Civil Engineering Reference
In-Depth Information
In large cities, where many activities are located nearer to each other than they
are in suburban areas, traveling options in addition to the car include walking,
biking, and transit. However, lower per capita car use in cities does not translate to
lower traf
c congestion because cities with higher population and employment
densities generate higher traf
c densities on their street networks.
However, although city and suburban travelers are both impacted by traf
c
congestion, the severity of impact on travelers might not be the same:
City streets provide access to more destination opportunities (per unit distance)
than suburban streets
Destination opportunities in cities can be reached by walking or biking while
these modes usually are not feasible in suburban areas.
Therefore, the negative impact of traf
c congestion on suburban travelers tends
to be greater than on city travelers.
23.2.1.1 Population Density
Population densities re
ect city size, city age, and physical features such as water
bodies and hilly terrain that can constrain development to a limited supply of
developable land. City age is also a factor. Older US cities that developed before the
automobile era are more densely developed than cities that grew in the automobile
era. The high rates of population growth and the availability and cost of land
typically determine land use densities. Population densities typically increase with
the cost of land.
fl
23.2.1.2 Employment Density
Employment concentrations initially developed in city centers and in established
suburban centers. They were created where major streets and transit lines crossed
and converged, and along water bodies. Transportation technology has also played
an important role in shaping densities and development. Until the 1929 Great
Depression, rapid transit lines helped to concentrate employment in the city center.
In the decades following World War II, major activity centers
—
largely auto
dependent
—
emerged in the suburbs of large cities.
23.2.2 Population Density, Mode of Travel, and Traf
c
Density
The effects of population and employment density on person trip rates, travel
modes, and per capita VMT have long been recognized.
