Revisiting Merge-Influence Area Empirically: Operations Inside Recurrent Freeway Bottlenecks

Abstract

The current effort aims to analyze traffic operations empirically at merge-influence areas positioned inside recurrent freeway bottlenecks. Four years of data at four bottlenecks in the Milwaukee County, Wisconsin, freeway system were used in the analysis. Bottlenecks were ranked on the basis of merge-area capacity use and were treated individually. The study evaluated how merge-area traffic mix influenced freeway breakdown characteristics such as prebreakdown flow (PBF), queue discharge flow (QDF), queue discharge speed, and congestion duration (CD). Merge-area traffic mix was evaluated with the ramp volume ratio (RVR), which was defined as the percentage of the ramp flow in the total merge-area traffic flow. For bottlenecks with heavily used merge areas, both capacities (PBF and QDF) and queue discharge speed were reduced when RVR increased in value. The value of CD appeared to be sensitive to RVR. The value of CD was found to double, triple, or almost quadruple when ramp flow composed a higher percentage of merge-area flow. Bottlenecks with lightly used merge areas sometimes showed different or even contradictory results. The study also modeled two capacity flows (PBF and QDF) stochastically at merge areas with the three-parametric Weibull distribution. From the current analysis, it appears that the maximum merge-area flow (4,600 passenger cars per hour) recommended by the Highway Capacity Manual is not a conservative design value.