Civil Engineering Reference
In-Depth Information
It is vital, therefore, that standards of tolerable congestion delay are related to
the size of the urban area and re
'
participation. When this is done the work products of transportation profes-
sionals will have a better chance of in
fl
ect community input via stakeholders
fl
uencing the decisions of transportation
policy makers.
8.2 The Dimensions of Congestion
Congestion can be characterized by four aspects of its occurrence:
intensity,
duration, extent, and variability
[
6
11
].
-
ects the amount of congestion expressed as a rate (e.g. minutes/mile).
Duration
refers to the amount of time the road/system is congested.
Extent
describes the miles of roads that are congested or the number of travelers
affected by the congestion.
Variability
measures the variation in the amount, duration, and extent of con-
gestion over time.
These indicators and their metrics are summarized in Table
8.1
.
The relationship between intensity, duration, and extent is shown in Fig.
8.1
where the variation in congestion intensity can be measured both in distance
(extent) and time (duration). Distance is shown on the horizontal axis, and time is
shown on the vertical axis. A series of contour lines, displaying various levels of
intensity, are displayed on this distance-time grid. Separate mapping can be used to
re
Intensity
re
fl
fl
ect
variability
in all three congestion criteria
—
one for average values, and
another for the 95 percentile values.
The above congestion indicators are further described below by various metrics.
Along with a description of these metrics, there is also a commentary of their
strengths and weaknesses.
8.2.1 Intensity
Intensity measures the amount of congestion delay experienced at an intersection
approach, along sections of a given route, several routes, or an entire urban area. Its
metrics include: (1) congestion delay rate, (2) vehicle-hours of delay, (3) person-
hours of delay, (4) travel time index, and (5) travel time tax.
8.2.1.1 Congestion Delay Rate
This is a measure of the amount of delay experienced on freeways and arterial
streets. The equation is as follows:
