Calibration of Kimber's roundabout capacity estimation model for local conditions.
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Roundabout entry flow is influenced a number of factors namely; traffic flow characteristics, geometric parameters of the roundabout, vehicle and driver characteristics, together with, environmental conditions. The major methods for estimating roundabout entry flow capacity are based on either gap acceptance theory or on empirical relations. Roundabout geometry is the individual aspect which can be manipulated by the designers to improve the entry flow. Data has been collected from twenty-one entries of five roundabouts, where heterogeneity in traffic is observed. Seven different vehicle categories are considered such as motorized two-wheeler, car, mini bus, light commercial vehicle (LCV), heavy commercial vehicle (HCV), trailer and bus. A linear regression model is proposed to predict entry flow capacity, based on the nature of variation with individual geometric elements. Various combinations of independent variables are used to estimate entry flow capacity. The linear correlations among the geometric variables are checked. In comparison with the existing empirical model such as the TRL regression model, the calibrated linear regression model produced better estimates with a much lower RMSE value compared to that of the TRL regression model. The entry flow increases with increase in entry width, approach width, entry radius, inscribed circle diameter and is found to be negatively influenced by entry angle.