Assigning user class link flows uniquely

Static user-equilibrium traffic assignment, the standard method for predicting traffic flows on urban road networks, is based on the principle that drivers choose their own least cost routes from their origins to their destinations. This principle may be formally described by user-equilibrium conditions, which correspond to the optimal solution to a nonlinear minimization problem. Although the total flows on links of the road network are uniquely determined by this formulation, multiple user-class link flows, as well as route flows, are not. Nevertheless, professionals frequently use class link flows output by their software for project evaluation, and class route flows to perform select link analyses. These applications may result in misleading findings, for example, in scenario analyses. An additional assumption may be imposed to determine these user class link flows and route flows uniquely. A possible assumption is the condition of proportionality, namely that the proportion of vehicles on each of two alternative, equal-cost segments (sequences of links) should be the same regardless of their user class, origin or destination. This paper compares two assignments in which the order of the car and truck trip matrices was specified as car followed by truck, and truck followed by car. Although the total link flows for the two assignments are effectively equal, substantial differences exist between the user class link flows determined by the different orderings of the matrices. Post-processing to impose the condition of proportionality on the class O–D flows removes about 90% of these differences. Analyses of class link flows of cars and trucks before and after imposing the condition of proportionality for one of these assignments reveal that about half of all links experience differences in user class flows, ranging up to ±300 vph. These findings offer insights into the magnitude of the differences arising from the non-uniqueness of class link flows, and identify which links are subject to differences. Imposing the condition of proportionality in multi-class traffic assignments is recommended in travel forecasting practice.


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  • Accession Number: 01493590
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Sep 20 2013 4:31PM