A Policy on Geometric Design of Highways and Streets

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Abstract:

American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Fifth Edition, (2004) (6th Edition to be released November 2011)

Access management is addressed in several sections of the AASHTO “Greenbook,” which was extensively updated in the Fourth Edition, published in 2001. Chapter 1 presents access needs and controls in the context of functional classification (p. 6) and reviews the functional characteristics (e.g. mobility versus access, roadway spacing) of rural and urban highway systems. The extent and degree of access control is identified as “a significant factor in defining the functional category of a street or highway.” (p. 6) The most basic factor in providing travel mobility is noted as “operating speed or trip travel time.” The importance of a complete functional design system that provides a hierarchy of movements is emphasized:

“The failure to recognize and accommodate by suitable design each of the different trip stages of the movement hierarchy is a prominent cause of highway obsolescence. Conflicts and congestion occur at interfaces between public highways and private traffic-generating facilities when the functional transitions are inadequate. Examples are commercial driveways that lead directly from a relatively high-speed arterial into a parking aisle without intermediate provisions for transition deceleration and arterial distribution, or more seriously, freeway ramps that lead directly into or from large traffic generators such as major shopping centers.” (p. 2)

It calls for functional classification as a design type, whereby “design criteria and level of service vary according to the function of the highway facility,” and indicates that “the use of functional classification as a design type should appropriately integrate the highway planning and design process.” (p. 13) This differs from traditional concepts of functional classification, whereby “highways with comparable traffic volumes are constructed to the same criteria and provide identical levels of service, although there may be considerable difference in the functions they serve.”
Chapter 2: Design Controls and Criteria includes the largest section devoted specifically to access management (pp 88-95). Access classification is identified as “the foundation of a comprehensive access management program” that “relates the allowable access to each type of highway in conjunction with its purpose, importance, and functional characteristics.” (p. 90) This section parallels the contents of the 2003 AMM, and therefore is not annotated in detail here. Reducing left and right turns is identified as method of reducing pedestrian-vehicular conflicts (p. 99). Chapter 7 addresses regulation of curb parking and notes that “curb parking is permissible on arterials when speeds are low and traffic demand is well below capacity,” adding that “eliminating curb parking can increase the capacity of arterials with four- or six-lane curb-to-curb widths by 50 to 80 percent.” (p. 491) Several other sections throughout the “Green Book” address design elements of importance to access management (e.g. auxiliary lanes, medians, frontage roads, bus turnouts, etc.)

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