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
. In Table 40, some cells are in the wrong row. An erratum
provides the correct table.