Civil Engineering Reference
In-Depth Information
Chapter 6
Insufficient Capacity, Growth
in Population, Employment, and Car Use
6.1 Historical Imbalance of Roadway Supply
and Travel Demand
Traf
c demand has spread in post-World War II metropolitan America as a result of
changing patterns of where people live and work, and how they travel. More people
occupying more land have created more activity dispersal and have increased car
dependency for mobility and access to activities. During this period, capacity
expansion was generally insuf
ciently serve growth in population,
employment, and car use. This chapter presents some of these trends and their
congestion implications.
Congestion increases when the investment in transportation facilities fails to
keep up with the growth in travel.
Today
cient
to ef
is roadway congestion is largely the result of the imbalance over time
between the growth of vehicles miles of travel (VMT) and the roadway capacity in
lane miles. Examples of this disparity are shown in Figs.
6.1
,
6.2
and
6.3
for
Interstate, other arterial highways, and local roads, respectively. The trends are
shown for the 20 year period between 1980 and 2000 [
1
]
While the VMT on urban interstate highways grew by 240 %, the lane miles in the
system grew by 150 %. The imbalance between VMT growth and roadway capacity
growth, however, was not limited to urban interstate highways but was also pre-
valent throughout the urban roadway system: the VMT for other arterials and local
roads grew by an average of 185 % compared to 135 % growth in lane miles.
Where this disparity has been the greatest, so has been its impact on congestion:
Fig.
6.4
and Table
6.1
([
2
], p. 16) compare the growth in congestion to the ratio of
change in demand to change in capacity over a 28 year period (1982
'
2010) for 101
-
urban areas ([
2
], Table 9, p. 52).
These trends show that urban areas where the increase in roadway capacity nearly
matched the increases in demand experienced a slower congestion growth than those
areas where capacity growth lagged substantially behind the growth in VMT.
