Roundabout Model Calibration Issues and a Case Study

Abstract

This paper discusses issues related to calibration of models for analysing roundabout capacity and performance. Two basic calibration methods that can be used for gap-acceptance and linear regression methods are described. A case study is presented to compare capacity estimates from the gap-acceptance and linear-regression methods, including a calibration example. A traffic model framework is presented to help with assessment of traffic models in a general framework, considering all aspects of models relevant to roundabout operation. While the discussion focuses on analytical models, the issues raised are also relevant to microsimulation models. Discussion on roundabout models should not concentrate on capacity alone, and instead, modelling requirements for estimating both capacity and performance (delay, queue length, etc) should be considered together. Various aspects of field observations relevant to the calibration effort are discussed. These include issues related to the definition and measurement of capacity, delay and queue length, including the effect of unequal lane utilization. Delay criteria for level of service definition are also discussed. Further aspects of model calibration discussed include the environment factor, adjustment for the arrival flow / circulating flow ratio, lane utilization factor, heavy vehicle factor, driver response time and calibration of models for operating cost, emissions and fuel consumption.