The standard method for predicting traffic flows on urban road networks, static user-equilibrium traffic assignment, is based on the principle that drivers seek their own least cost routes from their origins to their destinations. This principle corresponds to a user-equilibrium state in which all used routes have equal costs and no unused route has a lower cost for every origin-destination pair. This problem can be formulated as a convex optimization problem with linear constraints, and solved with an iterative algorithm. Although the total flows on links of the urban road network are uniquely determined by this formulation, multi-class link and route flows are not. An additional assumption, called the condition of proportionality, may be imposed to determine these flows uniquely. This condition was the basis for incorporating a post-processing adjustment into TransCAD and Visum. Analyses of class link and route flows from Visum assignments of cars and trucks to the Chicago regional network, with and without the condition of proportionality, are presented. Differences in class link flows with and without the condition of proportionality range from fairly large (+/- 300 vph) to zero. The largest absolute differences in link flows occur on links with small flows, which generally are arterial links. The largest relative differences pertain to links with class flows less than 2,000 vph. Differences in O-D flows over a pair of alternative route segments are then examined. The class flows without proportionality may be characterized as ‘all-or-nothing.’ Finally, select link analyses were performed to compare class O-D flows with and without proportionality, and observed to be substantially different. Whether the differences found are large enough to warrant concern in applications is a matter for practitioners to judge. The imposition of the condition of proportionality has the potential to improve the validity of traffic assignments applied in transportation planning practice by determining route flows and multi-class link flows uniquely. To be assured that class link flows are uniquely determined, especially in the case of special purpose facilities such as truck routes, however, practitioners would seem to be well-advised to consider imposing proportionality on their multi-class traffic assignments.