Enhancements to the Freeway Facilities Method in the Highway Capacity Manual to Enable Reliability Analysis

Abstract

The Highway Capacity Manual (HCM) methodology provides a fast and reliable approach for the analysis of freeway traffic operations; research is ongoing to add a reliability analysis component to the method. Several key methodological enhancements are needed to make the method ready for reliability analysis; these enhancements are described and illustrated in this paper and include (a) the incorporation of the drop in capacity in the queue discharge flow compared with the prebreakdown flow; (b) the addition of a speed adjustment factor (SAF) as a new calibration factor for the modeling of nonrecurrent congestion sources, such as weather and incidents; (c) the explicit incorporation of SAFs and capacity adjustment factors (CAFs) into the HCM methodologies for merge, diverge, and weave segments; (d) the development of new SAF and CAF default values for freeways; and (e) the addition of new congestion performance measures. The enhancements overcome key limitations in the existing HCM 2010 freeway facility method, which assumes a fixed capacity throughout undersaturated and congested regimes, ignores the free-flow speed–reducing effects of inclement weather, and treats weave and merge–diverge segments as basic freeway segments for CAFs less than 1.0. In addition to the existing performance measures generated by the computational engine, some new output variables are proposed: the travel time index and a denied entry queue length measure. The various enhancements are illustrated with computational examples, and a discussion is offered as to the effects of the enhancements on conventional freeway facilities analysis, even without the evaluation of reliability.