TM 5-822-2/AFM 88-7, Chap. 5
maintenance, and erosion.
Stability is required to
distance shown in tables 1-1 and 1-2. Horizontal curve
maintain the integrity of the pavement structure, and a
sight distance on single lane roads will be critical and will
slope stability analysis should be conducted for cuts and
be twice that required for a two or more lane highway.
(b) Passing sight distance. The passing
fills greater than 15 feet. For lower cut and fill heights,
erosion and maintenance. considerations control the
sight distance is the longest distance in which a driver
degree of slope. In general, side slopes should be no
can see the top of an oncoming vehicle, and the length
steeper than three horizontal to one vertical or two
of highway that must be visibly free of oncoming vehicles
horizontal to one vertical with a bench system. Additional
in order that the driver of a vehicle traveling at design
guidance for selecting degrees of slope is also presented
speed can overtake and pass a slower moving vehicle
in figure 3-2.
without hazard.
Passing sight distance should be
(8) Bridge clearance. Requirements affecting
provided as frequently as possible along two-lane, two-
highway safety are found in AASHTO publication, A
way roads, and a length equal to or greater than the
Policy on Geometric Design of Highways and Streets.
minimum values shown in table 1-1 should be provided.
(a) Horizontal at short bridges. The
The minimum passing sight distances in table 1-1
provide safe distances for a single isolated vehicle
minimum horizontal distance between curbs on short
traveling at design speed to pass a vehicle going 10
bridges must be equal to the width of the approaching
miles per hour less than design speed. It is desirable to
roadway including traffic lanes, parking lanes, full width
provide safe passing sections as frequently as possible
of shoulders, and medians (on divided highways). When
to provide each safe passing section with a sight
the cost of parapets and railings is less than the cost of
distance at least equal to but preferably greater than the
decking the median area, traffic lanes for traffic in
minimum passing sight distances shown in table 1-1.
opposing directions will be on separate structures. It is
Sight distances and safe passing sections should be
usually more economical to pave over the median area
shown on all construction and improvement plans to aid
on bridges with a median width less than about 15 feet.
(b) Horizontal at long bridges. Where a
in proper marking and sign placement. These distances
should not be confused with other distances used as the
long bridge is required, the designer should furnish a
warrants for placing no-passing zone pavement stripes
sketch of the proposed bridge and the basis for
on completed highways. Such values (e.g. section 3B-5
geometric and structural design to Headquarters,
of the Manual or Uniform Traffic Control Devices) are
Department of the Army (DAEN-ECE-G) Washington,
substantially less than design distances and are the
DC 20314-1000, or the appropriate Air Force major
result of operating control requirements based on
command.
(c) Vertical.
different assumptions from those for highway design.
The minimum vertical
(2) Horizontal alignment.
clearance will be at least 14 feet over all traffic lanes,
(a) General.
Where changes in
parking lanes, and shoulders. An additional 6 inches
horizontal alignment are necessary, horizontal curves
should be included to accommodate future resurfacing.
d. Design elements.
should be used to effect gradual change between
tangents. In all cases, consideration should be given to
(1) Sight distance. The length of roadway
the use of the flattest curvature practicable under
visible ahead of a vehicle along a highway is termed
existing conditions.
Adequate design of horizontal
"sight distance." Sufficient sight distance should be
curves depends upon establishment of the proper
available to enable a vehicle traveling at or near the
relations between design speed and maximum degree of
design speed to stop before reaching a stationary object
curvature (or minimum radius) and their relation to
in its path.
Discussions relative to sight distance
superelevation. The maximum degree of curvature is a
requirements for highway design in general are
limiting value for a given design speed and varies with
presented in AASHTO publications.
the rate of superelevation and side friction factors.
(a) Stopping sight distance.
The
(b) Maximum curvature (roads and
stopping sight distance is the distance traversed by a
streets). Desirable and absolute values for use in design
vehicle from the instant the driver sights an object
of horizontal curves on superelevated
requiring a stop to the instant the brakes are applied,
plus the distance required to stop the vehicle once the
brakes are applied. On single-lane roads the stopping
sight distance must be adequate to permit approaching
vehicles from either direction to stop. The sight distance
at every point along roads or streets must in all cases be
equal to or greater than the minimum stopping sight
3-13
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