A study into longitudinal driving behaviour on motorways in fog was carried out, based on an analysis of real traffic data. Traffic data were available on an individual vehicle level for a period of more than one year. They were obtained from inductive loop detectors on a two-lane motorway near the Dutch city of Breda. These data were combined with detailed visibility measurements from a sensor near the road. The analysis was mainly restricted to daytime fog periods. The results show that drivers reduce their free driving speed in fog, but not sufficiently to avoid a collision when they would be confronted with a stationary or much slower lead vehicle. The most critical behaviour was displayed in the visibility range between about 40 and 100 m. Following distances were lower in fog. However, this was mainly caused by the reduced speeds. An extra fog effect on following distance or headway has not been established. Time-To-Collisions (TTC's) were seen to increase in fog. This shows that speed differences between subsequent vehicles are reduced. (A) See also IRRD 898628.
Abstract