Design Speed Bill Gulick PE PLS Speed, Speed, Speed, Speed Running Speed Operating Speed Observed Speed (speed studies 85th percentile) Posted Speed Length / Time (used in cost analysis etc.)
White Sign (speed limit) Design Speed A selected speed used to determine the geometric features of the highway. (AASHTO 2011) Design Speed First Defined in 1936 (by Barnett) AASHO accepted 1938 Definition Changed over Time The maximum uniform speed which probably will be adopted by the
faster group of drivers but not, by the small percentage of reckless ones. 1936, 1938, 1940, 1941, 1945, 1954, 1973, 1977, 1984, 1988, 1994, 1997, 2004, 2011 AASHTO 1973 (1973 1994 + sort of) The maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. Design Speed Current definition is the 14th (I think) AASHTO Green Book 2011 A selected speed used to determine the various geometric design features of the highway. 14 Words to define and 19 paragraphs advising how to pick a good (appropriate) one. Logical
All feature related to obtain balance Above minimum where feasible Meet Driver Expectation (where practical) Within constraints of S.E.E. (social, environmental, economics) Design Speed More Advice on Design Speed On low speed facilities above minimum criteria may encourage higher speeds.
Higher Design Speeds are Appropriate for Higher Functional Class Roadways Design Speed is not particularly relevant in (true < 45mph)Urban Areas. (except for sight distances). Critical element in Rural Areas is Horizontal Curves and Superelevation. Design Speed should reflect the DESIRED Operating Speed Design Speed Other Factors (more advice) Posted Speed (PS) good correlation with Operating
Speed (OS) Design Speed (DS) little correlation with OS DS < PS = Lower Speeds (minor correlation) DS > PS = Higher Speeds (more correlation) Bigger Right Shoulder = Higher Speeds Bigger Median = Higher Speeds Note lack of the word safe in new definition.
Liability Associated with DS vs. PS over stated. Design Speed KYTC Highway Design Manual (2008 edition) Design Speed is a selected speed used to determine the various geometric features of the roadway. It is the highest continuous speed at which individual vehicles can travel with safety upon a highway when weather conditions are favorable, traffic density is low, and the geometric design features of the highway are the governing conditions for safe speed. 1970s again in mid 90s (not mid 80s). Until 1994 Design Speed
DESIGN SPEED Per revised NEW DESIGN MANUAL: KYTC HD-703 Design speed is a selected speed used to determine the various geometric features of the roadway. The selected design speed should be a logical one with respect to the anticipated operating speed, topography, the adjacent land use, and the functional classification of the highway. In selecting the design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental impacts, economics, project scope, aesthetics, and social or political impacts. The selected design speed should be consistent with the speed that drivers are likely to expect on a given highway facility. For a further discussion of the philosophy of design speed, see AASHTO's A Policy on Geometric Design of Highways and Streets. Design Speed 2011 AASHTO Green Book Minimum Recommended Design Speeds
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