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A S S I G N M E N T A D VA N C E S 41
time period was divided into 18 10-minute intervals. in different traffic routing. STA assigns more vehicles to
Three additional 10-minute intervals were used for net- freeways, VISTA assigns more vehicles to arterials and
work clearance. A total of 2.56 million vehicle trips were collectors, and the DTA approximator distributed traffic
assigned to the network. The TransCAD approximator more comparably among the roadway types. These dif-
cannot directly recognize tolls, so the delay-based tolls ferences result from the fundamental distinctions between
were added to the free-flow travel time for each link the link performance function-based approach and the
using an assumed value of travel time of $10 per vehicle capacity-constrained cell transmission model.
hour. The computation time for the three models varied, · The assessment shows that it is possible to com-
ranging from approximately 30 minutes for STA to 3 pare the results from the three different models based on
weeks for VISTA. global measures of system performance. It is more diffi-
· The most noticeable difference in the results from cult to compare the results on a link-by-link basis
the STA and the DTA approximator is in projection of because of the fundamental differences in the models.
links to be congested under static conditions. The DTA The results from both dynamic models showed much
approximator predicts a higher level of congestion. The higher congestion levels than the results from the static
STA predicted total system travel time of 1.27 million model. Additionally, the cell transmission model and
vehicle hours. The DTA approximator prediction of 2.53 link performance functions provided very different
million vehicle hours was nearly double the STA estimate. results. The suitability of each model depends on the
VISTA predicted a total system travel time of 3.09 million particular application.
vehicle hours. These results indicate that STA models
may underpredict congestion due to changes in demand
over the peak period. The three applications also resulted S. Travis Waller moderated this session.
