Civil Engineering Reference
In-Depth Information
Today the causes of traf
c congestion are more speci
cally known and include
(1) large concentrations of demand in time and space
including temporal surges in
travel demand on roadways of generally constant capacity physical, operational,
and design de
—
c demand that exceeds
roadway capacity, and (3) physical and operational bottlenecks.
Congestion generally increases with city size. This happens because activity
concentrations are larger, and travel distances are longer as cities grow.
Economists view chronic congestion as a pricing-induced problem. They argue
that the absence of marginal cost pricing contributes to congestion because average
cost pricing makes road use more attractive than it would be if prices would rise
with congestion [
2
,
3
].
ciencies that create bottlenecks, (2) traf
4.2.1 Concentration of Trips in Space and Time
If all travel demand were evenly distributed among the various sections of the urban
area, the traf
c congestion problem would be a rare event. Similarly if all travel
were evenly distributed to each hour of the day there would be little, if any,
congestion.
But travel demand patterns reflect the concentration in time and space of daily
activities: where and when people work, shop, recreate, move goods and provide
services. It is the peaking of these spatial and temporal travel patterns that con-
tributes to the recurring traf
c congestion problem.
4.2.2 Growth in Population, Employment, Car Use
and Insufficient Capacity
Growth in population, employment, and car use (vehicle miles of travel
VMT)
increase congestion on streets and highways where capacity growth has not kept
pace with growth in VMT. The factors that contribute to and shape the growth in
population, employment, and vehicle miles of travel (VMT) in urban areas are
discussed in detail in Chap.
6
.
—
4.2.3 Bottlenecks
Bottlenecks are perhaps the most common cause of congestion. They result from
the convergence of a greater number of lanes in the upstream roadways than are
available in the downstream roadways. Bottlenecks delay is typically found in
hours of peak flow where the number of lanes converging on a roadway, bridge or a
