The Effect of Network Structure and Signal Settings on the Macroscopic Fundamental Diagram

Abstract

Recently it has been proposed that the performance of a complete network can be represented graphically using only aggregated data for flow and density. The resulting graph is the macroscopic fundamental diagram (MFD), which relates the average flow (performance) to the number of vehicles in the network (accumulation). The resulting shape is often concave, meaning that the performance can be maximised if a fixed number of vehicles is maintained in the network (optimal accumulation). One of the most promising methods to achieve this, is by adapting signal timings. The objective of this thesis is to investigate if (1) a relationship between the shape of the MFD and the structure of the underlying network exists and on which factors this depends and (2) if the shape of the MFD of a subnetwork (neighbourhood) and its perimeter (ring of higher level roads (with signalised intersections) around a subnetwork) are affected by different signal timings and how this does affect the applicability of the MFD for control strategies. To assess the effect of the structure on the network, a tool has been developed to automatically generate fictitious networks with different structures, in which the amount of infrastructure and signal timings are matched dynamically to a randomly generated OD-matrix. The resulting networks are used as a base for the simulations, which are done in the microscopic traffic simulator VISSIM. Using various signal timings for signals located on the perimeter, the flow from the subnetwork to the perimeter is restricted and vice-versa, in order to assess how the MFD of both react to these changes. Regarding the relation between the shape of the MFD and the network structure, it is concluded that the structure of a network in itself does not have a strong influence on the shape of the MFD. Differences between MFDs are not caused by topological differences, but by the different characteristics of the roads (length, speed, capacity) and the amount and type of intersections in the underlying network. The different factors influencing the shape of the MFD could not be quantified – in order to construct MFDs based only on knowledge of the network and without the use of simulations - as the shape of the MFD is found to differ strongly, even for similar networks. As such it is concluded that the stochastic and dynamic nature of traffic has a strong influence on the shape of the MFD. Especially the accumulation is highly sensitive to differences in the distribution of traffic over the network, making the optimal accumulation particularly hard to predict. Regarding the effect of signal settings on the shape of the MFD of a subnetwork and its perimeter and its applicability for control strategies, it is concluded that a strong relation between the MFD of the subnetwork and its perimeter does exist, in which both react in the same way to changes in traffic demand and signal timings. The ratio between the performance of the subnetwork and perimeter is highly consistent and not strongly affected by changes in the signal settings, implying that signal timings exist, which optimise the performance of both the subnetwork and its perimeter. The optimal accumulation of the perimeter is found to be highly sensitive to changes in the signal timings. As a consequence it is concluded that the MFD is difficult to use for control strategies aiming to adapt signal timings to maintain the optimal accumulation in a part of the network, because these changed signal timings result in a different optimal accumulation.Transport and PlanningTransport & PlanningCivil Engineering and Geoscience