The following excerpt was taken from the 1994 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (pp. 233-237).

. . . Maximum grades of about 5 percent are considered appropriate for a design speed of 110 km/h. For a design speed of 50 km/h, maximum grades generally are in the range of 7 to12 percent, depending on topography. If only the more important highways are considered, it appears that a maximum grade of 7 or 8 percent would be representative for 50 km/h design speed. Control grades for 60-, 70-, 80-, 90-, and 100-km/h design speeds are intermediate between the above extremes.

The maximum design grade should be used infrequently rather than as a value to be used in most cases. At the other extreme, for short grades less than 150 m and for one-way downgrades, the maximum gradient may be about 1 percent steeper. For low-volume rural highways, grades may be 2 percent steeper.

Critical Lengths of Grade for Design

Maximum grade in itself is not a complete design control. It is necessary also to consider the length of a particular grade in relation to desirable vehicle operation. The term "critical length of grade" is used to indicate the maximum length of a designated upgrade on which a loaded truck can operate without an unreasonable reduction in speed. For a given grade, lengths less than critical result in acceptable operation in the desired range of speeds. If the desired freedom of operation is to be maintained on grades longer than critical, design adjustment such as change in location to reduce grades or addition of extra lanes should be made . . .