Observations of traffic pairs of flow vs. density or occupancy for individual locations in freeways or arterials are usuallyscattered about an underlying curve. Recent observations from empirical data in arterial networks showed that in some cases byaggregating the highly scattered plots of flow vs. density from individual loop detectors, the scatter almost disappears and welldefinedmacroscopic relations exist between space-mean network flow and network density. Despite these findings for theexistence of well-defined relations with low scatter, these curves should not be universal. In this paper we investigate if welldefinedmacroscopic relations exist for freeway network systems, by analyzing real data from Minnesota’s freeways. We showthat freeway network systems not only have curves with high scatter, but they also exhibit hysteresis phenomena, where highernetwork flows are observed for the same average network density in the onset and lower in the offset of congestion. Themechanisms of traffic hysteresis phenomena at the network level are analyzed in this paper and they have dissimilarities to thecauses of the hysteresis phenomena at the micro/meso level. The explanation of the phenomenon is dual. The first reason is thatthere are different spatial and temporal distributions of congestion for the same level of average density. Another reason is thesynchronized occurrence of transitions from individual detectors during the offset of the peak period, with points remain beneaththe equilibrium curve. Both the hysteresis phenomenon and its causes are consistently observed for different spatial aggregationsof the network.
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