Data from two portions of congested freeway have unveiled the mechanisms that cause traffic oscillations (i.e., stop-and-go driving conditions) to form and grow in amplitude over space. Oscillations formed due to driver lane-changing maneuvers. Vehicles that inserted themselves into relatively small spacings in adjacent (target) lanes triggered these formations by inducing temporary decelerations among vehicles immediately upstream. Once they formed, this same lane-changing mechanism triggered oscillation growth. And this mechanism perturbed traffic states in such ways as to extend the oscillating periods. Periods consequently reached several minutes in duration.; Of further note, oscillations often diminished in amplitude when they propagated past merge areas that had become fully engulfed in queues. The oscillations growth caused by lane-changing was countered by the inflows from queued on-ramps. This effect is explained here with a theory that applies both to the merging and the diverging maneuvers that occur near ramps. The theory was validated using observations from two freeway merges.; The findings thus indicate that readily measurable features of oscillatory flow are triggered by vehicular interactions between travel lanes. This contrasts the widely held belief that oscillations arise spontaneously in traffic due to driver car-following behavior. Such behavior turned out instead to play only very limited roles in this regard. These limited effects, moreover, were themselves observed to be triggered by the above-noted vehicle interactions between lanes.
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