Civil Engineering Reference
In-Depth Information
Chapter 8
Measuring Traf
c Congestion
8.1 Introduction
Congestion in transportation occurs when the occupancy of spaces (roadways,
sidewalks, transit lines and terminals) by vehicles or people reaches unacceptable
levels of discomfort and delay. For pedestrians, occupancy is expressed as the
number of pedestrians per unit area (or square feet per pedestrian). For vehicles, it is
expressed as the number of vehicles per unit length of roadway. As space occu-
pancy increases, the speed of movement decreases.
The traf
cde
nition of congestion has evolved over the years:
Mc Clintock in his 1925 topic
“
Street Traf
c Control,
”
de
nes congestion in
street traf
as a condition resulting from a retardation of movement below that
necessary for contemporary streets users
c
“
”
[
1
].
Alan Altshuler [
2
] indicates that
the term congestion denotes any condition in
which demand for a facility exceeds free-
“
fl
ow capacity at maximum design
speed
”
.
Homburger et al. [
3
]de
the level at which transportation
system performance is no longer acceptable due to traf
ned congestion as
“
c interference. This may
vary by type of transportation facility, geographic location, and time of day
”
.
“
”
“
The Institute of Transportation Engineers
con-
gestion means there are more people trying to use a given transportation facility
during a speci
Toolbox
(1996) states that
c period of time than the facility can handle with what are
considered acceptable levels of delay or inconvenience
”
[
4
].
The National Cooperative Highway Research Program Report No. 398 [
5
,
6
]
provides a number of de
nitions of congestion and its correlates of mobility,
accessibility, and reliability. These de
nitions provide a means to measure the
effects and consequences of traf
c congestion:
Congestion
—
the travel time or delay in excess of that incurred under light or
-
free-
fl
ow travel conditions.
