Developing Guidance for Calculating the Downstream Functional Area of an Intersection

About the project:

Appendix F of VDOT’s Road Design Manual (RDM) (VDOT, 2025) provides guidance and standards for several spacing standards including entrance, corner clearance, and intersection functional area.  Of these, the spacing standard with the least guidance is on the dimensions of the functional area.  VDOT’s RDM has guidance derived from AASHTO (2018) for upstream functional area dimensions but provides no guidance for the dimensions of the downstream functional area. The purpose of this project is to provide clearer guidance on calculating the downstream functional area for signalized intersections. 

The project tasks will include conducting a (1) literature review, (2) survey of best practices, and (3) exploratory crash analysis.  Based on the results of these tasks, guidance will be developed for inclusion into Appendix F of VDOT’s Road Design Manual (VDOT, 2025). This guidance will include methods and a spreadsheet-based tool for calculating appropriate areas (distances) for downstream functional areas as well as more descriptive details on calculating upstream functional areas.  The deliverable will be a final report documenting findings from the research tasks.  With clear appropriate guidance on functional area dimensions, more consistent entrance connection standards can be applied across the Commonwealth - thus improving predictability and saving time for VDOT, localities, and land developers in the approval process.   

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