The effects of reduced visibility and time pressure on drivers' distance-keeping behaviour

Preview allows drivers to build expectations and to anticipate future traffic circumstances and possible threats. In reduced visibility conditions, the span of preview is decreased and therefore uncertainty about the presence and behaviour of other traffic participants is increased. Drivers can compensate for this increased uncertainty by reducing speed and maintaining a larger headway, thereby giving themselves more time to react to hazards. However, if maintaining speed has a high priority, adaptation of speed and headway may diminish. Distance keeping behaviour in fog was studied in a driving simulator. The instruction to drive on a fixed time schedule was expected to reduce adaptation of driving behaviour in fog. In general, the results indicate that drivers increase headway when the possibilities for anticipation are reduced, thereby increasing the time available to react to decelerations of the lead car. If this compensatory strategy is not desired, drivers have to maintain high alertness in order to react adequately to unexpected decelerations of the lead car. Presumably, this is an effort-demanding strategy that imposes a high time-stress on the driver.