It has recently been demonstrated, both theoretically and experimentally, that the average flow and density of some urban traffic networks is related by a unique, reproducible curve known as the Macroscopic Fundamental Diagram (MFD). For networks in which vehicles cannot adaptively re-route to avoid congestion, this relationship is robust only when there are few vehicles on the network. As these types of networks become more crowded, trips are completed at a much lower rate than predicted by MFD theory. Thus, operating a network in congestion is extremely damaging on these networks. The...