Chapter 4 - Elements of Design

4.2. Horizontal Alignment

4.2.1. General Considerations

• Alignments should be as directional as practical, but should also be consistent with the topography, preserve developed properties, and take into consideration community values.

    

• In general, the number of short curves should be kept to a minimum.

    

• Long tangents are needed on two-lane highways such that sufficient passing sight distance is available on as great a percentage of the roadway as possible.

    

• In developing an alignment for a given design speed, the minimum radius of curvature for that speed should be avoided wherever practical. In general, flat curves should be used. The minimum radius should be used only  in the most critical conditions.

    

• In general, the central angle of each curve should be as small as the physical conditions allow, so that the highway will be as directional as practical. This central angle should be absorbed in the longest practical curve, but on two-lane highways the exception noted above applies.

  

• Consistent alignment should always be sought. Sharp curves should not be introduced at the ends of long tangents. Sudden changes from areas of flat curvature to areas of sharp curvature should be avoided, as they are contrary to driver expectancy. Where sharp curvature is introduced, it should be approached by a series of progressively sharper curves, where practical.

    

• Sharp curvature should be avoided on long, high fills. In the absence of cut slopes, shrubs, and trees that extend above the level of the roadway, it is difficult for drivers to perceive the extent of curvature and adjust their operation accordingly.

  

• Sharp curvature should be avoided near the following locations:  elevated structures; at or near a crest in grade; at or near a low point in a sag or grade; at or near intersections, transit stops, or points of ingress or egress; and at or near decision points.

  

• Abrupt reversals in alignment should be avoided. Such changes in alignment make it difficult for drivers to keep within their own lane. It is also difficult to superelevate both curves adequately, and erratic operation may result.

  

• The concepts of stopping sight distance, intersection sight distance, decision sight distance and driver expectancy should be considered during the development of horizontal alignments. If possible, the horizontal alignments of roadways should be free of curvature in and around intersections, interchanges, bridges, railroad crossings, toll plazas, drop lanes and roadside hazards.

  

• To facilitate pavement drainage, alignments should be laid out such that the 0% cross slope flat points associated with superelevation transitions on either end of a horizontal curve (if applicable) does not correspond to low points in the roadway vertical profile. Superelevation is discussed in this Manual in Section 4.5.

  

• The horizontal alignment should be coordinated carefully with the vertical profile design. This subject is discussed in further detail in this Manual in Section 4.4.

  

• The design speed of successive horizontal curves on ramps can vary as vehicles are often accelerating or decelerating. A common rule to apply to the speed design of ramps is that the design speed of the first curve of an exit ramp can be assumed to be 10 mph less than the design speed of the mainline. With each successive curve on the exit ramp, the design speed of the curve can be reduced based on computed vehicle deceleration. The process is to be reversed for entrance ramps, i.e., the design speed for curves will successively increase until the design speed of the last curve before the mainline is 10 mph less than that of the mainline.

For additional considerations in setting horizontal alignments, refer to of the AASHTO Green Book Chapter 3, Elements of Design - General Controls for Horizontal Alignment.