Chapter 4 - Elements of Design

4.3. Vertical Alignments

4.3.2. Maximum Vertical Grades

The grades selected for vertical alignments should be as flat as practical, and should not exceed the values listed in Table 4.5 Maximum Vertical Grades. Maximum values vary based on types of terrain, facility classification, and design speed. The maximum design grade should be used infrequently; in most cases, grades should be less than the maximum design grade. In Table 4.5, Maximum Vertical Grades, industrial roadways are defined as local and collector streets with significant (15% or more) truck traffic. Exceptions to the maximum vertical grades listed in Table 4.5. are as follows:

• For short sections less than 500-ft. and for one-way downgrades, the maximum grade may be 1% steeper than the values listed in Table 4.5.

• The maximum vertical grade for local streets, collectors and arterials may be increased by as much as 2% under extreme conditions

• Maximum values in Table 4.5 shall be reduced when upgrades cause a speed reduction greater than or equal to 10 mph.

For streets and highways requiring long upgrades, the maximum grade should be reduced so that the speed reduction of slow-moving vehicles (i.e., trucks and buses) is not greater than 10 mph. Where reduction of grade is not practical, climbing lanes should be provided to meet these speed reduction limitations. A design exception is required where a climbing lane cannot be provided and grade cannot be reduced.

Climbing lanes, speed reductions on upgrades and the critical lengths of grade associated with speed reductions are concepts that are discussed in detail in the AASHTO Green Book Chapter 3, Elements of Design. These concepts should be considered and appropriate provisions should be incorporated into any facility in which vertical grades will cause a significant (10 mph or more) reduction in the speed of a slow-moving vehicle.