VISSIM 5.30-05 User Manual

Optional Enhancements of the Model
differ only by a small section. In this case both routes have about the same
weight within the distribution, but the overall distribution is biased. This is a
general problem of Dynamic Assignment and know as the blue/red bus
paradox. The following graphics illustrate the problem:
Case 1
Two routes with identical cost. The trips
are split 50:50, no problem.
Case 2
Three routes with identical costs. Each
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route receives one third of the demand
from A to B, still no problem. A
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B
Case 3
Problem: Actually there are only two
distinct routes, but because of the slight
variations at the end, the route search
finds 3 routes.
Result: Distribution on 3 routes, but the
overlapping part of the two similar
paths contains too much traffic.
Case 4
Opposite problem of case 3: Actually
there are 3 routes but two have a short
part in common.
As in case 3 all three routes will get
about one third of the demand, which is
much more realistic than in case 3.
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A
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B
A
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B
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A
B
Solution in VISSIM
VISSIM offers an optional extension of the route choice model to correct the
biased distribution in case of overlapping routes. It is based on the idea of
the computation of a so called commonality factor of the routes. The
commonality factor expresses how much of a route is shared with other
routes. A high commonality factor indicates that a route has many edges in
common with other routes, and a low commonality factor indicates that a
route is quite independent of others. The commonality factor is then taken
into account in the route choice model in way that a high commonality factor
reduces the probability of the route to be chosen.
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