As noted in Section 7.3.1, a platoon, as used by Australasian traffic professionals, is a group of vehicles resulting from an interruption to traffic flow, such as the signal aspect turning red at a signalised intersection or crossing. When the green signal aspect appears, the vehicles that had been stopped move off in a group that we call a platoon.
If all the vehicles travelled at the same speed, the platoon would stay together as a tight group, as it moved along the road. In practice, however, there will be a distribution of speeds and the platoon will spread out, with faster vehicles moving further and further ahead of the centre of the group and slower vehicles dropping further and further behind. This spreading process is known as platoon dispersion and it is of interest because of its effect on efficiency of traffic flow, particularly in relation to coordination of traffic signals along a route or across a network. If dispersion did not occur and if downstream traffic signals were appropriately set, it would be possible for a much greater proportion of the platoon to move through the downstream signals without stopping than would be the case with the platoon dispersed.
Because of its importance to traffic management and control, there has been considerable research into platoon dispersion – see, for example, the historical review in Denney (1989). These studies have identified three principal ways of representing the dispersion process: in terms of kinematic wave theory, using diffusion theory and through recurrence modelling. These different approaches are discussed briefly in the following three sections.