  Road network junctions, such as merges and diverges, often act as bottlenecks
that initiate and exacerbate congestion. More complex junction configurations
lead to more complex driver behaviors, resulting in aggregate congestion
patterns that are more difficult to predict and mitigate. In this paper, we
discuss diverge configurations where vehicles on some lanes can enter only one
of the downstream roads, but vehicles on other lanes can enter one of several
downstream roads. Counterintuitively, these bifurcating lanes, rather than
relieving congestion (by acting as a versatile resource that can serve either
downstream road as the demand changes), often cause enormous congestion due to
lane changing. We develop an aggregate lane--changing model for this situation
that is expressive enough to model drivers' choices and the resultant
congestion, but simple enough to easily analyze. We use a game-theoretic
framework to model the aggregate lane choice behavior of selfish vehicles as a
Wardrop equilibrium (an aggregate type of Nash equilibrium). We then establish
the existence and uniqueness of this equilibrium. We explain how our model can
be easily calibrated using simulation data or real data, and we present results
showing that our model successfully predicts the aggregate behavior that
emerges from widely-used behavioral lane-changing models. Our model's
expressiveness, ease of calibration, and accuracy may make it a useful tool for
mitigating congestion at these complex diverges.
