Chapter 4 - Elements of Design

4.3. Vertical Alignments

4.3.1. General Considerations

The following are general considerations for vertical alignments:

• Maximizing sight distances should be a primary consideration when establishing vertical alignment.
   
• Long, gentle vertical curves should be used wherever possible and appropriate.
   
• “Roller coaster” or “hidden dip” profiles should be avoided by using gradual grades made possible by heavier cuts and fills or by introducing some horizontal curvature in conjunction with vertical curvature. The “roller coaster” may be justified in the interest of economy and may be acceptable in low-speed conditions, but is aesthetically undesirable.
   
• A single long vertical curve is preferred over "broken-back" grade lines (two crest or two sag vertical curves separated by a short tangent).
   
• Use a smooth grade line with gradual changes, consistent with the type of highway and character of terrain, rather than a line with numerous breaks and short lengths of tangent grades.
   
• On a long ascending grade, it is preferable to place the steepest grade at the bottom and flatten the grade near the top.
   
• Moderate grades should be maintained through intersections to facilitate turning movements. Grades should not exceed 6%, and 3% maximum is preferred.
   
• Sag vertical curves should be avoided in cuts as roadway flooding or ponding conditions may occur at these locations should the drainage system become clogged or overburdened.
   
• Vertical grades should be coordinated with required acceleration and deceleration areas, wherever possible. For instance, at an interchange, it is preferable for the crossing roadway to go over the limited access facility. That way, vehicles are on an upgrade as they decelerate towards a stop condition and are on a downgrade as they are entering the limited access facility.
   
• As much as possible, the vertical alignment should be closely coordinated with the natural topography, available right of way, utilities, roadside development, and existing drainage patterns.
   
• Vertical alignments should be properly coordinated with environmental constraints (e.g., encroachment into wetlands).
   
• When a vertical curve takes place partly or wholly in a horizontal curve, the vertical curvature should be coordinated with the horizontal curvature. See Section 4.4. Combined Horizontal and Vertical Alignments.

• When one roadway is in a tangent section and an intersecting roadway has a continuous vertical grade through an intersection, consideration should be given to rotating the pavement cross slope on the tangent roadway to a reverse crown to better match the profile of the intersecting roadway. Standard superelevation transition rates would apply.

Additional considerations for setting vertical alignments are detailed in the AASHTO Green Book Chapter 3, Elements of Design.