New Research on Roundabout Capacity Models

Aerial photograph of road

Mark Besley will present our analyses of HCM roundabout capacity research data at the TRB 6th International Conference on Roundabouts (15-18 May 2022).

These are now available in our Publications section.

AKÇELIK, R., SHIRKE, C., BESLEY, M., ESPADA, I. and BILLINGHURST, D. (2022). A Comparative Analysis of Exponential and Linear Roundabout Capacity Models Using HCM Research Data. Technical Note. Akcelik & Associates Pty Ltd, Melbourne, Australia.

Abstract

This report presents an investigation to assess basic exponential (non-linear) and linear model forms used in practice for roundabout capacity estimation. Best fit regression and anchored regression analyses were carried out using the HCM single-lane roundabout capacity research data. The full dataset as well as the Glens Falls and Carmel data subsets were analysed. These two data subsets for different roundabout geometry types represent a horizontal slicing of data with different capacity levels over the same range of circulating flows. To assess the applicability of basic exponential and linear models to low and high circulating flow ranges, additional data subsets were used by vertical slicing of the HCM capacity data. Using this method, two-segment linear and exponential models were analysed. Two-segment analyses were carried out using aggregate data as well. The models assessed also included the TRL-Kimber linear capacity model and the HCM exponential model with a new simplified version of the SIDRA geometry method added. The report includes discussions on frequency of data points in low, medium and high circulating flow ranges, the sum of entering flow (capacity) and circulating flow, the ratio of entering flow to circulating flow, issues related to anchored regression models, follow-up headways implied by best fit regression models being larger than measured values, and the quadratic model. As an issue related to underestimation of capacity at low circulating flows, a detailed single-lane roundabout example is given for unbalanced flow conditions under high demand levels. In conclusion, the assessments from various perspectives reported in this document demonstrate the non-linear characteristic of roundabout capacity data as a function of the circulating flow, and support the HCM exponential (non-linear) roundabout capacity model over the linear model form.

AKÇELIK, R. (2022). Searching for a Gap Acceptance Theory Basis for Linear Capacity Models. Technical Note. Akcelik & Associates Pty Ltd, Melbourne, Australia.

Abstract

This report presents an investigation to explore if a linear capacity model can be derived as a gap-acceptance capacity model assuming a uniform or linear arrival headway distribution of the opposing traffic stream. The uniform and linear arrival headway distributions are introduced and the gap-acceptance capacity models based on these headway distributions are presented. The derivations follow the signal analogy method used by the author in deriving the gap-acceptance capacity equations used in the SIDRA INTERSECTION software. These uniform and linear headway distributions are not realistic given the random nature of arrival headways including bunching considerations. It is shown that both headway distributions result in non-linear gap-acceptance capacity models. Although a form close to a linear model could be obtained by choosing low values of critical gap, these chosen critical gap values were not realistic when compared with observed values indicated by the HCM and Australian research data. The report also discusses application of a simplified SIDRA geometry method for estimating the parameters of the HCM (Siegloch) exponential roundabout capacity model. Results are promising when applied to the HCM roundabout capacity research data with favourable comparisons with the TRL linear empirical model. Alternative calibration methods for the TRL linear roundabout capacity model and HCM (Siegloch) exponential model are also discussed. It is shown that satisfactory results are obtained when the method was applied to the HCM model with a simplified version of the SIDRA geometry method added.

TRB 6th International Conference on Roundabouts
Monterey, CA - May 15-18 2022

The TRB 6th International Conference on Roundabouts gathers a professional community to share and consider experiences and knowledge with the goal to increase successful roundabout projects. Traditionally a triennial conference, a strong interest has continually grown since the first in 2005. This conference provides a forum for the exchange of ideas on all aspects of roundabouts as we work together to improve the safety and sustainability of our roadway network including: Innovative Roundabout Solutions, Non-Motorized User Considerations, Corridors, Mini/Compact Roundabouts, Multilane Roundabout Considerations, Design and Construction, Roundabout Planning, Public Outreach and Education, and more.

The sessions will be held in the Monterey Conference Center, situated right next to the Portola hotel.

SIDRA SOLUTIONS is a proud Platinum Sponsor of this event.

New SIDRA Online Training dates are now available for June 2022.. Upcoming Events