Civil Engineering Reference
In-Depth Information
1.
Type of Area
In small towns and rural areas the threshold speed of congestion is (and should
be) free-
ow speed. Travelers in large cities, however, are accustomed to expect
an environment of greater traf
fl
c intensity with more vehicle-vehicle and
icts. Therefore they are likely to tolerate a lower
threshold speed of congestion (Table
8.6
).
2.
Time of Day
In large cities lower threshold speeds are tolerated during peak periods than in
off-peak periods (Table
8.5
).
3.
Type of Facility
One expects faster speeds on a freeway than on a local street. Therefore the
congestion threshold speed for a freeway is higher than for a local street
(Tables
8.5
and
8.6
).
4.
Economic Considerations in the Choice of Threshold Congestion Speed
Some economists and transportation professionals have questioned the use of
free-
pedestrian-vehicle con
fl
fl
flowing traf
c speeds as a basis for calculating peak period traf
c condi-
tions in larger urban area. John Meyer and Jose
'
Gomez-Ibanez in their topic
“
Autos, Transit, and Cities
”
[
20
] state:
“
A highway large enough to allow free
fl
c during the rush hours in the center of large cities is seldom
optimal because the building of such highway capacity is very expensive and the
bene
flowing traf
ts in time savings and operating costs accruing only to a small group of
rush hour users will be comparatively small. In short, highway engineers and
planners understand that
traf
c congestion is almost advisable in a well-
”
'
designed highway system.
Countering this position is, perhaps, today
is, reality
that some freeways are congested for many hours each weekday.
8.4 Conclusions
Congestion in cities is a by-product of their success in attracting people to jobs and
other amenities, and the inability of cities to improve/expand transportation capacity
to keep pace with this growth. The cities
'
challenge is to keep congestion man-
ageable as their population and economies grow.
To be helpful in congestion management decisions, the de
nition of congestion
should be based on a comparison of
“
actual travel times
”
with
“
expected travel
times
for peak hour and off peak conditions. Expected travel times can vary from
area to area, by time of day, and by type of routes, and should be established with
the input of the area
”
s stakeholders (e.g., travelers and freight movers).
Drivers begin to experience congestion when a time increment of 0.4 min/mile is
added to their trip, and it becomes a signi
'
cant problem when the increment reaches
0.8 min/mile. Congestion mainly occurs in the peak weekday commuter hours
—
when about 60 % of all congestion is concentrated
but it is also found on summer
and holiday weekends when many people travel to or from beaches and other
recreational areas.
—
