Civil Engineering Reference
In-Depth Information
cantly reduce the per capita use of auto
travel, and increase the use of alternative modes to the private car.
Therefore, increasing density can signi
5.3.1 Population Density, Traffic Density, and Traffic Speed
Although increasing population density
reduces
per capita auto use, Table
5.5
shows that increasing population density also
increases
traf
c density (auto trips
per square mile)
—
an indicator of traf
c congestion.
5.3.2 Population Density and Traffic Congestion
Figure
5.1
shows how urban density affects average traffic speeds in the NY
metropolitan area [
5
], where average traf
c speed is shown to decrease with
increasing population density: from 25 mph in the outer areas with population
densities of 3,400 persons per square mile to 10 mph in the city
'
s core with a
population density of 65,000 persons per square mile.
Hence the paradox: increasing population density reduces per capita auto use
(a desirable social and environmental objective) but it also increases traf
c con-
gestion (e.g., VMT per square mile increases). However, as will be discussed later,
the negative impact of traf
c congestion on trip time in high density cities is
mitigated by the shorter trip lengths they generate and their bene
cial effect of high
density on accessibility.
Table 5.5 The impact of population density on traf
c density
(1) Population density midpoint
Table
5.4
(persons per square mile)
(2) Average daily
auto trips per person
(3) Traffic density (auto
trips per square mile)
175
3.50
613
375
3.53
1,324
625
3.52
2,200
875
3.44
3,010
1,500
3.48
5,220
2,500
3.46
8,650
3,500
3.34
11,690
4,500
3.51
15,795
6,250
3.29
20,563
8,750
2.92
25,500
30,000
1.90
57,000
0.59
a
65,000
38,350
Source
Calculated from Table
5.4
a
This drop in auto use may be explained by an increased use of transit, walking and other non-
auto modes achievable at population densities approaching those in Midtown Manhattan
