Civil Engineering Reference
In-Depth Information
Chapter 4
Overview of the Causes of Congestion
4.1 Introduction
Traf
c congestion results from the imbalance between the supply of and the
demand for transportation facilities:
The
supply
is constrained by history and geography, by transportation
management and operating practices, and by the level of investment on streets
and highways
The
demand
results from the concentration of travel in space and in time
Congestion can be classi
ed in two categories: recurring and nonrecurring
Recurring Congestion
is the delay travelers regularly experience/expect during
known travel times
such as the morning and evening rush hours
—
Nonrecurring Congestion
delay is caused by non-predictable (random) events
that disrupt traf
flow. These include incidents such as vehicle breakdowns or
crashes; road repair and inclement weather; special events that create sudden
surges in demand such as the end of a sports event; and natural or man-made
disasters. Nonrecurring congestion can either create
new congestion
(in the
off-peak periods), or can increase the delay experienced during periods of
recurring congestion.
c
fl
4.2 Summary of Causes
Nearly a century ago Miller McClintock [
1
] stated that congestion is due to three
general causes: (1) the inability of the streets to hold a suf
cient number of vehicles
and to process them at an adequate speed, (2) the inclusion of elements in the traf
c
stream which hamper its free
fl
flow, and (3) the improper or inadequate direction and
control of traf
c.
