TM 5-822-2/AFM 88-7, Chap. 5
CHAPTER 3
GEOMETRIC DESIGN
used by opposing traffic for passing. The design criteria
3-1.
General.
Geometric design deals with the
presented herein for any highway are generally
dimensions of the visible features of a facility such as
applicable to multilane highways also, except that
alignment, sight distances, widths, slopes, and grades.
passing sight distance is not required. The principal
Geometric design policies are listed in tables 1-1 and 1-2
justification for construction of a multilane roadway is the
and discussed in subsequent paragraphs.
capacity required to accommodate the anticipated traffic
volumes. If traffic volumes require construction of
3-2.
Definitions relative to geometric design.
multilane highways that are planned and designed for
Definitions for specific terms not included in paragraph 1-
relatively high speeds, then opposing traffic should be
2 relative to geometric design are presented by AASHTO
separated by properly designed medians or concrete
(A Policy on Geometric Design of Highways and Streets)
barriers. Of particular significance is the effect of wide
and the Transportation Research Board (Highway
medians in virtually eliminating head-on collisions. Rear-
Capacity Manual).
end collisions and other accidents related to left-hand
turns are also reduced by use of wide medians to
3-3.
Roads and streets.
separate traffic. Divided highways designed to serve as
a. Types.
expressways are seldom warranted within military
(1) Designations of types. Highways may be
reservations. This is due to the limited area of such
grouped into various types on the basis of physical
installations and to the large expenditure of funds that
characteristics and ability to accommodate traffic.
must be made.
Highways are generally typed according to the number of
b. Design controls.
traffic lanes as-single, two-.
and three-lane, and
(1) Topography and land use. The location of
undivided or divided multilane (four or more traffic lanes)
a highway and its design elements are influenced to a
highways. When information is available relative to
considerable degree by the topography, physical
volume and composition of traffic and type of terrain for a
features, excavation limits, and land use of the area
proposed highway, the type required can be readily
traversed. These conditions are positive design controls,
determined by comparing the traffic volume expected on
and information regarding them is essential. Tables 1-1
the proposed road or street with the design hourly
and 1-2 show appropriate design standards for roads
volume shown in tables 1-1 and 1-2.
and streets traversing flat, rolling, or mountainous terrain
(2) Single-lane roads.
Geometric design
in built-up areas or open areas.
criteria for single-lane roads are shown in table 1-1 under
Vehicle characteristics.
Table 3-1
(2)
"class F roads." Where shoulders are not sufficiently
shows dimensions of design vehicles on which the
stable to permit all-weather use and the distance
1
geometric design criteria presented herein are based.
between intersections is greater than /2 mile, turnouts
1
Tracked vehicles used by the military services will fit into
shall be provided at /4-mile intervals for use by
this group of design vehicles except for one dimension,
occasional passing or meeting vehicles. Single-lane
i.e., width. Some of these vehicles are wider than 8.5
pavements may be provided for fire lanes and approach
feet, which is the maximum width shown in table 3-1 for
drives to buildings within built-up areas, in which case
any of the design vehicles. The turning radii and
the pavement will be at least 12 feet wide. Access roads
dimensions of special vehicles will be obtained from the
to unmanned facilities at Air Force installations will be
operating agency. Methods for modification of these
classified as "class F roads" and shall be designed in
criteria for use on roads and streets subject to vehicles
accordance with the geometric design criteria presented
greater in overall width than 8.5 feet are presented in 3-
for class F roads.
3d. (2). The selection of a design vehicle for use in
(3) Two-lane roads and streets. The bulk of
design of grades is also discussed in 3-3d.(2)
the roads and streets at military installations are two-lane
highways. These include class B, C, D, and E roads and
class B, C, D, E, and F streets. Geometric design
criteria are presented elsewhere in this manual.
(4) Multilane (four traffic lanes or more,)
highways.
A four-lane undivided highway is the
narrowest highway on which each traffic lane is intended
for use of traffic traveling in only one direction, and is not
3-1
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