VISSIM 5.30-05 User Manual

12 Dynamic Assignment
Again the sensitivity k in the exponent determines how much influence the
differences in utility have. In this formula, the relative difference in utility
determines the distribution, so that we will see only a small difference
between the 105 and 110 minute routes, whereas the 5 minute route will
receive much more volume than the 10 minute route.
Actually the Kirchhoff distribution formula can be expressed as a Logit
function, if the utility function is transformed to be logarithmic:
k ⋅ logU
j
e
k ⋅ logU
e i
k
U j
p(
R j ) = =
k
∑U
i
i ∑
i
where Cj is the general cost of route j.
12.6.3 Route Search
−k⋅logC
j
e
− k logC
e i
In VISSIM we assume that the drivers do not use only the best routes from
one parking lot to another, but that the traffic volume is distributed among a
set of available routes. Obviously, one would like to know the set of the n
best routes for each origin-destination-pair. Unfortunately there is no efficient
algorithm to simply compute the n best routes but there are algorithms to find
the single best one. To solve this problem we search for the best route for
each OD-pair in each iteration of the Dynamic Assignment. Since the traffic
situation and thus travel times change from iteration to iteration (as long as
convergence is not reached) we will find different “best” routes in the
iterations. All routes found (i.e. all routes that have qualified at least once as
a best route) are collected in an archive of routes and are known in all later
iterations. These routes are all stored in the path file (extension WEG).
The criterion for the “best” route is the general cost. That implies that for
different vehicle types different best routes can be found, because the
parameters of the general cost function are type-specific. Route search is
done at the beginning of each evaluation interval and is based on the
expected general cost for this interval computed from the preceding
iterations.
Since in the very first iteration no travel time information from preceding
simulation runs is available the cost is evaluated by replacing the travel time
with the distance (in m). Thus for the initial route search also link/connector
costs are taken into account. For every subsequent iteration the edges in the
network that have not been traveled by any vehicle have a default travel time
of only 0.1 second. This way it attracts the route search to build routes
including unused edges. This method might result in some useless routes
being found initially but by encouraging vehicles to try new paths the process
of finding new routes is speeded up. You might want to control the courage
of the vehicles to discover new routes by adding some weight to the distance
in the general cost function so that they do not try obvious detours. However,
626 VISSIM 5.30-05 © PTV AG 2011
=
∑
i