Use of microsimulation for examination of macroscopic fundamental diagram hysteresis patterns for hierarchical urban street networks

This study used microsimulation to analyze traffic performance on various idealized hierarchical urban street networks. Each network consisted of local streets and arterial streets, which represented the micro- and macrostructure of an urban network, respectively. Arterials were differentiated from local streets through additional green time at intersections and additional travel lanes. An idealized peak period was simulated for completely uniform demand patterns. Observed relationships between average network flow and density - known as the macroscopic fundamental diagram (MFD) - were used to compare the performance of arterial structures. Specifically, the size and shape of hysteresis loops that emerged in the MFD were used, since the networks were found to have more uniform congestion patterns during the onset of congestion than during congestion recovery. The presence of arterials was found to affect significantly the spatial distribution of congestion on the network. Arterials that passed through the congested center of the network and arterials that divided the network resulted in increased congestion inhomogeneity and larger hysteresis loops in the MFD. Arterials placed near the periphery of the network helped to attract vehicles to less-used areas of the network and reduce congestion in the center, which reduced congestion inhomogeneity. Furthermore, limited opportunities to access the arterials also contributed to dense pockets of congestion on nearby streets. For the network and demand conditions studied here, arterial ring roads appeared to distribute congestion more evenly and have had better network performance than arterial grids.