Defining, Measuring, and Modeling Capacity for the Highway Capacity Manual

AbstractThe highway capacity manual (HCM) is the publication used most often to estimate capacity. Field measurements in previous research have shown that the capacities at several freeway bottlenecks are noticeably lower than the values recommended in the HCM (highway capacity manual), by an order of a few hundred vehicles per lane. In addition, several articles have shown that maximum freeway throughput may differ between undersaturated and oversaturated conditions. This paper assembles capacity flows before and after the initiation of oversaturation, discusses recommended definitions of capacity, and provides capacity values and capacity prediction models for various types of freeway segments. It is recommended to define prebreakdown capacity as the 85th percentile of the 15-min average prebreakdown flow and the postbreakdown capacity as the 85th percentile of the 15-min average discharge flow. A clear drop in throughput between prebreakdown and discharge capacity values was observed. The paper concludes by providing recommended capacity values as a function of the number of lanes and the segment type. These values are significantly lower than those recommended in the HCM (highway capacity manual) for basic freeway segments.