The control of motorway reliability

This paper presents an analysis of the instability phenomenon on motorways, with the aim of arriving at the definition of a control strategy suitable for keeping the flow stable. By using some results of the motorway reliability theory, a relationship between reliability and some flow characteristics is obtained, which shows the existence of a reliability threshold critical for flow stability. The macroscopic flow characteristics corresponding to this threshold are very different in different situations, so that the control of flow stability requires the analysis of speed and time gap microscopic processes surveyed on a cross section of the motorway carriageways to be controlled. A method is presented, based on the analysis in real time of these processes, by which it is possible to detect the approach of instability before its effects become manifest, and to single out the proper control strategy in different situations.