The National Academies

NCHRP 15-22 [Completed]

Safety Consequences of Flexibility in Highway Design

  Project Data
Funds: $499,407
Research Agency: University of Kentucky
Principal Investigator: Nikiforos Stamatiadis
Effective Date: 9/11/2001
Completion Date: 12/30/2004

Background: The AASHTO publication, A Policy on Geometric Design of Highways and Streets, a.k.a. the "Green Book," provides detailed guidance and control values for the design of new alignments or of those undergoing major reconstruction. Most of the design controls represent limits (i.e., maximums or minimums); however, in some instances, a range of design values or even a single value is recommended. For most controls, the "Green Book" indicates that a combination of theory, measurement, and practice has shown the recommended control to provide a safe, comfortable, and aesthetically pleasing roadway.

When highways pass through rural communities, questions arise as to the appropriate values for geometric design elements. Research is needed to provide information on the safety consequences (positive and negative) of varying geometric design values in rural communities ( The term "rural communities" as used in this project refers to developed areas, which may or may not be incorporated, as well as to the transition zones between high-speed operations in undeveloped surroundings and the lower speed operations in the developed area). Emphasis should be given to the major geometric elements. Flexibility in design is one of the tools designers can use to achieve context-sensitive design. Information from this study will be useful to designers involved with resurfacing, restoration, rehabilitation, and reconstruction projects where flexibility in design is often employed.

Objective: The objective of this project is to develop guidance to enable project planners and designers to estimate the safety consequences of varying geometric design values for highways through rural communities.

Tasks: To accomplish the objective the following tasks are envisioned.

PHASE I: (1) Synthesize available information on the safety consequences of varying geometric design values on highways passing through rural communities. (2) Survey state departments of transportation and others to identify projects on highways through rural communities that demonstrate geometric design flexibility. The survey shall identify the geometric design elements where flexibility was used, why flexibility was used, and the availability and extent of before-and-after data. (3) Submit an interim report that summarizes the information from Tasks 1 and 2 regarding alignment, cross-section, and intersection geometric design elements such as (a) lane and shoulder widths, (b) design speed, (c) horizontal and vertical alignment, (d) superelevation, (e) intersection sight distance, (f) median presence, (g) clear zone, (h) shoulder versus curb and gutter, (i) sidewalk width, and (j) intersection curb return radius. The interim report shall include a data collection and analysis plan to develop information to estimate the safety consequences of design variations for highways through rural communities. The interim report shall also outline the final format in which the information will be presented. (4) Meet in Washington, D.C., with the project panel to review the interim report approximately 1 month after its submittal. Submit a revised interim report reflecting the panel's review comments.

PHASE II: (5) Conduct the data collection and analysis as outlined in the approved work plan, and develop the information in the format outlined in the interim report. (6) Submit a final report documenting the entire research effort. The final report shall describe how the project was conducted. The final report should include clear, practical guidance in a format suitable for incorporation into design and safety documents (e.g., "Green Book") and processes (e.g., Interactive Highway Safety Design Model) for resurfacing, restoration, rehabilitation, or reconstruction projects. The information should enable project planners and designers to estimate the safety implications of design decisions for specific projects. This information may be in the form of charts, tables, and estimation procedures.

Status: The revised final report has been submitted by the contractor. The report will not be published

Product Availability: The revised final report is available for loan from the NCHRP.

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