OPERATING SPEED APPROACH TO GEOMETRIC DESIGN OF LOW-SPEED URBAN STREETS

Design speed is one tool used to attain a consistent design. It is also the most basic, fundamental criterion in highway and street design. The selection of design speed has evolved to be a method used for the design and correlation of design elements. This is reasonable in concept, but research has documented discrepancies between design speed and actual vehicle speed on existing roadways. The American Association of State Highway and Transportation Officials (AASHTO) Design Policy presents the design consistency approach. The determination of horizontal and vertical alignment is based on the selected design speed. For a given design speed, the AASHTO Design Policy presents design values for geometric elements such as minimum curve radius, stopping sight distance, and length of vertical curve. Designing geometric features using the same design speed should produce a segment of roadway with like characteristics. However, there are deficiencies in this design speed approach, especially when applied in the low-speed environment. This paper presents the analysis and interim findings from a Federal Highway Administration study on the Relationship of Operating Speeds to Roadway Geometric Design Speeds. Under this study, data were collected on low-speed urban streets with varying roadway, roadside, and driver/vehicle characteristics. The objective of the study was to develop an operating speed approach that would result in design criteria yielding vehicle speeds more consistent with the intended vehicle speeds.