OBJECTIVES

The objectives of this project were to (1) determine appropriate default values for inputs to Highway Capacity Manual (HCM) analyses and (2) to develop a guide to select default values for various applications.

STATUS

The report has been published as NCHRP Report 599.

BACKGROUND

The Year 2000 Highway Capacity Manual (HCM 2000) is the most extensively referenced document on highway capacity and quality-of-service computations in the United States. While the HCM 2000 focuses on providing state-of-the-art methodologies for operational analyses, it is also used in planning and preliminary engineering applications.

To assist engineers and planners in applying HCM methodologies, the HCM 2000 includes default values for many of the more difficult-to-obtain input parameters and variables. "A default value is a representative value that may be appropriate in the absence of local data" [HCM 2000]. As a result of insufficient field data, the HCM 2000 recommends only a single default value for many key data items, inadequately reflecting the variety of traffic and facility conditions across the United States. Because of limited resources or inexperience, analysts often use these default values inappropriately.

TASKS

PHASE I

Task 1. Review both national and regional literature and databases to identify prior research and sources of data that may be used to determine HCM default values.

Task 2. Inventory input parameters and variables for each of the HCM 2000 analysis methodologies. Submit a working paper summarizing the current knowledge on the ranges and typical values for each of the inputs; categorized by region, metropolitan area size, or other factors as appropriate. Revise the working paper based on comments from the panel and selected members of the TRB Highway Capacity and Quality of Service Committee and resubmit it in a form suitable for electronic publication.

Task 3. Recommend and rank inputs that deserve further study given the available project resources based on how sensitive the methodologies are to the input and the difficulty of obtaining a non-default input value. Justify these rankings in a conference call with the panel.

Task 4. Develop a detailed work plan for determining appropriate default values for the inputs recommended in Task 3. It is expected that this work plan will depend heavily on mining existing data sources supplemented, as necessary, by limited field data collection. The work plan should provide for an assessment of the reasonableness of analysis results using the recommended defaults.

Task 5. Prepare a detailed outline of the guide that will be developed in Task 8.

Task 6. Within 9 months of contract execution, submit an interim report documenting Tasks 1 through 5 for review by the NCHRP. The panel will meet with the contractor to review the interim report and approve the work plan prior to proceeding to Task 7. The research plan shall provide a 1-month period for review and approval of the interim report.

PHASE II

Task 7. Carry out the approved work plan developed in Task 4.

Task 8. Develop the guide outlined in Task 5. The organization of the guide should be consistent with that of the HCM 2000. Each input parameter and variable identified in Task 2 should be briefly described. For inputs that should not be defaulted (e.g., facility type, area type, basic geometry, demand volumes), describe why default values should not be used and identify possible sources of local data. For inputs for which defaults can be appropriate, provide default values or ranges of values and a rationale for selecting a default value for a particular analysis. Describe situations where it is more appropriate to use a measured or derived value rather than a default and situations where agencies should establish their own default values. Demonstrate how changes in default values affect service volume tables and typical analyses.

Task 9. Develop specific recommendations for incorporating the project results into updates of the HCM. Present current thinking on these recommendations to the TRB Highway Capacity and Quality of Service Committee near the end of the project.

Task 10. Develop recommendations for incorporating the project results into software that implements the HCM methodologies. Incorporation of these recommendations into the software should assist analysts in using default values appropriately and reviewers of analyses in understanding whether default values were used appropriately.

Task 11. Submit a final report that documents the entire research effort and includes the Task 8 guide as a stand-alone document. The products of Tasks 9 and 10 should be included as appendices to the final report.