Two experiments were carried out in a driving simulator in order to study driving behaviour in reduced visibility conditions. Both studies show that drivers maintain larger time headways when preview is limited. The results suggest that the adaptation of time headway was related to difficulty of anticipation rather than perceptual degradation due to reduced visibility. Drivers who were instructed to drive on a fixed time schedule did not increase their time headway in adverse visibility circumstances. However, they reacted very accurately to decelerations of lead cars, which prevented the occurrence of critical situations. In general, the results indicate that drivers apply a hierarchy of adaptive strategies which are aimed at the control of time pressure in driving. In normal visibility conditions, drivers adopt an anticipatory driving strategy. When the possibilities for anticipation are reduced, drivers compensate by means of speed reductions and increases in time headway, in order to increase the time available to react to potential threats. When this compensatory strategy is impossible or undesirable, drivers have to maintain high alertness in order to react accurately to unpredictable hazardous events. This strategy imposes a high time pressure on the driver. Presumably, drivers try to restrict time pressure and therefore the effort costs of driving when they have the opportunity to do so.
Abstract