Study On Capacity Of Roundabouts Based On Gap Acceptance Theory

Roundabouts are the important nodes of urban traffic network. The vehicles operation regular and service characters at roundabouts should be learned for improving urban traffic status. As a key parameter of roundabouts, capacity’s research is helpful to take some effective measures of controlling and managing traffic.The main researches have been finished in the following aspects.(1) Study on traffic operation performances at roundabout weaving sectionsBased on the survey of Shuma Square roundabout using video cameras, some parameter performances were analyzed, which included the velocity distribution, the distance distribution of changing lane, the headway distribution of confluence vehicles and vehicles on circulating lanes, rejected headway and accepted headway. Some universal conclusions about traffic operation performances at roundabout weaving sections were drawn by analyzing the statistics regular.(2) New methods of critical gap and comparison to former methodsTwo calculation methods of critical gap are proposed including definition method (3-7), revised Raff’s method (3-17). Both definition method and revised Raff’s method use total rejected coefficient βr. The definition method is simpler than maximum likelihood method and Hewitt’s method. Revised Raff’s method improves theory of Raff’s method and calculates truly. Based on the calculation and comparison of critical gap in single-lane roundabout and multi-lane roundabout, Revised Raff’s method ensures true calculation values.(3) Assumption and preliminary verification of exponential rejected proportion functionThe exponential rejected proportion function (3-19) is proposed. Based on preliminary survey and statistics, the rejected proportion coefficient at single-lane roundabout is a fixed value under a certain traffic condition. The assumption that rejected proportion function is exponential function f(t)=e-r(t-tm) is tenable. The different rejected proportion coefficients should be derived for different type of roundabouts in field survey. The exponential rejected proportion function gives a theory function for distribution of rejected gap and accepted gap.(4) Improvement of capacity methods and comparisonBased on gap acceptance theory, the research improves the classical capacity methods and compares revised methods with former methods by use of field survey and emulation values. The revised methods can more exactly reflect entry capacity than former methods. For useing total rejected coefficient and total accepted coefficient, the assumptions should be verified in field calculation.(5) Improvement and verification of entry capacity of two-lane roundaboutHagring’s method and Xiang’s method are the major calculation methods of two-lane roundabouts. The research revised Xiang’s equation. There is hardly any difference among the equations under different circulating flow condition. The applicability of revised equation is better than Xiang’s method because the former uses fewer accumption.(6) Entry capacity of multi-lane roundabout and full capacity of roundaboutBased on Hagring’s capacity method of multi-lane roundabout, the result is credible to calculate capacity of field roundabouts. Full capacity of roundabout can be derived by computer iteration based on gap acceptance theory. It is the ending condition of iteration that all entries become saturated, and the full capacity has been verified by field roundabout.Based on the classical gap acceptance theory and some assumptions, survey method of rejected gap and accepted gap was designed. Some new methods calculating critical gap were proposed. The probability density function of rejected gap and accepted gap can be deduced by introducing exponential rejected proportion function. The relation among variables of these functions can also be obtained, and the classical capacity methods are revised. The revised methods of capacity and critical gap are compared with the former methods based on a great deal of field roundabout surveys in Dalian and Liaoyang.