The effect of driver behaviour on motorway reliability

The motorway transport system possesses an internal control mechanism that governs the stability of traffic flow in the presence of vehicle speed fluctuations. When traffic density exceeds a limit value, the control mechanism enters into crisis and flow becomes unstable. This paper describes an analysis of the manner in which individual driver decisions influence the onset of this crisis. A simulation procedure was employed to examine the behaviour of traffic on a motorway carriageway in correspondence of several driver choices. Analysis of the results of this simulation and comparison with data obtained from experimental measurements made in real traffic situations showed that the reliability of a motorway, and hence its transport capacity and safety, depends to a great extent on driver behaviour. It is thus possible to improve motorway function by imposing speed limits when traffic approaches conditions of instability.