TM 5-822-2/AFM 88-7, Chap. 5
focus. Traffic facilities generally operate poorly at or
in available lanes of a facility and the directional
near capacity, and facilities are rarely designed or
distribution of traffic.
(d) Control
conditions.
Control
planned to operate in this range. Capacity analysis is
also intended to estimate the maximum amount of traffic
conditions refer to the types and specific design of
that can be accommodated by a facility while maintaining
control devices and traffic regulations present on a given
prescribed operational qualities.
facility. The location, type, and timing of traffic signals
(c) Capacity analysis is, therefore, a set
are critical control conditions affecting capacity. Other
of procedures used to estimate the traffic-carrying ability
important controls include STOP and-YIELD signs, lane
of facilities over a range of defined operational
use restrictions, turn restrictions, and similar measures.
(6) Levels of service. The concept of levels of
conditions.
It provides tools for the analysis and
improvement of existing facilities, and for the planning
service is defined as a qualitative measure describing
and design of future facilities.
operational conditions within a traffic stream and their
(d) The definition of operational criteria
perception by motorists and/ or passengers. A level-of-
is accomplished using levels of service. Ranges of
service definition generally describes these conditions in
operating conditions are defined for each type of facility,
terms of such factors as speed and travel time, freedom
and are related to amounts of traffic that can be
to maneuver, traffic interruptions, comfort and
accommodated at each level.
convenience, and safety.
(e) The following presents and defines
(7) Six levels of service are defined for each
type of facility for which analysis procedures are
the two principal concepts of this manual: capacity and
available. These definitions are general and conceptual
level of service.
(5) Capacity. In general, the capacity of a
in nature, and they apply primarily to uninterrupted flow.
Levels of service for interrupted flow facilities vary widely
facility is defined as the maximum hourly rate at which
in terms of both the user's perception of service quality
persons or vehicles can reasonably be expected to
and the operational variables used to describe them.
traverse a point or uniform section of a lane or roadway
They are given letter designations from A to F with level-
during a given time period under prevailing roadway,
of-service A representing the best operating conditions
traffic, and control conditions. The time period used in
and level-of-service F the worst. In general, the various
most capacity analysis is 15-minutes, which is
levels of service are defined as follows for uninterrupted
considered to be the shortest interval during which stable
flow facilities:
flow exists.
(a) Level-of-service A represents free
(a) Capacity is defined for prevailing
flow. Individual users are virtually unaffected by the
roadway, traffic, and control conditions, which should be
presence of others in the traffic stream. Freedom to
reasonably uniform for any section of facility analyzed.
select desired speeds and to maneuver within the traffic
Any change in the prevailing conditions will result in a
stream is extremely high. The general level of comfort
change in the capacity of the facility. The definition of
and convenience provided to the motorist, passenger, or
capacity assumes that good weather and pavement
pedestrian is excellent.
conditions exist. It is also important to note that capacity
(b) Level-of-service B is in the range of
refers to a rate of vehicular or person flow during a
stable flow, but the presence of other users in the traffic
specified period of interest, which is most often a peak
stream begins to be noticeable. Freedom to select
15-minute period. This recognizes the potential for
desired speeds is relatively unaffected, but there is a
substantial variations in flow during an hour and focuses
slight decline in the freedom to maneuver within the
analysis on intervals of maximum flow.
(b) Roadway conditions.
traffic stream from level-of-service A. The level of
Roadway
comfort and convenience provided is somewhat less
conditions refer to the geometric characteristics of the
than level-of-service A because the presence of others in
street or highway, including the type of facility and its
the traffic stream begins to affect individual behavior.
development environment, the number of lanes (by
(c) Level-of-service C is in the range of
direction), lane and shoulder widths, lateral clearances,
stable flow, but marks the beginning of the range of flow
design speed, and horizontal and vertical alignments.
(c) Traffic conditions. Traffic conditions
in which the operation of individual users becomes
significantly affected by Interactions
refer to the characteristics of the traffic stream using the
facility. This is defined by the distribution of vehicle types
in the traffic stream, the amount and distribution of traffic
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