Capacity of 2-lane Roundabouts

af Poul Greibe og Belinda la Cour Lund


The number of roundabouts has increased significantly in Denmark over the last two decades. The majority of roundabouts are single-lane roundabouts (used in rural and urban areas), but due to increasing traffic volumes, more and more roundabouts in rural areas are now being built as 2-lane roundabouts.

In Denmark, the capacity of roundabouts is traditionally based on gap acceptance theory and a number of previous studies have measured critical gap and follow up times for varies types of roundabouts in order to set up a general capacity model to be used in Denmark.

This presentation will mainly focus on 2-lane roundabouts and will be based on a new study from 2-lane roundabouts (mainly with “turbo design”) that was completed in 2008. The results from the study include new values for critical gap and follow up times to be used in 2-lane roundabouts. The study also re-estimates the values for passenger car units (pcu) for two different categories of heavy vehicles. The new pcu-values indicate that capacity of roundabouts is more influenced by heavy vehicles than previously expected. The study also estimates the effect on capacity from exiting vehicles (vehicles leaving the roundabout at the adjacent arm). It is well known, that high traffic volumes of exiting vehicles reduce the capacity of entering vehicles. The results indicate that the capacity is reduced by up to 20% in situations with high traffic volumes of exiting vehicles. Empirical data from observed capacity at 2-lane roundabouts have been used to test different theoretical and empirical capacity models. In general, the theoretical capacity models seem to have certain limitations when it comes to describing the observed data. Methods to overcome these limitations are sought and will be presented.

Finally, an analysis of drivers lane use when entering roundabouts has been examined. The majority of entering drivers use the right lane (outer lane), even at high traffic volumes. It is assumed that better signing and marking might lead to a more balanced use of the entrance lanes, which could lead to higher capacity.

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