Capacity of Large Dual and Triple-Lanes Roundabouts During Heavy Demand Conditions

摘要

A multivariate model for the prediction of roundabout maximum entry flow during forced flow condition is developed considering the circulating flow, exiting flow and roundabout geometric characteristics. None of the current available methods are specifically developed for forced flow conditions. The data were collected from 13 large roundabouts. Beside circulating and exiting flows, number of lanes and lateral position of the vehicles, as they approach and cross the roundabout, showed significant influence on roundabout entry capacity. The developed model matched the field data reasonably well and fall well in between other methods. Substantial differences in capacity estimates were observed between various considered methods. Such differences make the judgment of accepting or rejecting capacity estimates in the region very difficult. They also lead to confusions to practicing engineers, officials, consultants and contractors. Traffic assignments and gating policies might be affected by improper assessment of capacities. Capacities estimated through the two most famous methods, UK and aaSIDRA, showed poor match with field data during forced flow conditions and with those estimated through the developed model. It is quite essential to recalibrate such methods to meet regional needs. Non-calibrated existing models, such as aaSIDRA, RODEL and French methods, did not show better capacity estimates than those much simpler ones, such as HCM, FHWA or Bahrain models. This indicates that it is essential to calibrate models for local conditions. This paper describes a new model for capacity estimation of roundabouts derived under heavy demand conditions at large roundabouts.