Under All-or-Nothing Assignment, all traffic flows between O-D pairs are assigned to the shortest paths connecting the origins and destinations. This model is unrealistic in that only one path between every O-D pair is used, even if there is another path with the same or nearly the same travel time or cost. Also, traffic on links is assigned without considering whether or not there is adequate capacity or heavy congestion travel time is a fixed input and does not vary depending on the congestion on a link.
The following are traffic assignment methods encountered in transportation planning practice, all of which are available in TransCAD:
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