Exploratory analysis of signal coordination impacts on macroscopic fundamental diagram

The macroscopic fundamental diagram (MFD) of urban traffic is a recently developed tool to describe traffic on large-scale urban networks. A network's MFD can be significantly affected by various properties, including signal settings, block lengths, travel speeds, and routing behaviors. However, current understanding of the impact of signal offsets (i.e., signal coordination) on the MFD is limited to idealized one-dimensional networks, and little is known about how signal coordination affects the MFDs of more realistic two-dimensional networks. To overcome this gap, the present study examined the impacts of signal coordination on the MFD of an idealized two-dimensional grid network using both an analytical method and a microscopic traffic simulator (Aimsun). Seven coordination strategies were considered, including the simultaneous, alternating, double-alternating, one-dimensional green-wave, two-dimensional green-wave, MAXBAND, and random strategies. In general, the impacts of coordination were highly sensitive to the signal cycle length chosen. The results also revealed that poor coordination can significantly decrease the network capacity and the free-flow travel speed. However, good coordination offers little advantage over simultaneous offsets, even with directional travel demands moving in the prioritized directions. The reason is that the benefits provided to vehicles in the prioritized directions are negated by the disadvantages to vehicles traveling in the nonprioritized directions. These results suggest that the insights from other idealized simulations with simultaneous offsets may be generalizable to more realistic situations with more realistic coordination strategies.