The distribution of speeds and gaps in traffic systems

This paper has introduced some free speed and headway distributions that can be applied to represent the flow of vehicles in traffic system. It has described a series of theoretical models for speeds and headways, and has compared these models with observed data. A fundamental conclusion is that different models must be applied to flows under different flow regimes, and that these regimes may be defined in terms of the flow concentration diagram for a road. Traffic flow depends greatly on vehicle characteristics and driver behaviour, and governments are continually trying to improve vehicle and driver 'performance' through regulation and legislation. This paper has indicated a number of ways in which observed traffic behaviour has changed in recent years (eg, changes in the form of free speed distributions). These changes will be reflected in a continuing need to reconsider theoretical models of traffic flow to ensure their relevance.