Civil Engineering Reference
In-Depth Information
If Capacity Is Not Increased
In a growing region with increasing traf
c demand, nearly every road will expe-
rience traf
c growth (from development activities with direct access to the road, or
from traf
c passing through) in the area.
In the example cited, if the congested road in question does not increase its
capacity (S
0
), the corridor will receive less future years development with a smaller
traf
c demand limited by natural growth of the metro area (D
3
) than it would
experience (D
1
) with added capacity This condition is shown in Fig.
17.17
where
the added volume from
“
natural
”
growth [(V
0,3
)
(V
0,0
)]
increases
the generalized
-
cost of travel from P
1,1
to P
0,3
.
Are Communities Better or Worse Off by Adding Road Capacity?
The preceding example shows that the future traf
c growth in the area served by the
existing road will be less than that expected with its expanded capacity (V
0,2
vs. V
1,1
).
From these expected outcomes it is clear that not increasing the capacity of the route
will increase traf
c congestion in future years (P
0,2
) versus (P
1,1
) even if the growth in
traf
c volume is less than that resulting with expanded capacity (V
0,2
) versus (V
1,1
).
This example shows that reducing traffic growth by avoiding capacity expansion
strategies does not necessarily lead to less congestion but, on the contrary, con-
tributes to its increase.
Opposing road capacity expansion projects solely on the argument that they
induce additional private vehicle trips ignores the fact that most of these trips are the
product of increased human activities undertaken by people who bene
t from them.
“
Economic studies that measure traveler bene
ts comprehensively
—
not just in
terms of travel time saved
—
show that there is indeed a positive bene
t to road users
resulting from congestion relief, even when traf
c increases as a result of the
project
”
[
1
,
22
].
17.5 Conclusions
This chapter has shown various ways to expand the capacity of the existing
roadway systems to provide congestion relief. These include the removal of bot-
tlenecks caused by lane imbalance, reducing con
flows at intersections and
interchanges as well as adding new capacity to meet increasing travel demands.
However, adding new capacity is a two edged strategy: it reduces congestion in the
short term, but it also generates additional vehicle traf
fl
icting
fl
c in the long term. This effect
promotes endless public debate about the merit of this strategy
—
especially in built up
areas where it creates major disruptions to communities and neighborhoods.
In his 1979 topic on
The Urban Transportation System
—
Policies and Policy
Innovation [
17
], Alan Altshuler indicates that
“
Congestion tends to be greatest
