VISSIM 5.30-05 User Manual

12 Dynamic Assignment
12.5 Simulated Travel Time and General Cost
12.5.1 Simulation Period and Evaluation Interval
The microscopic simulation of the traffic flow is used during the Dynamic
Assignment to determine travel times in the network. This travel time
measurement is performed per edge and per evaluation interval.
In Dynamic Assignment, as opposed to static assignment, travel demand
and network infrastructure are not assumed to be constant in time. Therefore
the traffic situation and as a result the travel times will change during the
assignment time period. To cover these changes the total simulation period
is divided in smaller evaluation intervals in which travel times are observed
separately. The appropriate size of the evaluation interval depends on the
dynamics of the travel demand. The evaluation interval should be smaller
than the interval in which the demand changes.
Example: If you have OD matrices with intervals of 1 hour, the evaluation
interval should not be longer than half an hour.
As a rule of thumb, evaluation should have at least the double temporal
resolution of the demand changes.
On the other side, an evaluation interval below five minutes does not make
sense because the fluctuation of the values will increase with smaller
intervals. Especially when signal controls are used the evaluation interval
must be significantly longer than the cycle times used.
In most cases, evaluation intervals ranging from 5 to 30 minutes will be
appropriate.
12.5.2 Simulated Travel Times
During a simulation, travel times are measured for each edge in the network.
All vehicles that leave the edge report the time they have spent on the edge.
All travel times during one evaluation interval are averaged and thus form the
resulting travel time for that edge. There is a special treatment of vehicles
that spend more than one evaluation period on an edge, e.g. during
congestion. They report their dwell time as well although they have not left
the edge. That is necessary to get information about heavily congested links
even if there is - because of congestion - no vehicle able to leave.
The travel time measured in the current iteration n is actually not used
directly for route search and route choice in the same iteration. Instead it
influences next iterations. This behavior is sensible since we normally do not
want the travel times during 8 a.m. and 9 a.m. on Tuesday to influence route
choice for the time from 9 a.m. to 10.a.m. the same day but rather to
influence the same period, i.e. 8 to 9 on the next day.
620 VISSIM 5.30-05 © PTV AG 2011