Once trips have been split into highway and transit trips, the specific path that they use to travel from their origin to their destination must be found. These trips are then assigned to that path in the step called traffic assignment. The process first involves the calculation of the shortest path from each origin to all destinations (usually the minimum time path is used). Trips for each O-D pair are then assigned to the links in the minimum path and the trips are added up for each link. The assigned trip volume is then compared to the capacity of the link to see if it is congested. If a link is congested the speed on the link needs to be reduced to result in a longer travel time on that link. Changes in travel times mean that the shortest path may change. Hence the whole process is repeated several times (iterated) until there are equilibrium between travel demand and travel
supply. Trips on congested links will be shifted to uncontested links until this equilibrium,
condition occurs. Traffic assignment is the most complex calculation in the travel modeling
sequence and there are a variety of ways in which it is done to keep computer time to a minimum
[3].