Differences among route flow solutions for the user-equilibrium traffic assignment problem

User-equilibrium (UE) traffic assignment is widely used in travel forecasting models. Analyses of assignment results often require detailed information about route flows. For a given UE model, total link flows are uniquely determined, but there can be many route flow solutions. Our goal is to evaluate the potential disagreement between different route flow solutions for the same UE model. We considered a case study of a UE model for the Chicago region. We examined 1,000 different UE route flow solutions. Out of 127,248 routes used by UE solutions, 53,134 have alternatives. For 51,748 routes (97%) the flow is completely undetermined, meaning that it is zero in one UE solution and equal to the total origin-destination (O-D) flow in another UE solution. Significant differences were also found in the distribution of users of a given link among O-D pairs ("select link analysis"). We conclude that in analyses that require route flows, it is important to make a proper choice of a specific route flow solution, such as the maximum entropy user equilibrium route flow solution.