Phase diagram hints at traffic solution
May 28, 1999
Various types of traffic jams can be explained by modelling traffic flow as a gas according to group of European physicists. Dirk Helbing, Ansgar Hennecke and Martin Treiber from the University of Stuttgart in Germany and Eotvos University in Budapest found that the equations that explain the kinetics of gases can also model stop-and-go waves, traffic jams and other traffic patterns seen on motorways (Phys. Rev. Lett. 82 4360). Transitions between the different "phases" of the traffic can be triggered by cars joining the motorway at intersections or other disturbances such as a change in the number of lanes.
Helbing and co-workers have identified six different traffic phases: homogeneous congestion, oscillatory congestion, triggered stop-and-go traffic, moving localised clusters, pinned localised clusters, and free traffic. One axis of their phase diagram represents the flow of vehicles already on the motorway, while the other represents vehicles joining the motorway. Transitions between the states are triggered by localised event - such as a convoy of slow moving lorries passing the intersection. The boundaries between the different phases are related to various properties of the traffic flow such as the density of vehicles.
According to group's simulations, the occurrences of traffic jams can be reduced by adjusting various properties of the intersections where vehicles join the motorway.