Time gap is by far the most widely accepted measure for drivers' gap acceptance behaviours in gap acceptance studies. Nonetheless, time gap, as conventionally measured after a vehicle clears a fixed reference line in an intersection, is sometimes inadequate in representing gap acceptance behaviours, because it is measured well after a driver's decision-making time. Projected time gap (time gap 'projected' to a fixed reference line at the instance when a driver makes the crossing or merging decision), though intuitively a better measure, is beyond the capability of conventional methods. This paper, adopting CAMDAS, a Video Image Processing (VIP) software package, presents a practical approach, which is capable of extracting the distance gaps as well as the associated major-road vehicle approach speeds, and hence the projected time gaps. The relationships between time gap, distance gap, projected time gap and the major- and minor-road vehicle speeds are also established and evaluated. (a).
Abstract