An Examination of the Geometric Model on the Capacity Prediction of Two-Lane Entry Roundabouts
by Mark T. Johnson, P.E., and Ting-Li Lin, Ph.D.
Abstract
Roundabout capacity is primarily estimated by gap-acceptance or by geometric models. The 2016 Highway Capacity Manual (HCM6) implements a form of the gap-acceptance model developed by Siegloch in 1973. The HCM7 update retains the HCM6 roundabout equations. A geometric capacity model was developed in the UK by Kimber and Hollis in 1980.
In 2012 capacity data was collected as part of the FHWA project, Assessment of Roundabout Capacity Models for the Highway Capacity Manual. This data was used to produce HCM6, an update of the HCM 2010. In HCM6, a capacity curve was fitted through all the capacity data for both single- and multi-lane roundabouts. This paper’s research was applied to two-lane data to ascertain if the geometric model may help explain the wide scatter in the multi-lane data and improve capacity estimation.
investigate this hypothesis, we conducted statistical analysis on data from different geographical areas. A capacity line was derived using the geometric capacity model and the HCM6 model with global anchor and local calibration for comparison with the capacity data.
The results showed that the uncalibrated geometric model gave a better fit to local data. This indicates that the between-site difference in driver behavior is primarily due to the between-site differences in geometry. Consequently, when roundabouts are grouped by their geometry the wide data scatter is significantly reduced.
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