An iterative model for computing capacities at all-way stop-controlled (AWSC) intersections has been included in the new Highway Capacity Manual (HCM) 2000. The model is based on five saturation headway values, each reflecting a different level of conflict faced by the subject-approach driver. From this model the capacity and service time at any approach can be computed using iterative calculations. The model in the HCM is a so-called approach-based model, which only takes into account the conflicting cases among the approaches. The effect of turning streams or movements is not modeled in sufficient detail. In contrast, a theoretical, stream-based model for determination of capacities at AWSC intersections has been developed. This model is based on the addition-conflict-flow (ACF) method developed from graph theory and takes into account all the traffic streams at the intersection. This allows a systematic and realistic analysis of the traffic process at AWSC intersections. The computational procedure included in the model can be conducted without iterative calculation steps. The ACF and the HCM models for intersections with single-lane approaches were comprehensively validated, as well as a modified version of the HCM model that significantly enhances its features. The results of the validation indicate that the total capacity of an AWSC intersection with single-lane approaches based on the HCM model ranges between 1,450 and 1,550 passenger cars per hour (pc/h), whereas the total capacity based on the ACF model ranges between 1,600 and 2,000 pc/h. The modified HCM model yields total capacities ranging between 1,700 and 2,000 pc/h. The ACF model and the modified HCM model yield similar capacity results under normal traffic flow conditions.
Abstract