Determinations of critical gap and follow-up time at roundabouts in Jordan

Critical gap and follow-up times are two important fundamental traffic parameters used in the design and operational analysis of roundabouts and un-signalized intersections. Moreover, they are used to assess capacity and level of service at minor roads. In Jordan, roundabouts are used extensively, and therefore, there is a need to conduct studies on follow-up and critical gap time. The driver behaviour in Jordan can be described as aggressive in terms of their choice of follow-up headway at roundabouts. In this study, the roundabouts were located in mountainous terrain which allowed the collection of critical gap observations for approach slopes ranging from 6% up to 8%. The critical gap and follow-up times were recorded and analyzed. This research also identified other factors affecting critical gap, lag and follow-up times such as geometry of roundabout, slope of the approach, culture and behaviour of drivers. This research also proposes a significant relationship between approach slope at roundabouts and the observed gap. This relationship may be incorporated in the estimation of capacity and level of service determination at roundabouts. The results of this study should assist highway and traffic designers in the design and the performance evaluation of roundabouts.