The usual criterion for differentiating between `free' and `following' vehicles is by using a critical headway. Since the degree of `following' is indicative of service quality on a highway, the critical headway is used as a means of determining the level of service. This paper presents the results of an analysis of critical headways from data collected at 58 different rural highway sites in New Zealand using ten different techniques. Three techniques were identified as being the most suitable for determining the critical headway - the mean of the relative speeds, the mean relative speed ratio and the exponential headway model. The latter is recommended as the best approach. The analyses showed that the critical headway varies between sites and, to a lesser degree, by traffic volume - the vast majority were in the 3.0 s to 4.5 s range. This is lower than what has been generally observed in other countries. (A)
Abstract