Geography Reference
In-Depth Information
that, all else being equal, a larger prism is better since it affords greater potential
mobility and therefore higher accessibility. However, there are costs required to
achieve the accessibility; some of these burden broader society. Take the work-
to-home journey using private owned vehicles as an example (see Fig.
12.1
a): if
using motorized conveyances such as the automobile, the individual consumes non-
renewable resources (gasoline or diesel fuel) and generates noxious emissions to the
environment. Therefore, a prism with larger size may also imply higher potential
costs both to the individual and society at large.
In this section, we illustrate the general analytical framework to calculate
expected environmental costs of accessibility. Specially, we focus on the engine-
based costs, including consumption of non-renewable resources such as fossil fuels
and emissions of harmful pollutants such as NO
X
, PM-2.5, and
greenhouse gases
(GHG) such as CO
2
.
The past two decades have witnessed increasing attention to the negative
impacts of transportation and concerted efforts towards a sustainable transportation
paradigm (Bannister
2008
; Black et al.
2002
; Ewing et al.
2008
). The goal of
transportation planning is no longer recognized as maximizing but rather managing
mobility. However, widely-applied emissions models such as MOBILE or MOVES
(United States Environmental Protection Agency (EPA)
2009
) operate at the
aggregate level; this is not appropriate for travel demand management policies that
focus on individual-level activity and mobility patterns. Calculating the accessibility
cost associated with a space-time prism considers the operation of an individual
vehicle; this can evaluate microscale traffic scenarios and individual-level mobility
and accessible patterns (Barth et al.
1996
).
Barth et al. (
1996
) identify three essential parameters that determine the engine-
based cost of vehicle movements: spatial reference, temporal reference, and velocity
profile. The spatial reference and temporal reference cover the external factors that
affect the engine performance such as the slope of road surface and the ambient
temperature at each moment in time. Given these external factors, the engine-based
cost of a vehicle is non-decreasing with respect to velocity. A general function
C
(
v
j
(
x
,
y
,
t
)) to calculate the instantaneous vehicle cost should have the following
properties: (i)
v
<
v
0
)
C
(
v
j
(
x
,
y
,
t
))
C
(
v
0
j
(
x
,
y
,
t
))
8
(
x
,
y
,
t
) (non-decreasing with
respect to velocity); and, (ii)
C
(0
j
(
x
,
y
,
t
))
D
0 (no cost while the vehicle is station-
ary). The cost function can be modified by characteristics of individual vehicles (e.g.
engine type, weights, mileage), road conditions (e.g. slope and surface material),
and environmental factors (e.g. humidity, temperature).
Given the cost function
C
(
v
j
(
x
,
y
,
t
)), along with the velocity distribution and the
visit probability at accessible locations within the prism, we can calculate engine-
based cost of a prism as:
8
<
9
=
2
4
3
5
dxdy
t
j
Z
“
v
max
Z
P
.x; y/
ˇ
ˇ
ˇ
t
P
t
v
ˇ
ˇ
ˇ
.x; y/
C
v
ˇ
ˇ
ˇ
.x;y;t/
d
v
C
ij
D
dt
:
;
t
i
v
min
.x;y/
2
Z
ij
.t /
(12.40)
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