Civil Engineering Reference
In-Depth Information
7.2 Recurring Congestion
7.2.1 Physical Bottlenecks
Topographic barriers and physical bottlenecks on streets and highways represent
choke points that reduce road capacity and cause peak hour traf
c to back up and
create congestion of the upstream roadways.
7.2.1.1 Topographic Barriers
A city
s physical features can create congestion. Topographic barriers such as hills,
mountains, steep grades, and water bodies constrain street patterns and concentrate
travel on a limited number of available crossings. Balancing the capacity of the
approach roadways with the capacity provided at a limited number of crossings is
usually a dif
'
cult task seldom achievable. Therefore, in these areas peak hour
congestion is a common event on the roadway approaches to bridges, tunnels, and
other roadways that traverse such crossings.
Some US examples of topographic barriers in cities illustrate their impact on
congestion.
Manhattan Island
in New York City requires motorists to cross the Hudson and
East Rivers to reach the business district. AM peak hour inbound traf
c backs
up forming long queues on the roadways leading to the CBD, often requiring up
to 40 min before reaching the crossing. Likewise PM peak hour traf
c backs up
forming long queues over many blocks on city streets that often require up to
40 min waiting time before reaching the bridge or tunnel crossing.
San Francisco
is located on a peninsula that is separated from Marin County and
East Bay communities by San Francisco Bay. Road access from the north and
east is limited to the Golden gate and Bay Bridges that constrain traf
c demand
from the converging freeways leading to the Bay Bridge.
Los Angeles
San Fernando Valley is separated from the rest of the city by the
Santa Monica mountains.
'
Seattle
is hemmed in by Elliot Bay and Lake Washington.
Pittsburgh
s
Golden Triangle is located between the Allegheny and Mongon-
ohela Rivers, and nearby hills to the east and south.
'
New Orleans
is bounded by the Mississippi River and Lake Ponchatrau.
The Bronx
in New York City, has few continuous east
—
west streets because of
its dif
cult terrain.
Most cities also have man-made barriers to travel. These include large ceme-
teries, railroad embankments with infrequent crossings, and large private
developments.
