Q2 Z2,(Q2) Z2(Q2) - Institute for Water Resources - U.S. Army

truck sample is small ( 148 origins in January). This is due to the fact
that truck transportation is particularly favorable for short haul, and
little corn is grown within a short distance of Chicago. On the other
hand, the few truck origins spread far away from Chicago have a lot of
weight (35%) in the sample, although they hardly can be considered as
typical origins of truck shipments. In that respect, it is worthwhile
to point out that most of these origins are also shipping by rail. They
probably use truck shipments occasionally, or have unusual characteristics.
Now, the parameters of the population of truck origins have been computed
on the basis of the whole sample and have been biased by the peculiar
characteristics of these origins. Therefore, the estimates of the money
equivalents of quality differences between road transportation and the
two other modes have also been biased. It is likely that more realistic
estimates would have been obtained on the basis of a truck sample reduced
to the "typical" origins. For the boundaries an example shows well how
sensitive they are to variations of the money equivalents of quality
differences. In the case of Railway C a change of the money equivalent
from -3.54 to -2.54 would have reduced the radius of the circumference
limit between road and rail areas from 85.31 to about 70 millimeters.
Variations of the same order of magnitude have been found for the other
railways.
In view of these remarks our confidence in the spatial model is
increased. On the basis of correct information it can produce fairly accurate
predictions of the choice of modes. As the results take the form of market
areas for transportation modes they are particularly convenient to generate
estimates of demand for transportation and evaluate investment projects
in ways of transportation.
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