Civil Engineering Reference
In-Depth Information
EIGHT
TRAFFIC ACROSS THE BRIDGE
The most important practical function of a bridge is to serve as a link con-
necting transportation routes across an obstacle, such as a river or limited-
access highway, or to elevate the road so it does not interfere with activity
below, such as a local street or natural ecosystem. For decisions on whether
a bridge should be built, expanded, or rehabilitated, its effect on transporta-
tion is usually the foremost consideration.
For these decisions, the operative question is: What effect will the
bridge have on improving transportation, say by decreasing travel times
and reducing congestion? As we examine ways of answering the question
in this chapter, we will look mainly at road transportation with cars and
trucks, because that is the preponderant way in which people and goods
move in the United States. We do care about subways, railways, trolleys, and
pedestrian travel. We understand the advantages that transportation modes
other than the automobile bring for better air quality, safer commuting,
reduced use of fossil fuels, and streets made more pleasant for pedestrian
enjoyment.
Still we will speak here of cars and related motor vehicles, because
cars are not going away soon. For those who can afford to drive and are
capable of driving, in places where road capacity is available, the car maxi-
mizes personal options for choosing destinations and choosing the time and
route of travel. This flexibility is all the more important for businesses. In
principle, if all flowed freely, and everyone could afford a vehicle, and there
were no side effects like pollution, the car would maximize freedom, business
and work opportunity, and personal satisfaction. Though for many urban
planners, the first impulse is to want to reduce automobile dependence, we
should not forget the value, even pleasure, that cars provide to millions of
people.
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