The Effect of Traffic Signals on the Macroscopic Fundamental Diagram

Since the recent empirical evidence of the existence of the macroscopic fundamental diagram (MFD), there are already numerous applications for it, ranging from traffic management to traffic flow modelling. However, little is known what effect internal network control has on the shape of the MFD. This research will investigate the shape of an MFD on a regular network with and without traffic lights. To this end, we consider a regular grid network of infinite length. This is represented in a microscopic traffic simulation model as a two-ring network. We compare the situation without traffic lights to the situation with traffic lights with fixed timing. The uncontrolled case shows a higher flow for lower densities, while the controlled case shows a higher flow for higher densities. Analysis of the underlying process shows that this is due to the fact that traffic lights keep the vehicles spread more homogeneously over a network. In contrast, uncontrolled intersections result in an unstable situation where one part of the network becomes fully congested and the other part almost empty. This shows that traffic lights are reducing the performance for the low-density situations, but improving the traffic performance in high-density situation. In particular, the stability of a homogeneous spatial traffic distribution can be improved, even with fixed traffic light settings.