TM 5-822-2/AFM 88-7, Chap. 5
or streets, again taking into account the relative traffic
highway, and the operators of the vehicles. Safety is
frequencies where there are tracked vehicles from more
related to lane width, conditions and width of shoulders,
than a single traffic-category.
distance to lateral obstructions, maximum curvature,
(b) Special-purpose
vehicles.
sight distance, and allowable speeds. The geometric
design policies set forth in tables 1-1 and 1-2 have been
Information regarding pavement design requirements for
established to ensure roadway conditions adequate to
special-purpose vehicles producing loadings significantly
accommodate design volumes and permit operating
greater than those defined in this manual will be
speeds approaching the design speeds in a safe and
requested from Headquarters, Department of the Army
efficient manner.
(DAEN-ECE-G), Washington, DC 20314-1000; or
(12) Designations of design control factors.
Headquarters, Air Force Engineering and Services
The major controls used in design of highways should be
Center (AFESC/DEMP), Tyndall AFB, Fla. 32403-6001.
(10) Speed.
shown on the title sheet of construction plans for each
project. The present ADT, the future average daily traffic
(a) Factors
geometric
(design ADT), the DHV, the percentage of trucks during
design. Vehicular speed varies according to the physical
the DHV (T), and the design speed (V), plus any other
characteristics of the vehicle and highway as well as its
major design control factors should be shown for each
roadsides, the weather, the presence of other vehicles,
project.
and speed limitations (either legal or because of control
c. Cross-section elements.
devices). On streets, the speed generally will depend on
(1) Pavement.
traffic-control devices when weather and traffic
(a) Type surface. Pavement type is
conditions are favorable. On roads, the physical features
seldom an important factor in geometric design;
of the roadway usually control speed if other conditions
however, the ability of a pavement surface to retain its
are favorable. Therefore, speed is a positive control for
shape and dimensions, its ability to drain, and the
geometric design. Consideration must be given to the
possible effect of pavement surface on driver behavior
selected design speed and average running speed if
should be considered in geometric design. Use of the
adequate designs are to be developed.
(b) Design speed. The speed selected
geometric and structural design criteria presented herein
and in TM 5-822-5/AFM 88-7, chapter 3, and TM 5-822-
for design is the major control in design and correlation
6/AFM 88-7, chapter 1, will provide suitable pavements
of the physical features of highways. Practically all
for classified roads and streets at military installations.
features of a highway will be affected to some extent by
(b) Normal cross slope. Selection of
the design speed. Maximum curvature, superelevation,
proper cross slope depends upon speed-curvature
and minimum sight distance are automatically
relations, vehicle characteristics, curb requirements, and
determined by the selected design speed.
Other
general weather conditions. Cross slope for sharp
features such as pavement and shoulder width, and
curves (superelevation) is discussed in AASHTO, A
lateral clearance to obstructions are not directly affected
Policy on Geometric Design of Highways and Streets.
by design speed but do affect vehicle speed. The design
Cross slope on tangents and flat curves is shown in
speed should be selected primarily on the basis of terrain
tables 1-1 and 1-2. Where two or more lanes are
characteristics, land use, and economic considerations.
inclined in the same direction on class A roads and
The geometric design policies presented herein are
streets, each successive lane outward from the crown
based on the design speeds shown under "Design
line shall have an increased cross slope. The lane
Controls" in tables 1-1 and 1-2.
(c) Average running speed.
adjacent to the crown line should have the minimum
The
cross slope shown in tables 1-1 and 1-2, and the cross
average running speeds on which the geometric design
1
slope of each successive lane shall be increased /16
policies are based are shown under "Design Controls" in
inch per foot. Where pavements are designed with
tables 1-1 and 1-2. These values were selected on the
barrier curbs, it is recommended that a minimum cross
basis of information presented in AASHTO's A Policy on
3
slope of /16 inch per foot be used on class A, B, and C
Geometric Design of Highways and Streets and the
1
roads and streets and that a minimum cross slope of /4
Highway Capacity Manual.
inch per foot he used on class D, E. and F roads and
(11) Safety. Geometric features of a highway
streets.
are designed for the safe, economic, and efficient
passage of the( using traffic. Highway safety depends
upon the proper arrangement of the physical features of
the roadway, the characteristics of the vehicles using the
3-8
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