Abstract
Capacity models based on the gap acceptance theory are widely used in unsignalised intersections and roundabout capacity analysis. These models are based on the statistical distribution of major vehicle headways. In this field, 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. The SNE method was used to develop a calibrated relation between parameters, using field 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. The paper also addresses the importance of each input variable and parameter in the resulting capacity model, through a sensitivity analyses.
Highlights
IntroductionAs in many other countries, the use of roundabouts has spread throughout the country, namely to solve intersections safety and/or capacity problems (Antov et al 2009; Mauro, Branco 2010)
The vast majority of intersections in Portugal are un-signalized
This paper addresses the application of gap-acceptance methods in roundabout capacity analyses
Summary
As in many other countries, the use of roundabouts has spread throughout the country, namely to solve intersections safety and/or capacity problems (Antov et al 2009; Mauro, Branco 2010). Developing highly accurate capacity models is very important. In the scope of capacity analyses, is classified in two main groups: empirical (regression analysis) and stochastic (gap acceptance theory). Current Portuguese practice relies mainly on gap acceptance models to analyze conventional at-level intersections and on regression models to analyze roundabouts. This paper addresses the application of gap-acceptance methods in roundabout capacity analyses. Within this approach each roundabout entry is treated as a normal T-intersection which allows the direct application of general capacity formulas
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