The information on grades reflects design practices related to cost and operational efficiency. The AASHTO Green Book refers to “reasonable guide values for maximum designs,” and further indicates that such guidelines are based primarily on traffic operational considerations as opposed to direct safety impacts. While designers should be encouraged to stay within the AASHTO Green Book guidelines, flexibility may be acceptable to meet unique local conditions.
For example, steeper than maximum grades may be acceptable if they are short, if heavy vehicle traffic is a relatively small proportion of total traffic, or if the total traffic volume is sufficiently low such that the adverse capacity effects of grade would be negligible. Also, steep grades can be acceptable if the designer is able to provide vertical curves long enough to enable sufficient stopping sight distance.
Less than minimum grades may be acceptable where the cross slope can be designed to compensate for drainage and where alignment is primarily tangent. Climate conditions may also allow for flexibility, i.e., where rainfall and/or freezing is relatively infrequent.
Source: A Guide for Achieving Flexibility in Highway Design p. 55
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.
