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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.

Mark Johnson is the Principal Engineer for MTJ Roundabout Engineering. Mark founded MTJ in 2005, after 12 years of public- and private-sector transportation engineering positions, including four years for the City of Loveland, Colorado, and four years at the Wisconsin DOT where he was a key contributor to WisDOT roundabout program. MTJ's work is focused on roundabout solutions to complex traffic problems. Mark works with agencies and consultancies on roundabout feasibility analysis, design, review, and public outreach/education. Mark is co-author of the 2010 FHWA Roundabout Guide (NCHRP 672), is an authorized FHWA Peer-to-Peer Roundabout Reviewer, and an active member of the ITE and TRB Roundabout Committees.
Ting-Li Lin has a Master's degree and a PhD degree in statistics from the University of Wisconsin-Madison. He has 12 years of experience in statistical consulting, working with researchers and scientists from various fields, including engineering, biomedical science, agricultural science, social science, and the pharmaceutical industry. Ting-Li recently joined the Frontier Science and Technology Research Foundation as a research scientist.


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.

Read the full paper here (PDF)