Some Observed Features of Freeway Traffic Oscillations

Abstract

Some recent published works on freeway traffic have focused on congested traffic conditions in which oscillations form and propagate upstream through vehicle queues. Some features of oscillating waves were examined through empirical evidence. The data measured on multiple days along an approximately 3.5-km three-lane freeway in Istanbul, Turkey, present pronounced features of oscillations, such as their formation, propagation, and growth. It was identified that oscillations form immediately upstream of a bottleneck, they propagate at certain speeds against the direction of traffic flow, and their amplitude grows while propagating. In reports published earlier, it was stated that oscillating waves propagate backward at nearly constant speed with no dependence on flow. It has been observed in this study that propagation speeds cluster into two groups with averages of 21.29 and 15.52 km/h at the downstream and upstream study segments, respectively. It was shown that any difference in propagation speeds may be attributable to the features of the segment studied and, in turn, to the slope of the congested part of the associated fundamental diagram. A theory on reduction of oscillation amplitude at merge sites proposed in the literature was tested, and predictions of the theory were validated with some acceptable error.