Freeway Facility Methodology in Highway Capacity Manual 2000

The next edition of the Highway Capacity Manual (HCM 2000) will contain for the first time an operational analysis procedure for directional freeway facilities up to 20 to 25 km long. At the simplest level, this procedure integrates the proposed HCM 2000 methods for the analysis of basic, ramp, and weaving segments to enable the analysis of an entire facility. But the proposed facility methodology goes much further. It allows the user to analyze multiple, contiguous time intervals with timevarying demands and capacities. It can handle both undersaturated and oversaturated traffic conditions (with some limitations). In the latter case, both the spatial and time extent of congestion are estimated. Finally, the method permits the investigation of the effect of many traditional and intelligent transportation system–based freeway improvement strategies such as full or auxiliary lane additions, ramp metering, incident management, and a limited set of high-occupancy-vehicle designs on facility performance. Described here is the conceptual model for and computational steps of the methodology, with emphasis on the components for analysis of oversaturated conditions. The scope and limitations of the methods are also highlighted. Reference is given to a companion paper that describes how the results of the method were validated in the field and how they compared with those obtained from widely used freeway simulation models.