The AASHTO publication A Policy on Geometric Design of Highways and Streets
(Green Book) defines "design speed" in several locations throughout the text; in at least one location, design speed is described as the maximum safe speed. The Green Book also defines or uses other terms including operating speed, average running speed, and 85th percentile speed. Other policies and practices govern the posting of regulatory and advisory speeds on highways. Current definitions are unclear, are difficult to apply, and include language that can be difficult to explain and defend to the public and in legal proceedings.
There is a need to reevaluate how AASHTO defines speed-related terms and uses speed as a control in its technical policy and guidelines on geometric design. This evaluation needs to fully account for the relationships among speed, geometric design elements, and highway operational practices such as setting posted and advisory speeds. For example, some geometric design elements are based directly on design speed, others are related indirectly; some influence drivers' speed choice, others do not. These elements need to be identified and evaluated, and alternatives to speed-based design criteria need to be identified and evaluated.
The objectives of this research were to evaluate the relationships among speed, geometric design elements, and highway operational practices and recommend appropriate revisions to the AASHTO technical policies and related highway operational guidelines.
The revised final report is available as NCHRP Report 504
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provides the correct table.
To accomplish these objectives, the following tasks are envisioned: (1) Review current practice. (a) Synthesize the latest information on various speed designations (e.g., running, design, operating, posted, advisory, 85th percentile) and their relationships to geometric design elements and highway operations. Include past legal case studies regarding design speed. (b) Survey current state and local practices for establishing design speed and posted speeds. Include experiences with the relationship of operating speed to design and posted speeds. (c) Analyze speed definitions and applications found in the literature search and surveys. Identify inadequacies and inconsistencies with recommendations for solutions. (2) Critically review design elements to determine if they need to be based on speed. Identify alternative design-element-selection criteria other than speed or suggest refinement of the existing speed-related methods. (3) Prepare an interim report summarizing the results of Tasks 1 and 2. The interim report shall recommend procedures for determining design-element values, identify data needs to validate this procedure, and propose a detailed work plan for collecting the needed field data and for completing the remaining tasks. (4) Collect and analyze the necessary field data to refine and support the procedures outlined in the approved interim report. (5) Modify as needed and validate the recommended approach. Select a representative sample of recently constructed or reconstructed roadways that met the then current AASHTO technical policies where selection of design speed was an issue and evaluate the impact the recommended procedures would have had on design and operations. (6) Based on the approach validated in Task 5, develop a comprehensive set of recommended design guidelines and suggested modifications to the AASHTO technical policies and related highway operational practices. The products of this task shall include text, tables, and illustrations. (7) Prepare a final report that documents the entire research effort. The product of Task 6 will be included as two stand-alone appendices, one dealing with the AASHTO Green Book and the other with other policies and practices.