The fundamental diagram : a macroscopic traffic flow model. Paper and shortened version presented to the OECD Symposium on Methods for determining geometric road design standards 1976, Helsingor, Denmark, 10-12 May 1976.

In the theory and models of traffic-flow the emphasis lies on the interaction between vehicles, based on the characteristics of the road, the drivers and vehicles, and not on the interaction between a single vehicle and the road. Three levels of models, arranged in the order of increasing detail can be distinguished: macroscopic, mesoscopic and microscopic, which are concerned with mean values of traffic-flow characteristics, distribution of traffic-flow characteristics around their mean values and individual vehicle trajectories. To illustrate this, the traffic-flow characteristic speed will be considered: on macroscopic level it is the mean speed, for example, of vehicles passing a given point in a given period of time; on mesoscopic level, this characteristic indicates the speed distribution, while on microscopic level, it means the speed profiles of a number of vehicles over a road section. Before discussing the fundamental diagram - a macroscopic model - information will be given concerning the significance of the traffic-flow theory in general