Fundamental Diagram for an Intersection: Application for a Roundabout

Abstract

A method to describe traffic flow at a road junction is the application of a special form of the fundamental diagram (FD). This intersection fundamental diagram (IFD) is an intermediate version between the traditional FD for a single road section and the macroscopic fundamental diagram (MFD) for an urban street network. The derivation of the IFD from basic definitions, together with some mathematical consequences, is described. As a result, rather simple formulas are given which allow the determination of traffic performance parameters from the results of simple traffic counting. Thus, the IFD provides a method to estimate measures of effectiveness for an intersection, such as average velocity or average loss time, based on automatic counting equipment. The derivations start with rather simplified assumptions, which then are extended to account for more complex situations. An example, applied to a roundabout, demonstrates how an IFD can be formed based on real-world data. For more comprehensive evaluations, the roundabout has been modeled by microsimulation. Thus, IFDs can be presented based on a wider sample size for varying external conditions. The results demonstrate several influencing factors for the shape of a roundabout-IFD. Here, for example, the effect of pedestrians who cross the entries and exits of the roundabout, can be demonstrated. These initial studies underline that the IFD can become a tool to assess the performance of an intersection based on simple data acquisition techniques. It can be expected that, because of further improved automated counting techniques, also based on floating cars, this approach may play a more important role in the future.