In a car-following experiment in a fixed-based driving simulator the relationship between velocity of the lead vehicle and headway was studied. 54 subjects completed the experiment. The experiment consisted of two parts. In one part of the experiment the velocity of the lead vehicle was manipulated. Subjects were presented with lead vehicles driving with a constant velocity of 40, 50, 60 or 70 km/h. Distance headway increased as a function of velocity of lead vehicles. Time headway, however, was constant over all velocities of lead vehicles. Time headway appeared to be very stable within the subjects over all speed conditions. Also in the second part of the experiment, the lowest distance to the lead vehicle after deceleration of the lead vehicle, was strongly related to the time headways as found in the first part of the experiment. There was a relation between time headway and the average deceleration. Subjects with a smaller time headway started to decelerate at a shorter distance to the lead vehicle and decelerated stronger than subjects with a larger time headway. Also, the deceleration was stronger when the initial speed difference was larger. In a second part of the experiment the urgency of the situation was manipulated by letting the lead vehicle decelerate from different initial velocities to different lower velocities. Brake reaction time was not related to time headway. It was however strongly related to the urgency of the situation. It was concluded that brake reaction time in this experiment was not a reflection of the ability of subjects to react fast, but of the necessity to react fast. Subjects with a smaller time headway braked harder than subjects with a larger time headway. This effect was completely determined by the larger velocity of subjects with smaller time headway at the moment the lead vehicle started to decelerate. Subjects with larger time headways already had decreased their velocity to a greater extent at that stage. This confirmed the results of the first part of the experiment. Time headway was not related to age, driving experience or sensation seeking. The lack of an age effect was attributed to the limited range of age in this experiment. The lack of an effect of sensation seeking on THW was attributed to the number of subjects and the type of instructions given to the subjects with respect to the driving task.
Samenvatting