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This paper examines the behavior of multi-lane freeway traffic past on-ramps, building on the continuum model of part I and
focusing on the onset of congestion. The main complication with merges is that rabbits (fast vehicles) entering from an
on-ramp usually stay on the shoulder lane(s) of the freeway for some distance before merging into the fast lane(s). An
idealization is proposed where this distance is taken to be the same for all vehicles. As a result, the system behaves as if
there was a fixed buffer zone where entering rabbits cannot change lanes. The model of part I is extended to capture the
peculiarities of traffic within such a buffer zone, including its two end-points: the "entrance" and the "merge". The onset
of congestion is described by means of waves. The paper shows that this highly idealized model (and its more realistic
cousins) explain qualitatively all the puzzling facts discussed in part I without introducing obviously unreasonable
phenomena.
A typical sequence of events during the onset of congestion is predicted to be as follows: (i) increased on-ramp flows and the
ensuing merging maneuvers into the passing lane generate a (fast-moving) queue at the merge that grows on the passing lane(s)
of the buffer; (ii) when the speed in this queue drops past a critical level (vf) a 1-pipe queue forms at the merge, which
then grows upstream across all lanes; (iii) if the front of this queue moves slowly forward, as per the model of part I, then
the lane-flows at the merge point would be at "capacity" from then on (with roughly the same speed across all lanes) but
downstream of the front there would be a "discharge state" with less total flow. Therefore, an observer downstream of the
merge would see this discharge state before the capacity state, and would record a drop in flow followed by a recovery. If the
front of the queue would move slowly backward, then the sequence of events following (ii) is somewhat different, as described
in the paper.
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