Developing empirical capacity estimation model for highway roundabouts in Bangladesh

Abstract

Transportation infrastructure plays a significant role behind economic growth of a country. Particularly, in developing countries, heavy vehicles act as the backbone in transporting goods and people via highways while sharing the same carriageway with other vehicles. However, presence of heavy vehicles in traffic stream lowers operating speed and flow in rural highways especially at the vicinity of intersections. With the increasing portion of heavy vehicles in a traffic stream, the capacity of roundabouts decreases drastically. This research aims at developing an empirical model for estimating and predicting capacity of highway roundabouts considering both geometric and traffic parameters. Six roundabout intersections along the national highways owned by Roads and Highways Department (RHD), Bangladesh have been selected as the study area, and relevant geometric and traffic data have been collected. The collected traffic flow data was extracted by using pixel-based heterogeneous traffic flow measurement technique. The geometric data was extracted by applying AutoCAD and ArcGIS. Using multivariate regression method, an empirical model for capacity estimation and prediction of roundabout has been developed as a function of entry width, circular road width, distance between entry and nearest exit, central island diameter, and circulating traffic flow. The presence of heavy vehicles in traffic stream has been incorporated in estimating and predicting capacity of roundabouts by converting all types of vehicle to passenger car equivalent units. The developed model was evaluated with observed capacity collected from the selected six roundabouts. The evaluation has shown that the model performed well with the shortlisted explanatory variables. The model was based on negative exponential with circulating flow (Qc) and positively linear with geometric parameters. In the developed empirical model, circular road widths and central island diameters were found to be more useful predictor variables compared to others used in the established models (e.g., flare length, entry radius, and entry angle). A negative exponential relationship was found between entry capacity and circular flow. The entry width and entry to the nearest exit distance of roundabouts were found to have a positive linear relationship with entry capacity. A comparative study with HCM 2016 model, TRRL model, IRC model, and German model for estimating roundabout capacity has been performed to assess the suitability of the developed model for estimating capacity of highway roundabouts. The comparison results reveal that the entry capacity of a roundabout estimated using HCM 2016, German, TRRL, and IRC model differs significantly from the observed capacity values but the estimation using the developed regression model does not differ significantly from the observed values. Hence, the developed regression model is statistically better in estimating entry capacity of highway roundabouts in Bangladesh. Microscopic simulation model was developed in a state-of-art microsimulation tool VISSIM for further investigation of the effect of independent variables. The VISSIM coded model was calibrated and validated with the field collected data of selected sites. The calibrated VISSIM model then used to exploring the developed model with different traffic flow scenario and geometric configurations. The extracted capacity from VISSIM was compared with predicted capacity of developed empirical model capacity. This analysis was important as the variability of traffic and geometric data had not been explored through analytical approach. The capacity variability within the VISSIM simulated roundabouts has proved the accuracy of the empirical model. Furthermore, a Python language-based program called PyNomo has been executed with the help of several add-on packages (e.g., numpy, scipy etc.) to generate a compound parallel scale nomograph consisting of multiple variables using the developed empirical model to aid the practitioners and engineers as a quick tool to estimate and predict roundabout capacity. Considering the impact of heavy vehicles in social and economic development of a country, the model developed in this study will be useful for policymakers and practitioners while planning and designing roundabouts in rural highways to keep in pace with the ever-increasing future traffic demand.