E 430 Combination of Horizontal and Vertical Alignment

The horizontal and vertical alignment of a high-beams go straight ahead into space. The danger is way should not be designed independently. Rather, somewhat reduced by commencing the curvature the horizontal and vertical controls are determined, at a distance well before the top of the curve as and their influence is considered jointly to arrive at the optimum highway location. The general controls to be considered for the proper combination of horizontal alignment and profile are as follows:

1. Long, flat grades at the expense of having to use excessive horizontal curvature, or long, flat curves or tangent alignments with long, steep grades are unsatisfactory extremes in design. A compromise is appropriate when, by sacrificing some of the ideal qualities of either a good horizontal alignment or a good grade, a relatively balanced design is achieved.
2. A vertical curve superimposed upon a curved horizontal alignment or vice versa is more pleasing esthetically than a series of humps visible to the driver for some distance. However, there are some attendant hazards with this type of design, and its effect on traffic operations should be carefully considered, as discussed below.
3. Horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve because it is difficult for the driver to see an alignment change. This is true especially at night where there are no other lights and as the vehicle approaches the summit, the headlight beams go straight into space. The danger is somewhat reduced by commencing the curvature at a distance well before the top of the curve as an advance warning of an alignment change, i.e. the horizontal curve is made longer than the vertical curve. 
4. Sharp horizontal curvature should not be used at or near the low point of a pronounced sag vertical curve, because the road ahead is foreshortened and any sharp horizontal curvature appears warped or distorted. In addition, trucks, in particular, tend to pick up speed at the bottom of grades, and vehicular control is more difficult to maintain.
5. On two-lane highways, the need for safe passing sections at frequent intervals, and for an appreciable percentage of the length of the highway, overrides the general desirability of the horizontal and vertical alignment combination.
6. Horizontal curvature and profile should be made as flat as feasible at highway intersections, where sight distance along both highways is important and vehicles may have to slow down or stop.
7. On divided highways, variation in the width of median and the use of separate profiles and horizontal alignments should be considered to obtain the design and operational advantages of independent one-way roadways.