TM 5-822-2/AFM 88-7, Chap. 5
roads are shown in table 1-1. The absolute maximum
access to adjacent property, and economics. It is
curvature for roads without superelevation is the same
generally agreed that design grades for roads and
as shown for streets with normal crown sections in table
streets is primarily dependent on vehicle characteristics,
1-2. Absolute maximum values for degree of curvature
rate and length of grade, drainage, and safety. Control
on streets in built-up areas are shown in table 1-2.
grades for design of roads and streets are shown in table
(c) Superelevation.
A
practical
1-1. The values shown were established in accordance
with AASHTO grade design methods and as presented
superelevation rate together with a safe side friction
in the Transportation Research Board (TRB) Highway
factor determines maximum curvature. Superelevation
Capacity Manual. The objective in selecting maximum
rate and side friction factors depend upon speed, degree
grades for use in design is to determine the length of a
of curvature, frequency, and amount of precipitation and
designated upgrade (critical length) upon which a
type of area, i.e., built-up or open. Superelevation rates
particular vehicle (design vehicle) can operate safely
will be determined in accordance with AASHTO
without reducing its speed below a specified speed
methods.
(d) Widening of roads and streets.
(generally 30 mph). The term "maximum grade" in itself
has no significance unless length of grade and type
Pavements on roads and streets will be widened to
vehicle are stated. In grade design, gradeability of the
provide operating conditions on curves comparable to
vehicle is the most important factor. For comparative
those on tangents. Widening is necessary on certain
purposes, gradeability may be expressed by the weight-
highway curves because long vehicles (see WB40,
power ratio of a vehicle. According to AASHTO, and
WB50, WB60) occupy greater width, and the rear wheels
confirmed for military vehicles, a loaded truck of
generally track inside the front wheels. The added width
40,000pounds gross weight powered so that the weight-
of pavement necessary can be computed by geometry
power ratio is about 400 (100 horsepower) is
for any combination of curvature and wheel base.
representative of the size and type of vehicle which
Generally, widening is not required on modern highways
should be used for control of grade design. The
with 12-foot lanes and high type alignment, but for some
maximum grades recommended for use in design of
combinations of speed, curvature, and width, it may be
roads and streets, shown in table 1-1, were selected on
necessary to widen these highways also. The amount of
the basis of these values. Since capacity of a road or
widening required on horizontal curves on roads is
street is directly affected by reduction in speed, there
shown in table 3-3. This is the widening normally
must be restrictions on speed reduction if the capacities
required for off-tracking and may not provide clearance
shown in tables 1-1 and 1-2 are to be used for design;
where sight is restricted. The additional width should be
therefore, the distance the design vehicle can travel up a
added to the inside of the curve, starting with zero at the
designated grade before vehicle speed is reduced to a
tangent-spiral (TS), attain the maximum at the spiral-
specified value must be determined. This distance is
curve (SC), and diminishing from the maximum at the
termed critical length of grade. Critical lengths for
curve spiral (CS) to zero at the spiral-tangent (ST) as
grades shown in tables 1-1 and 1-2 are extracted from
shown in figure 3-3. Increased sight distance may be
AASHTO publications.
It is emphasized that the
provided by additional widening or by removal of sight
capacities (DHV) shown in tables 1-1 and 1-2 no longer
obstructions.
The latter is normally recommended
apply on roads or streets where the length of designated
because it is generally more economical. Figure 3-4
grades is in excess of the critical lengths. In instances
shows the relation between sight distance along the
where the length of grades is longer than the critical
center line of the inside lane on horizontal curves and the
length, the designer has three alternatives: change
distance to sight obstructions located inside these
location to reduce grades, reduce capacity, or provide
curves. The clear sight distance along the center line of
climbing lanes for heavy vehicles. Where the average
the inside lane on horizontal curves should equal the
daily traffic varies from two or three vehicles to none, the
minimum stopping sight distance shown in table 1-1 for
geometric design should be in accordance with the
the design speed.
(3) Vertical alignment.
criteria provided for class F roads. except that the
(a) Grade. It is essential that proper
maximum grade should be determined on the basis of
capability of vehicles required to use these roads. For
consideration be given to selection of grades for use in
instance, if all
design of roads and streets at military installations.
Selection of design grades involves traffic volumes,
composition of traffic, average running speed, capacity,
vehicle characteristics, drainage, safety, appearance,
3-14
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