Civil Engineering Reference
In-Depth Information
Table 2.4 Transport mode and urban form
Item
Type of city
Pedestrian
Electric transit
Automobile
Population
3,000,000
3,000,000
3,000,000
Area (square mile)
30
200
500
±
Density (persons/sq.mi)
100,000
15,000
6,000
Jobs in city center
200,000
300,000
150,000
Development pattern
Compact
Radial with major corridor
Dispersed
Example
Paris pre 1900
Chicago 1930
Dallas 1990
Assuming an average of 225 square feet of
fl
floor space per commuter, the of
ce
fl
s downtown commuters would amount to
approximately 79 million square feet, and to accommodate Austin
floor space needed to hold New York
'
'
s downtown
commuters, 18 million square feet.
2.3 Conclusion
Urban development and congestion patterns re
ect the available transportation
technologies. Each advance in the speed of travel has increased mobility, in
fl
fl
uenced
land development, the form of cities, and patterns of congestion.
Walking limited the radius of cities to the distance one could cover in 30
40 min
-
(an average of about 2 miles).
The electric street car extended the radius of the city, focused development
along street car lines, reduced residential density in city centers and spread
congestion outward. Large cities such as Boston and Philadelphia placed their
street car lines underground to avoid congestion in city centers.
A handful of cities built rapid transit lines that complemented suburban rail lines
in improving mobility. These facilities had the dual effects of further concen-
trating development in the city center and extending urban development out-
ward along the rapid transit lines. In a few cases, parallel rapid transit lines were
built to accommodate the increased demand.
Automobiles and the roadways that were built to serve them further decen-
tralized development and traf
c congestion.
The changes in transport
technology progressively
fl
flattened the population
—
density gradient
the decline in population density with increasing distances
from the city center. These changes are illustrated in Table
2.4
that gives
illustrative population and employment densities for pedestrian, electric transit
and automobile cities [
5
].
