Where it is necessary to accept a steeper than normal grade, designers should evaluate the operational effects of the grade on heavy vehicles. The AASHTO Green Book (2) includes a set of design curves that enable estimation of vehicle speeds on upgrades.

Where it is necessary to accept a steeper than normal grade, designers should evaluate the operational effects of the grade on heavy vehicles. The AASHTO Green Book (2) includes a set of design curves that enable estimation of vehicle speeds on upgrades. Use of these curves can help uncover where short, steep grades will have little adverse operational effect, and hence may be acceptable. Where it is necessary to design longer upgrades, mitigation measures can include provision for wider shoulders or climbing lanes. Mitigation of steep downgrades can include design of truck escape ramps, increased shoulder widths and clear zones at the bottom of the grade, and increased superelevation rates. Finally, designers should strive for greater than minimum horizontal curves at the bottom of steep downgrades. Where design values outside the normal ranges are considered, mitigation efforts should focus on the potentially adverse operational issues discussed above. Acceptance of grades below the minimum can be accompanied by steeper than normal yet traversable cross slopes, special drainage designs, and special attention to the design of pavement edges at superelevation transitions to horizontal curves. Avoidance of flat spots along the pavement should be the objective. Designers can mitigate the use of a sub-minimum grade by carefully checking pavement contours and edge of pavement elevations, making local adjustments to ensure proper drainage as part of the final design process.

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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