Civil Engineering Reference
In-Depth Information
Fig. 11.5 Distribution of person trip lengths for the NY metropolitan area (all modes, all
purposes, 1997/1998).
Source
Reference [
4
]
“
”
given distance and
that number by the intervening travel time, cost or
distance [
1
] can be estimated by Eqs.
11.1
and
11.2
:
discounting
DO
j
k
.
tt
ij
X
X
b
z
n
A
i
¼
m
ð
11
:
1
Þ
j¼
1
;k¼
1
X
z
j¼
1
ðDO
j
Þ
k
=ððtt
ij
Þ
b
m
A
ik
¼
ð
11
:
2
Þ
where:
1. A
i
= an accessibility index of a reference zone (
i
) to destination opportunities
(
j
) aggregated across all of types of destination opportunities (
k
n
) (e.g., work,
shopping, etc.) in the area. This zonal accessibility index is a metric used to
compare and rank zones by their aggregate accessibility using a given mode
(m). Thus the accessibility value of each zone is mode-dependent.
2. A
ik
= the accessibility index of reference zone (
i
) to destination opportunities (j)
of a given type (
k
). The value of A
ik
is used to compare the modal accessibility
of one zone to that of another zone or to compare the zonal accessibility
provided by different transportation modes.
3.
i
= the reference zone
4. j = the zone of destination opportunities
5. k = the type of destination opportunities (work, shopping, medical, etc.)
6. (DO
j
)
k
= the number of destination opportunities of a given type
k
at zone j,
7. (tt
i
−
j
)
b
m = travel time measures separating zone
i
and zone
j
using mode
m
. The
exponent of travel time differs with trip purpose [
5
]. Thus for work travel
b
= 2.0; while for shopping travel
b
could = 3.0, to re
−
ect less willingness to
tolerate longer travel times for shopping trips than for work trips. As was
discussed earlier, (tt
i
−
j
)
m
is determined by the door-to-door travel speed of the
mode used.
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