Car-following is defined as being that phenomenon in which a vehicle follows a lead vehicle which is travelling at an arbitrary speed. If the velocity of the lead vehicle which is travelling at an arbitrary speed. If the velocity of the lead vehicle is designated by V1 and the rear vehicle velocity by V2 then the relative velocity v is defined as the difference V1-V2. It is the threshold of this velocity that has been investigated. Here "velocity threshold" is defined as that relative velocity which the driver of the rear vehicle can detect with a 50 per cent probability at a given headway for a given for presentation time. The headway is taken as the distance from the drivers eyes to the rear bumper of the lead vehicle. This study is concerned with the determination of velocity thresholds under night driving conditions, and is basted on the premise that the information available to the driver of the following car concerning the state of the lead car, is primarily provided by the taillights of the lead car. When a relative velocity exists between the two cars, this visual information appears as a change in the visual angle suspended by the two taillights of the lead car and apparent changes in the brightness and area of the taillights. For the presentation times of relative velocity used in this study, the changes in headway were so small, that it was assumed taillight brightness and area could be taken as fixated. Hence, velocity thresholds were obtained by considering only the change in visual angle. This, of course, may not be the only cue of consequence in detecting relative velocities, but it is almost certainly a major one.
Abstract