Civil Engineering Reference
In-Depth Information
Chapter 9
The Impacts of Congestion on Trip Time
9.1 Introduction
Congestion metrics that focus only on network performance (e.g., see Chap.
8
) are
necessary but not suf
cient in addressing the impact of congestion on travelers.
Reliance just on network congestion metrics provides only a partial understanding
of the congestion problem. This is because network performance metrics are not
connected with trip length. A delay rate of 1 min per mile has a different impact on
a 5 mile trip (common in compact cities) than on a 10 mile trip (common in
suburban areas). Therefore, one should not ignore the effect of trip length in ana-
lyzing the impact of congestion on travelers.
9.2 Travelers with Different Trip Lengths
How trip time is affected by congestion depends on the length of the trip. The length
of the trip, in turn, depends on the size of the urban area, its population density, and
its development patterns.
The effect of area size on trip length is shown in Fig.
9.1
[
1
]. This relationship is
from data of the 60s and 70s, which does not account for growth in car ownership
and population in the expanding suburbs of metropolitan areas in the last 50 years,
thus the trend shown would underestimate average trip lengths for current condi-
tions (see Chap.
6
).
The effect of population density on trip length is shown in Fig.
9.2
.
When population densities are less than 5,000 people per square mile, the
average trip length exceeds 16 miles. As densities increase there is a corresponding
decrease in trip lengths
a decline to about 10 miles per average trip at densities of
25,000 persons per square mile, and to less than 5 miles when densities exceed
40,000 people per square mile.
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