Civil Engineering Reference
In-Depth Information
A recent Transportation Research Board study requested by the US Congress,
TRB Report 298 [
1
], examined the relationship between the built environment and
motor vehicle travel. The study assessed whether petroleum use, and hence emis-
sions of carbon dioxide (CO
2
)
could be reduced by
more compact, mixed use development. The most reliable studies estimate that
doubling residential density across a metropolitan area could lower household VMT
by 5
—
the primary greenhouse gas
—
12 %; and perhaps by as much as 25 % if coupled with higher employment
concentrations, signi
-
cant public transit improvements, mixed uses, and other
supportive demand management measures.
The general effects of population density on daily person-trip rates, travel modes,
and per capita VMT are shown in Table
23.1
[
2
]. As density increases above 10,000
persons per square mile, there is a pronounced decrease in daily per capita person-
miles, vehicle miles, and auto trips. And there is a corresponding increase in the use
of other modes.
However, while some compact land uses generate less VMT per capita, they can
generate higher traf
c density (VMT per square mile) resulting in higher traf
c
congestion. This is especially true for concentrations of of
ce buildings in city
centers. Therefore some of the most compact cities also tend to be the most con-
gested. An illustration of this phenomenon is shown in Table
23.2
, for the San
Francisco Bay Area [
3
].
This table clearly shows why traffic congestion increases with population den-
sity: VMT decreases less than a corresponding increase in population density.
Therefore, more compact (high density) developments do not necessarily lead to
Table 23.1 Daily trips per person in the US by population density and mode
Density
(persons per
square mile)
Daily person trips by mode
Daily
person
miles
Daily
VMT per
person
Auto Bus Rail
Taxi Walk
and bike
Other Total
0
99
3.55
0.02
0.00
0.00
0.24
0.16
3.77
31.58
21.13
-
100
-
249
3.50
0.02
0.00
0.01
0.24
0.13
3.90
29.95
20.73
250
-
499
3.53
0.02
0.00
0.00
0.29
0.12
3.96
29.33
20.40
500
749
3.44
0.05
0.01
0.01
0.21
0.12
3.88
29.00
20.99
-
750
999
3.44
0.05
0.01
0.01
0.26
0.13
3.90
26.25
18.35
-
1,000
-
1,999
3.48
0.03
0.01
0.00
0.23
0.11
3.86
26.17
18.63
2,000
2,999
3.46
0.06
0.01
0.00
0.28
0.11
3.92
23.45
19.04
-
3,000
3,999
3.34
0.06
0.02
0.01
0.29
0.09
3.81
23.11
16.89
-
4,000
-
4,999
3.51
0.05
0.01
0.00
0.30
0.08
3.95
24.77
17.24
5,000
7,499
3.29
0.09
0.02
0.01
0.36
0.06
3.83
24.56
16.28
-
7,500
9,999
2.92
0.11
0.05
0.02
0.45
0.07
3.62
20.59
14.15
-
10,000
-
49,999
1.90
0.29
0.21
0.03
0.95
0.04
3.42
17.02
8.73
50,000<
0.59
0.42
0.61
0.16
1.55
0.07
3.40
12.55
2.31
Source
Reference [
2
]
