Vasconcelos, LuísSilva, Ana BastosSeco, ÁlvaroSilva, João2025-11-102025-11-102012-12-13Vasconcelos, L., Silva, A. B., Seco, Álvaro, & Silva, J. (2012). Estimating the Parameters of Cowan’s M3 Headway Distribution for Roundabout Capacity Analyses. The Baltic Journal of Road and Bridge Engineering, 7(4), 261-268. https://doi.org/10.3846/bjrbe.2012.351822-427X1822-4288http://hdl.handle.net/10400.8/14579Capacity models based on the gap acceptance theory are widely used in unsignalised intersections and roundabout capacity analysis. !ese models are based on the statistical distribution of major vehicle headways. In this "eld, Cowan’s M3 distribution is usually recognized as the most adequate, but the estimation of its parameters is not trivial. In this paper, the main estimation methods are reviewed and a new method (Simultaneous Numerical Estimation – SNE) is proposed. !e SNE method was used to develop a calibrated relation between parameters, using "eld data collected in Portuguese roads and roundabouts. It was determined that the new formula improves capacity estimates, either in one-lane or in two-lane roundabouts. !e paper also addresses the importance of each input variable and parameter in the resulting capacity model, through a sensitivity analyses.engRoundaboutCapacityCowan M3Statistical distributionSensitivity analysesHeadwayjournal article10.3846/bjrbe.2012.35