Board Committee on Highway Capac-
ity and Quality of Service.
More than 60 professionals directly
participated in substantive reviews of
the procedures through the TRB com-
mittee and its several subcommittees.
More than 30 NCHRP panel and TRB
committee meetings were held during
the 8 years of development. Each of the
14 chapters of the 500-page manual
went through at least three drafts and
cycles of review and revision, and was
submitted to at least 32—and as many
as 200—reviewers. More than 750
pages of reviewers' comments were
received. This effort culminated in the
publication of the third edition of the
Highway Capacity Manual by TRB in
August 1985.
Application
The 1985 Highway Capacity Manual. a
collection of procedures and method-
ologies for calculating highway capac-
ity and level of service, neither consti-
tutes nor attempts to establish legal
standards for highway construction. It
provides methods for analyzing in ad-
vance the quantity of service a highway
can provide as well as the quality of
that service. In particular, the 1985
manual will change the way analysts
consider service quality and will pro-
vide greater flexibility in adapting pro-
cedures to actual conditions. Designed
for use in planning, design, and opera-
tional analyses, the manual is particu-
larly relevant to the current concerns of
maximizing the utility of existing
streets and highways.
Unlike previous editions, the High-
TR News, March–April 1987 27
way Capacity Manual will be updated
continuously on a chapter-by-chapter
basis, so that users may benefit from
the latest research as soon as it is prac-
ticable. To permit this continual updat-
ing, the 1985 manual was published in
loose-leaf form.
Because of the complexity of some of
the new analysis methods, there was a
need for microcomputer software. The
FHWA responded by developing mi-
crocomputer software to provide an
official set of programs and greater
uniformity in their application. The
FHWA is also developing a set of
training materials for use in short
courses. Many universities have been
using the new Highway Capacity Man-
ual since September 1985 in both short
courses and undergraduate and graduate
curricula.
Benefits
The 1985 Highway Capacity Manual
represents the latest approach to making
major decisions about highway con-
struction and operational improve-
ments. Quantifying the benefits of a
better approach is practically impossi-
ble; however, even without looking at
numbers, it is clear that, with the most
conservative estimates of savings, an
impressive benefit-cost ratio exists for
the research that led to the new manual.
Even less quantifiable but equally
important are benefits in the form of
consistency, reliability, and recognized
credibility. With nearly 20,000 copies
of the publication already in use, deci-
sions will not only be better, they will
be more consistent from project to proj-
ect, from city to city, and even among
different countries. Furthermore, people
with limited technical training will be
better able to conduct technical analy-
ses, and will be able to do so even more
easily with introduction of the new
software. Finally, in an age in which the
availability of a recognized set of
credible procedures is a requirement in
convincing the courts that acceptable
techniques are being used, the manual
has proved to be a valuable tool with
significant benefits to the highway pro-
fession.
To obtain a copy of the 1985 Highway
Capacity Manual, published as TRB Special
Report 209, contact the Business Office,
Transportation Research Board, 2101 Con-
stitution Avenue, N.W., Washington, D.C.
20418 (telephone: 202-334-3218) (price:
$40.00). The FHWA computer programs
can be ordered from the McTrans Center,
512 Weil Hall, University of Florida,
Gainesville, Fla. 32611 (telephone: 940-
392-0378) (price: $50.00). Information on
the FHWA training courses can be obtained
from Dennis Lee, FHWA, HTO-31, 7th and
D Streets, S.W., Washington, D.C. 20590
(telephone: 202-366-1993).
Suggestions for "Research Pays Off" ar-
ticles are welcome. Contact Nancy A.
Ackerman, Editor, TR News, Transpor-
tation Research Board, 2101 Constitu-
tion Avenue, N.W., Washington, D.C.
20418 (telephone 202-334-2972).
