The other major assumption in traffic assignment concerns the manner of representing
traffic flow and conditions—that is, the way that travelers’ route choices are related to
networkwide congestion and travel times. Historically, traffic assignment methods focused on
representing average or steady-state conditions over an analysis time period that was long
compared to the time scale of traffic dynamics. In these problems, travel times and volumes on
links and routes can be considered to be constant over the analysis period, meaning there is no
need to account for their variations over time. Within this approach, the relationship between the
average traffic level on a particular network facility and its average travel time is represented by
a VDF, which typically is either a closed-form mathematical function or sometimes a user specified piecewise linear curve.