Nicolosi et al. [49] investigated three roundabouts: one single-lane roundabouts and two multi-lane roundabouts and measured the values of critical and follow-up headways; the method proposed by Dawson [50] and the regression method proposed by Siegloch [51] were applied.
The critical headways determined using method 2 varied between 3.4 s and 4.9 s in the right lane and 4.2 s and 5.5 s in the left lane (with a weighted mean of 4.3 s and 4.8 s, respectively), whereas the critical headways determined using method 3 varied between 3.2 s and 4.9 s in the right lane and 3.7 s and 5.5 s in the left lane (with a weighted mean of 4.2 s and 4.6 s, respectively).
Further investigations at an existing roundabout were carried out by Guo [77] which estimated the value of mean critical headway using different conventional methods (i.e. maximum likelihood method [19, 41], Raff's method [47], Ashworth's method [78]).
Table 1 Critical and follow-up headways values for single-lane roundabouts study name country estimation method applied critical headway follow-up headway mean [s] st.
Despite its simplicity in calculating the critical headway the Ashworth's method [78] assumes the exponential distribution for the headway of circulating stream and the normal distribution for the accepted gaps; this situation is often difficult to satisfy on field because traffic streams can result influenced by upstream traffic conditions or low flow rates in undersaturated conditions.
In the TCQSM, capacity factors are divided into Vehicle Characteristics, Right of Way Characteristics, Operating Characteristics, Passenger Traffic Characteristics, Street Traffic Characteristics and Methods of Headway Control (gap between two consecutive vehicles on a route) [1.3 18].
Table 2 Critical and follow-up headways values for double-lane roundabouts study name country estimation method applied entry lane critical headway follow-up headway mean [s] st.
However, the critical headway estimated by such a method represents an average value of all the observed drivers.
The minimum and maximum values of the mean critical and follow-up headways at single- and multi-lane roundabouts with their standard deviations are reported in Tables 1 and 2. Li et al. [71] also estimated the critical headway through the maximum likelihood method [19]; the assumption of log-normal distribution for the critical headway was made.
The present study, thus, creates a starting point of further initiatives aimed at establishing a robust method of modeling headways on two-lane rural highways with mixed traffic.
