4.9 Combination of Horizontal and Vertical Alignment

Due to the near permanent nature of roadway alignment once constructed, it is important that the proper alignment be selected consistent with design speed, existing and future roadside development, subsurface conditions, topography, etc. The following factors are general considerations in obtaining a proper combination of horizontal and vertical alignment:
  • The design speed of both vertical and horizontal alignment should be compatible with longer vertical curves and flatter horizontal curves than dictated by minimum values;
  • Design speed should be compatible with topography with the roadway fitting the terrain where feasible; and
  • Alignment should be as flat as possible near intersections to maximize intersection sight distance.
For rural divided facilities:
  • Independent mainlane profiles are often more aesthetic and economical;
  • Where used on non-controlled access facilities with narrow medians, median openings should be placed at locations that minimize crossover grades and provide adequate sight distance for vehicles stopped therein; and
  • When designing independent vertical and horizontal profiles on divided facilities, considerations should be given to the impact these profiles may have on future widening into the median.
For rural two-lane highways and Super 2 highways:
  • The need for safe passing sections at frequent intervals should be carefully considered in developing horizontal and vertical alignments.
Refer to
AASHTO's A Policy on Geometric Design of Highways and Streets
for examples of good and poor practices in alignment and profile relationships.