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Bridges: Flexibility in the AASHTO Guidelines

The replacement or retention of bridges having historic or aesthetic value or the design of bridges on very low-volume roads may justify traveled way widths less than the indicated minimum AASHTO Green Book (2) values. The width evaluation should be made by a design professional on an individual basis as part of the retention or removal decision-making process. Once the retention or replacement width decision is made, structural details, including bridge rails, should conform to AASHTO bridge specifications. The replacement or retention of bridges having historic or aesthetic value or the design of bridges on very low-volume roads may justify traveled way widths less than the indicated minimum AASHTO Green Book (2) values. The width evaluation should be made by a design professional on an individual basis as part of the retention or removal decision-making process. Once the retention or replacement width decision is made, structural details, including bridge rails, should conform to AASHTO bridge specifications.

The AASHTO Green Book (2) offers flexibility by presenting ranges of bridge deck widths depending on functional classification and traffic volumes. Consideration of pedestrian and/or bicycle needs is in addition to these width recommendations. The AASHTO Green Book (2) further recognizes that there are situations where existing structures marginally meet the recommended width criteria, and makes provisions for structures to remain in place under certain circumstances. Where full bridge width is not achievable, interim measures (sufficient until complete reconstruction is necessary) are cited as appropriate, including signing and pavement marking. Likewise, where full vertical under-clearance cannot be achieved, lesser dimensions may be acceptable if an alternative route exists for trucks.

From A Guide for Achieving Flexibility in Highway Design, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. AASHTO publications may be purchased from that organization's bookstore at 1-800-231-3475 or online at http://bookstore.transportation.org.

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