Deriving Capacity and Critical Density of Basic Freeway Segments

Abstract

The variability of freeway capacity has been extensively analyzed in numerous studies. Recent research has also focused on the critical density at capacity and its relationship to breakdown occurrences across different segment types to better quantify the quality of service for freeways. Despite recent progress, identifying a representative value for the capacity and critical density of basic freeway segments remains challenging. These segments seldom reach capacity, because breakdowns often result from downstream bottlenecks. This paper aims to estimate the capacity and critical density of basic freeway segments by implementing a methodology that quantifies how variations in the number of merging ramp vehicles affect the total downstream capacity and critical density. The assumption was made that low onramp volumes would cause merge segments to operate in a similar manner to basic segments. The results demonstrated that decreasing the number of merging ramp vehicles reduced the total critical density and increased the total capacity, implying that basic segments may have a lower critical density and higher capacity compared with merge, diverge, and weaving segments.