The AASHTO Green Book presents horizontal curve design values for a range of maximum superelevation rates ranging from 0.04 to 0.10. Agencies are free to adopt one or more ranges consistent with their terrain, climate, and other factors. Within a given design range for maximum superelevation, the full range of curvature and recommended superelevation is presented for the range of design speeds.
Designers are encouraged to use a range of curvature to establish a best alignment. Rigid adherence to minimum radius curvature is not recommended; rather, use of the full range of curvature is encouraged to fit the terrain, land use constraints, and desired operating speeds. Indeed, designers should be encouraged to use above minimum values (i.e., milder curvature) while keeping an appropriate balance of tangent and curve alignment. Although design guidelines specify one minimum value for radius as a function of design speed and superelevation, the above discussion suggests that design values less than those specified within the Green Book may be acceptable under certain conditions.
For reconstruction projects, when proposals are made to reduce the operating speed of the alignment in order to reduce construction costs or environmental impacts, the designer should carefully evaluate the crash history and alignment consistency. When less than minimum radius values are used, every effort should be made to employ appropriate mitigation measures to assure that safety will not be degraded. In some instances, actual operating speeds might be greater than the intended or nominal curve design speed. Mitigation measures may be appropriate at such locations.
Source: A Guide for Achieving Flexibility in Highway Design p. 53
Published: 2004
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 https://bookstore.transportation.org.
