Estimation of critical headways at unsignalized intersections – a microscopic approach

Understanding gap-acceptance behavior is fundamental to safety and capacity analyses of two-way controlled intersections and roundabouts. Capacity models based on gap-acceptance theory rely on the critical headway parameter, whose estimation raises some difficulties: it depends on several external variables and cannot be observed directly. Existing methods, based on field observations, produce estimates that are mostly site-specific, which limits their applicability under different geometric and operational conditions. In this paper an alternative estimation method is proposed. The model is based on the interactions between the entry and opposing vehicles, described at microscopic level. It captures the complex interactions between the driver/vehicle dynamics and the intersection geometry, but is simple enough to be easily implemented in a spreadsheet. The model was calibrated based on a video recording of a roundabout in Coimbra, Portugal, and validated at other roundabouts, with different geometric and traffic characteristics. The resulting estimates are close to the results of conventional estimation methods, suggesting that the proposed model is a promising alternative to field observations, particularly for the study of non-conventional intersections.