Estimating critical gap of roundabouts by different methods

Critical gap is an important parameter which affects the capacity and delay of unsignalized intersections. There are different critical gap values for drivers in different geometry parameters and traffic conditions. Critical gap is generally considered to follow Erlang distribution, lognormal distribution, etc. Various methods calculating critical gap of roundabouts have been presented. Many studies on gap acceptance at roundabouts have been carried out in America, Australia and some countries in Europe; whereas the same works have rarely been done in China. This paper gives an overview of many methods calculating critical gap. On the base of video survey of Shuma Square roundabout in Dalian, we acquired a large number of rejected gaps and accepted gaps in rush hour. Critical gap can be calculated by use of different methods including Ashworth´s method, Raff´s method and the maximum likelihood method (MLM). As a result, the maximum likelihood method and the revised Raffs method can be recommended for practical application, and Ashworth´s method will result in larger result because it only uses values of accepted gap.