Searching for a Gap Acceptance Theory Basis for Linear Capacity Models

Abstract

This technical note 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. It is shown that both the uniform and linear 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.