Civil Engineering Reference
In-Depth Information
Chapter 11
The Impact of Traffic Congestion
on Accessibility
11.1 Introduction: De
ning Accessibility
Accessibility is a widely used term. In transportation, it can refer to a traveler
s
physical or economic ability in using a given travel mode; it can refer to describe a
traveler
'
'
s access to one or more destination opportunities available within a speci
c
distance, travel time, or travel cost from the traveler
s origin; or it can be used by
the marketing department of a retail store chain to describe/quantify the number
potential customers within a 20 min travel time to a store.
The number of destination opportunities accessible from a given location is
determined by (1) a traveler
'
s mobility (the door-to-door distance one can cover
within a travel time and cost budgets), (2) the connectivity of the street network that
determines the directness of travel between an origin and a desired destination, and
(3) by the number of desired opportunities located within this distance.
Therefore, the same zone or area can have different accessibility measures: one
for those who walk; another for those who ride transit, and yet another for car users.
As shown in Fig.
11.1
, a traveler
'
s
trip distance
is determined by her/his trip
time budget, the modal door-to-door trip speed, and the degree of network con-
nectivity; while the number of destination opportunities is determined by the land
use density and mix of the desired activities located within this distance.
For example, a drug store located 10 min away by a slow mode (e.g., walking) is
just as accessible as one located 10 min away by a faster mode (e.g., auto). The
reason why the walking mode provides the same accessibility than a faster mode is
because the two drug stores are located in areas of different land use density: the
city drug store with a walk access of 10 min is located
'
mile away while the
suburban drug store with a drive access of 10 min is located 5 miles away.
Therefore, focusing on mobility alone to improve accessibility ignores the role
played by land use policies in the urban area. This chapter describes the impact of
traf
½
c congestion on accessibility via its impact on mobility
and
the patterns of
activities in the urban area.
