TM 5-822-2/AFM 88-7, Chap. 5
with others in the traffic stream. The selection of speed
capacity occurs when average running speed is between
is now affected by the presence of others, and
30 and 45 miles per hour. Any factors which reduce or
maneuvering within the traffic stream requires substantial
increase the average running speed will also reduce
vigilance on the part of the user. The general level of
capacity. It is anticipated that there may be instances
comfort and convenience declines noticeably at this
where the average running speed may be reduced
level.
substantially in which .case the capacity will also be
(d) Level-of-service D represents high-
reduced. In these instances the capacities (DHV) shown
in tables 1-1 and 1-2 no longer apply. The capacities
density but stable flow. Speed and freedom to maneuver
(DHV) shown in tables 1-1 and 1-2 for class A, B, and C
are severely restricted, and the driver or pedestrian
roads and class A, B, C, and D streets will be reduced in
experiences a generally poor level of comfort and
accordance with the following tabulation in all cases
convenience. Small increases in traffic flow will generally
where it is anticipated that the average running speed on
cause operational problems at this level.
(e) Level-of-service
a substantial length of a road or street will be appreciably
E
represents
less than 30 miles per hour.
operating conditions at or near the capacity level. All
speeds are reduced to a low but relatively uniform value.
Capacity (DHV) In Percentage
Freedom to maneuver within the traffic stream is
Average Running
of Values Shown
extremely difficult, and it is generally accomplished by
Speed, mph
in tables 1-1 and 1-2
forcing a vehicle or pedestrian to "give way" to
accommodate such maneuvers.
Comfort and
30
100
convenience levels are extremely poor, and driver or
25
95
pedestrian frustration is generally high. Operations at
20
87
this level are usually unstable because small increases in
15
72
(8) Vehicle loads on Army and Air Force
flow or minor perturbations within the traffic stream will
cause breakdowns.
pavements. Relations between load, load repetitions,
(f) Level-of-service F is used to define
and required pavement thickness developed from
forced or breakdown flow.
This condition exists
accelerated traffic tests of full-scale pavements have
wherever the amount of traffic approaching a point
shown that, for any given vehicle, increasing the gross
exceeds the amount which can traverse the point.
weight by as little as 10 percent can be equivalent to
Queues form behind such locations. Operations within
increasing the volume of traffic by as much as 300 to 400
the queue are characterized by stop-and-go waves, and
percent. On this basis, the magnitude of vehicle loading
they are extremely unstable. Vehicles may progress at
must be considered more significant in the design of
reasonable speeds for several hundred feet or more,
pavements than the number of load repetitions. For
then be required to stop in a cyclic fashion. Level-of-
forklift trucks where the load is concentrated on a single
service F is used to describe the operating conditions
axle, and for tracked vehicles where the load is evenly
within the queue as well as the point of the breakdown.
divided between the two tracks, the severity of the
It should be noted, however, that in many cases
vehicle loading is a function of the gross weight of the
operating conditions of vehicles or pedestrians
vehicle and the number of load repetitions.
The
discharged from the queue may be quite good.
magnitude of the axle load is of greater importance than
Nevertheless, it is the point at which arrival flow exceeds
gross weight for most other multiaxle vehicles since axle
discharge flow which causes the queue to form, and
spacings are generally large enough that there is little or
level-of-service F is an appropriate designation for such
no interaction between the wheel loads of one axle and
points.
the wheel loads of an adjacent axle. Thus, for multiaxle
(g) Capacity for uninterrupted flow. The
vehicles having uniform axle loads, the increased
Highway Capacity Manual presents methods for
severity of loading produced by four- or five-axle trucks
determining highway capacity for uninterrupted flow and
compared to two- or three-axle trucks is largely a fatigue
methods by which this capacity is modified for interrupted
effect resulting from a larger number of axle-load
flow. Therefore, it is necessary to determine the capacity
repetitions per vehicle operation.
(a) Pneumatic-tired vehicles. To aid in
for uninterrupted flow of both roads and streets. The
capacity for interrupted flow should then be determined
evaluating vehicular traffic for the purpose of pavement
as described therein. The DHV shown in tables 1-1 and
design. pneumatic-tired vehicles have been divided into
1-2 is equal to the capacity for uninterrupted flow for
the following three groups:
each class of road and street on the basis of the
geometric design criteria presented. Highway capacity is
directly related to the average running speed. Maximum
3-5
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