Default Values for Highway Capacity and Level-of-Service Analyses.

The Highway Capacity Manual (HCM) is the authoritative source providing state-of-the-art methodologies for evaluating highway, transit, bicycle, and pedestrian facilities at both the operational and planning levels. These methodologies require input parameters that depend on, in many cases, detailed site-specific data. Default values are used to represent input parameters when the input parameters are difficult to measure or estimate. A default value is a representative value that may be appropriate for estimating an input parameter in the absence of local data. No nationwide research effort had been conducted to assemble field measurements to determine if the default values in the HCM represent typical field conditions. For this reason, this research study was conducted to develop guidance to assist users of the HCM in the selection of default values for various Highway Capacity Manual applications. A Guidebook was developed to provide this assistance. The results of this research are being considered by the NCHRP Panel that oversaw this work. These revised default values have not yet been formally adopted by the Committee on Highway Capacity and Quality of Service. A sensitivity analysis was conducted for 63 input parameters in HCM 2000 where default values are suggested. Nineteen of these input parameters were determined to have a high degree of sensitivity in influencing the service measure results. These sensitive input parameters influence the service measures for the following HCM chapters: Urban Streets, Signalized Intersections, Pedestrians, Bicycle paths, multilane highways, and Basic Freeway Segments. The Guidebook recommends that default values for some input parameters continue to be developed based on existing HCM guidance. Other input parameters should be based on field measurements obtained from on design plans. Recommendations were made for new default values based on the assembly of a nationwide database. Additional guidance is provided to aid in the selection of default values based on the review of recent research that reported on field measurements.